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Spinal cord blood flow measured by 14C-iodoantipyrine autoradiography during and after graded spinal cord compression in rats
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Spinal Cord Blood Flow Measured by 14C-Iodoantipyrine Autoradiography During and After Graded Spinal Cord Compression in Rats* Anders Holtz, M.D., Bo Nystr/Sm, M.D., and Bengt Gerdin, M.D. Departments of Neurosurgery and Surgery, University Hospital, and Department of Experimental Medicine, Pharmacia AB, Uppsala, Sweden
Holtz A, Nystr6m B, Gerdin B. Spinal cord blood flow measured by z4C-iodoantipyrineautoradiography during and after graded spinal cord compression in rats. Surg Neurol 1989;31:350-60. The relations between degree of thoracic spinal cord compression causing myelographic block, reversible paraparesis, and extinction of the sensory evoked potential on one hand, and spinal cord blood flow on the other, were investigated. This was done in rats using the blocking weight-technique and ~4C-iodoantipyrine autoradiography. A load of 9 g caused myelographic block. Five minutes of compression with that load caused a reduction of spinal cord blood flow to about 25%, but 5 and 60 minutes after the compression spinal cord blood flow was restored to 60% of the pretrauma value. A load of 35 g for 5 minutes caused transient paraparesis. Recovery to about 30% was observed 5 and 60 minutes thereafter. During compression at a load of 55 g, which caused almost total extinction of sensory evoked potential and irreversible paraplegia, spinal cord blood flow under the load ceased. The results indicate that myelographic block occurs at a load which does not cause irreversible paraparesis and that a load which permits sensory evoked potential to be elicited results in potentially salvageable damage. WORDS: Autoradiography; 14C-iodoantipyrine; Myelography; Rats; Spinal cord blood flow; Spinal cord injury KEY
Introduction T h e introduction o f the weight drop-technique by Allen in 1911 [2] was the beginning o f the modern era of spinal cord trauma research. The kinetic compression
model used, in which the spinal cord is struck by a falling weight, has since been the most frequently employed trauma model in research in this field. It has great similarities to spinal cord trauma in humans, which has been a strong argument for its use. This model was later followed by others mimicking various factors considered to be involved in the pathophysiology o f spinal cord damage. Compression injuries have been studied with the balloon compression technique [11,16,18], the clip compression model [14], and the method used in the present s t u d y - - t h e blocking weight-technique [4]. The impact o f compression on the spinal cord blood flow (SCBF) has been evaluated in a few studies. Rivlin and Tator [14] observed generalized hypoperfusion in the compressed area after clip compression in the rat, and Dolan et al [5] reported a decrease in SCBF rostral and caudal to the damage. Palleske et al [13] claimed that the caudal SCBF was decreased after balloon cuff trauma, and Sandler and Tator [16] found that there was a relation between the posttraumatic SCBF and neurologic deficit. There ap° pear to have been no studies hitherto, however, on the relation between different degrees of compression trauma, the extent o f SCBF deterioration during compression, and the subsequent neurological damage. In the present study we have investigated the effects o f three different intensities o f spinal cord compression, each with its defined clinical outcome, on SCBF in the rat. The blocking weight-technique o f Croft et al [4] was used and SCBF was measured during and after compression trauma by means o f the 14C-iodoantipyrine (*4C-IAP) quantitative autoradiography technique.
Materials and Methods * The experimentalprotocol was approved by the EthicalCommittee of Uppsala University. Address reprint requests to: Anders Holtz, M.D., Department of Neurosurgery, University Hospital, S-751 85 Uppsala, Sweden. Received May 31, 1988; accepted November 25, 1988. © 1989by ElsevierSciencePublishingCo., Inc.
Animals
Male Sprague Dawley rats (ALAB, Sollentuna, Sweden) were used. T h e animals were fasted over night but had free access to tap water. 0090-3019/89/$3.50
Spinal Cord Compression in the Rat
Anesthesia and Surgery The animals were anesthetized with 3% halothane in 70/30 N20/O2, orally intubated with a polyethylene tube (PE 160) and artificially ventilated in a small-animal respirator (Braun-Melsungen, FRG). The halothane concentration was thereafter decreased to 0.5% and muscular relaxation was achieved by repeated doses of suxamethonium. A P E 50 catheter was inserted into the tail artery for repeated blood sampling and for measurement of mean arterial blood pressure (MABP). PE 50 catheters were also inserted into the carotid artery and jugular vein for blood sampling and 14C-IAP infusion. A midline longitudinal incision extending from Th4 to T h l 0 was made and a dorsal laminectomy of Th7 and Th8 was performed under microscopic guidance, leaving the dura intact. In animals subjected to methylene blue myelography (see below), the laminectomy was extended to Th9 and Thl0. The rats were placed prone on a heating pad and the body temperature was kept close to 37°C with the aid of a rectal thermistor. The halothane was now discontinued and before proceeding with the compression injury a steady-state period of 30 minutes was allowed, during which the animals were anesthetized with a 70/30 N20/O2 mixture. The acid-base status was measured at least twice and the ventilation was adjusted to obtain values within the following ranges: pH 7.35-7.45; PCO2 4.5-5.5 kPa; PO2 > 9.5 kPa; and base excess -+ 3 mmol/1 L (pH blood gas analyzer, IL 1302). Animals not fulfilling these criteria at the end of the steady-state period or showing signs of surgical injury to the spinal cord were excluded. There was concern throughout the experiment that there would be no suffering by the animals, and the animals were on anesthesia during the steady-state period when the halothane was discontinued. There are several reasons to think that the animals did not suffer pain during that period. First, during the steady-state period the mean arterial pressure was unaltered, indicating the N20/O2 anesthesia is sufficient to maintain the animals unconscious and without stress. During the steady state, the animals did not respond with an increase in blood pressure when stimulated by pain. Second, in later experiments yet to be published, spinal cord compression was carried out under combined chloral hydrate/barbiturate anesthesia, and in those animals the spinal cord compression caused a similar increase in MABP to that seen in the present study. In conclusion, there is no evidence that the animals perceived pain during the spinal cord compression.
Surg Neurol 1989;31:350-60
35 l
Spinal Cord Injury An apparatus for applying a predetermined load on the dorsal surface of the spinal cord was designed (Figure 1). An adjustable aluminum rod (A) is mounted horizontally on the main upright (B). A short weight-bearing rod (C) passes freely through the end fixture (D) on the horizontal rod. A slightly curved rectangular baseplate (E), 2.2 × 5.0 mm, area 11.0 mm 2, is fitted at the lower end of the rod and constitutes the contact area against the dura. A small plate is fixed to the upper end of the rod (F) and, by loading this plate with given weights, a defined compression force can be applied to the spinal cord. The spine was stabilized on a fixed position by clamping the spinous process of Th6 with a specially designed tissue forceps mounted on the framework. A screw (G) on the framework permits this position to be varied. At the time of injury the spine is adjusted horizontally and a predetermined weight is applied on the plate of the weight-bearing rod (F).
Methylene Blue Myelography After laminectomy a small incision was made in the dura at the T h l 0 level and a PE 50 catheter was inserted in the subarachnoid space and directed rostrally a few millimeters, whereafter the dural incision was closed by cyanoacrylate glue. The spinal cord was then exposed to a load of 20 g at the usual Th7-8 level and methylene blue (1 mg/mL in physiological saline) was infused at a pressure of 20 cm H20. The compression was then gradually reduced and the passage of contrast medium was observed and photographed.
Measurement of Sensory Evoked Potentials After laminectomy a midline scalp incision was made and recording and reference electrodes were placed frontally in the midline. Bipolar stimulating electrodes were placed in the tail. Stimulation was performed with a pulse duration of 0.1 msec and a repetition rate of 2 Hz. The stimulus intensity was gradually increased until muscle twitches were noted. The cortical activity was monitored on a Disa Averager (Type 1 4 G l l , Dantec, Skovlunde, Denmark) and Disa Electroencephalograph (Type 14All, Dantec, Skovlunde, Denmark). Cortically evoked responses before and after laminectomy served as controls.
Determination of Spinal Cord Blood Flow Spinal cord blood flow was measured by the 14C-IAP autoradiography method according to the technique described by Abdul-Rahman et al [31 and presented in detail previously [7]. Briefly, 40 IzCi of ~4C-IAP (Du-
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imml
F
Figure 1. An artist's view of the weight-holding equipment. The construction makes it possible to expose the spinal cord parallel to the table and to apply the weight perpendicularly. For an explanation of the letters, see Materials and Methods.
Pont, N E N Products, Boston, MA) was infused intravenously in 45 seconds. Repeated blood samples were obtained and the blood sampling times were recorded and used together with the isotope concentration of each blood sample to describe the temporal profile of the arterial 14C-IAP concentration. At the end of the infusion access to the spinal cord was obtained with a guillotine procedure using a double-edged chisel. The part o f the spinal cord located between the two incisions, including at least the compressed area and 5 mm above and below this area, was removed and immediately frozen in isopentane chilled to -50°C. Autoradiographs were obtained from 20 izm thick cryostat sections and from standards of known activity. Local tissue 14C activities were then determined from the optical densities of areas representing gray and white matter, using a densitometer with an aperture of 0.1 mm (Microdensitometer 3 CS, Joyce Loebel, Gateshead, England). In both gray and white matter 10 to 12 determinations were made in two to three adjacent sections and the mean of these values was recorded for each rat. Determinations were made at three levels in the compressed area: in the center of the 5.0 mm compression length, and 2 mm rostral and caudal to that point. Rostral and caudal to the compression area determinations were made 1, 3, and 5 mm from the edge o f the compression plate. Totally, nine different levels were evaluated.
Statistical Methods A multifactorial analysis of variance with multiple range testing, according to the method of least significant differences, was employed. A difference at the 5 % level was considered significant and is denoted by an asterisk. Values in Table 1 and Figures 6 and 7 are given as mean - SEM.
Experiments and R e s u l t s The experiments were divided into two main sessions. First, it was determined which three degrees of spinal cord compression would cause myelographic blockage, transient paraparesis with recovery over weeks, and extinction of sensory evoked potentials, respectively. Subsequently, the relation between these three degrees of spinal cord compression and SCBF was studied. Determination of the Graded Spinal Cord Compression
Evaluation of the Compression Requiredfor Myelographic Block Four rats were investigated with myelography. In none of the animals was there any passage of contrast medium over the compressed spinal cord area at weight loads of 8 g or higher (Figure 2). Reduction of the weight to 7 g
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Table 1. PhysiologicalVariablesat SteadyState After 5 and 60 Minutes of Compressionand 5 and 60 Minutes after Decompression Group
Treatment
Arterial pH
1
Controls
SS
2
9g5'C
3
9 g 5'DC
4
9 g 60'DC
5
9 g 60'C
SS 5'C SS 5'DC SS 5'DC 60'DC SS 5'C 60'C
6
35 g 5'C
7
35 g 5'DC
8
35 g 6 0 ' D C
9
55 g 5'C
Arterial PCO2 (kPA)
7.39 -+ 0.02 5.0 + 0.4 Myelographic block compression (9 g) 7.40 + 0.02 5.0 -+ 0,2 7.37 -+ 0.03 5.3 -+ 0.4 7.40 -+ 0.02 5.0 -+ 0.4 7.38 -+ 0.04 5.3 -+ 0.6 7.39 +- 0.01 5.3 -+ 0.4 7.40 -+ 0.03 5.3 +- 0.3 7.40 + 0.03 5.2 -+ 0.3 7.41 -+ 0.03 4.9 + 0.2 7.33 -+ 0.03 5.3 + 0.3 7.39 -+ 0.0 5.2 + 0.3 Transient paraparetic compression (35 g)
SS 5'C SS 5'DC SS 5'DC 60'DC
7.38 7.29 7.41 7.35 7.43 7.42 7.38
± 0.02 -+ 0.1 +_ 0.04 -+ 0.10 -+ 0.01 -+ 0.02 + 0.02 Extinguished SEP
SS 5'C
7.40 +- 0.03 7.30 -+ 0.06
Arterial PO2 (kPa) 18.0 -+ 2.9
Base excess (ram) - 0 . 7 -+ 1.8
16.9 14.3 17.2 17.0 14.2 15.9 16.6 16.6 16.8 17.2
-+ 2.9 -+ 4,3 -+ 3,9 -+ 3,9 -+ 2,3 -+ 2,6 -+ 1,7 -+ 2.0 +- 1.7 -+ 3,8
-1.0 -1.2 -0.4 -0.4 0.0 0.6 0.2 -0.3 -1.1 0.0
-+ -+ -+ +-+ -+ -+ -+ + ---
1.8 2.2 1.9 1.9 1.9 1.4 1.9 1.5 1.3 1.0
4.9 -+ 0.3 5.9 -+ 0.7 5.1 -+ 0.3 5.8 -+ 0.8 4.9 -+ 0.3 4.8 -+ 0.5 5.6 -+ 0.2 (55 g)
17.9 11.6 15.1 11.1 16.7 15.3 14.0
++-+ -+ -+ -+ -+
-2.1 -4.7 0.8 0 1.3 -0.1 0.2
+-+ ++ + -+ +-
0.9 2.0 2.8 2.9 1.4 2.2 0.8
4.9 + 0.3 6.3 -+ 0.8
17.9 -+ 2.0 12.8 -+ 4.5
1.6 5.1 2.7 5.2 2.9 3.1 3.4
- 1 . 1 -+ 1 - 3 . 2 -+ 2.3
Abbreviations: SS, steady state; 5'C, 5 minutes of compression; 5'DC, 5 minutes after decompression; 60'DC, 60 minutes after decompression; 60'C, 60 minutes of compression.
resulted in minor but visible passage of contrast medium bilaterally to the compression plate, and at a load of 6 g the contrast flow was unlimited. To be sure of a reproducible myelographic block in the subsequent experiments, the 9 g load was chosen.
Evaluation of the Compression Required for Transient Paraparesis The relation between different loads, duration of compression, and the resulting motor disturbance has previously been determined in our laboratory [12]. The evaluations of the motor performance were made on the inclined plane as described by Rivlin and Tator [15]. Figure 3 shows the motor performance after compression of the spinal cord for 5 minutes with loads of 20, 35, and 50 g [12]. Following the 20 g compression the rats showed only a minor disability during the first postoperative days, whereas compression at a load of 35 g caused transient paraparesis with a maximum after 24-48 hours followed by almost complete restitution in 10-14 days. On compression at 50 g the animals became paraplegic, with only slight restitution during the 3 postoperative weeks investigated. In subsequent experiments the group subjected to the 35 g load was therefore selected as the transient paraparesis group.
Evaluation of the Compression Required for Extinction of Sensory Evoked Potentials The effect of increasing compression weights on sensory evoked potentials (SEP) was evaluated in four animals. As shown in Figure 4, increasing loads from 15-45 g resulted in a progressive weakening of SEP and an increase in conduction time. At 50 g SEP was virtually extinguished in all four rats. To ensure that a reproducible extinction of SEP would have occurred in the subsequent studies, a load of 55 g was chosen. Relationship B e t w e e n Spinal Cord C o m p r e s s i o n a n d S p i n a l Cord Blood F l o w The animals were divided into nine experimental groups.
Controls Group 1. Spinal cord blood flow was determined at the end of a 30-minute steady-state recovery period.
Myelographic Block Compression (9 g) Group 2. At the end of the steady-state period a compression of 9 g was applied. After 5 minutes of
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A
Figure 2. Representative photographs of methylene blue myelography at various loads. Contrast medium is injected from the right (single arrow). (A) A t 8 g no contrast medium has passed under the plate and the left part of the spinal cord is white (double arrow). (B) A t 7 g there is a minor contrast medium passage (double arrow) and (C)at 6 g the contrast medium flow is unlimited (double arrow).
C
compression, SCBF was measured without removal of the load. Group 3. As in group 2, but SCBF was determined 5 minutes after decompression. Group 4. As in group 2, but SCBF was determined 60 minutes after decompression. Group 5. As in group 2, but SCBF was measured
after 60 minutes of compression without removing the
load.
Transient Paraparesis Compression (35 g) Group 6. As in group 2, but the compression weight was 35 g.
Spinal Cord Compression in the Rat
Surg Neurol 1989;31:350-60
steady-state pressure was significantly higher in the 35 and 55 g compression groups than in the 9 g group (Figure 5 B). At the end of compression M A B P had returned to the steady-state level and it remained so until sacrifice in all the groups• Simultaneously, with the normalization of blood pressure the bradycardia and ectopic beats disappeared.
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T h e results concerning SCBF are presented in Figures 6 and 7. 1 '2 3 4 5
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A s s e s s m e n t Day F i g u r e 3. The motor performance measured at the inclined plane on different days postoperatively in three series of rats, all subjected to spinal cord compression for 5 minutes, but with different loads (20, 35, and 50 g). The motor performance is defined as the maximum angle of the plane of which the animal can maintain its position. Data from [11]. H = 20
g: C) - - C) = 35 g,' i o o o l
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F i g u r e 4. Representative recordings of senso~ evoked potentials during compression with increasing weights.
50 g.
Group 7. As in group 3, but the compression weight
0g
was 35 g.
Group 8. As in group 4, but the compression weight was 35 g.
Extinguished Sensory Evoked Potentials (55 g) Group 9. The same type o f compression as in group 2, but the compression was 55 g.
15g
Physiological Variables T h e values for the physiological variables before, during, and after the different grades o f compression are given in Table 1. T h e r e were no differences in body weight, duration o f operation, p r e - s t e a d y - s t a t e blood glucose concentration, or body temperature between the groups (data not shown). During compression there was a rise in PCO2 and a decrease in p H and base excess. This decrease was significantly m o r e pronounced at loads of 35 and 55 g than at 9 g. These alterations were transient, however, and were fully normalized 5 minutes after decompression.
_
35g
45g Cardiovascular Response A rapid increase in both systolic and diastolic pressure was noted within a few seconds after the onset of compression. T h e pressure response was followed by a bradycardia and ectopic beats somewhat after the initial pressure peak, which as a rule did not exceed 60 seconds (Figure 5 A). The rise in M A B P measured as the difference between peak pressure and preload
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Figure 5. (A) A n arterial pressure recording from an animal exposed to a 35 g load for 5 minutes. Top left, the rapid increase in blood pressure is seen and, top right, the bradycardia and frequent extrasystoles that occur shortly after the pressure peak. (B) Mean arterial blood pressure during and after spinal cord compression at different loads. - - = 9 g," : 3 5 g, e e e e = 5 5 g. Figure 6. Gray matter spinal cord blood flow in the different experiments. (A) A t the end of a 5-minute compression at different loads. Significant differences: * = versus control," @ = 9 g versus 35 g; 0 = 9 g versus 55 g; @ = 35 g versus 55 g. (B) A t the end of 9 g compression for different times. Significant differences: * = versus control; • = 5 minutes versus 60 minutes. (C) 5 and 60 minutes after 9 g decompression. Significant differences: * = versus control; • = end of compression versus 5 minutes of decompression," A = end of compression versus 60 minutes of decompression. (D) 5 and 60 minutes after 35 g compression. Labels as in (C). a
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Spinal Cord Compression in the Rat
Surg Neurol 1989;3 l:350-60
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Figure 7. White matter spinal cord blood flow in the different experiments. Labels as in Figure 6.
During compression at all loads used there was a significant reduction of SCBF in both gray and white matter. Figure 8 shows the appearance of autoradiographs from animals exposed to the three different loads. After 5 minutes of compression with a 9 g load there was a reduction in the gray matter SCBF in the central compression area from a control value of 85 -+ 11 to 19 -+ 11 mL/min/100 g, and in the white matter SCBF from 16 to 4 mL/min/100 g, i.e., to 22% and 25% of the precompression values, respectively (Figures 6 A and 7 A). There was also a less pronounced decrease in SCBF in the noncompressed spinal cord 1 and 3 mm from the edge of the compression plate. An extension of the compression time to 60 minutes was not followed by any further reduction of SCBF (Figures 6 B and 7 B). At the end of the 60-minute compression, however, SCBF was more deteriorated in the noncom-
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pressed gray matter, while the white matter SCBF was similar after 5 and 60 minutes of compression. Compression weights of 35 and 55 g resulted in blood flow values in both gray and white matter that were significantly lower than in the 9 g group (to mean values of 9 -+ 5 and 0.4 -+ 1 mL/min/100 g for 35 g and 4 - 5 and 0 - 0 for 50 g, respectively) (Figures 6 A and 7 A). The 35 g load was followed by a decrease in SCBF, which on an average was between that seen with loads of 9 and 55 g. However, there was no difference in SCBF of the compressed area between 35 and 55 g loads, while SCBF was more affected in the adjacent spinal cord during compression with a 55 g than with a 35 g load.
Spinal Cord Blood Flow After Trauma In the 9 g group, SCBF in both gray and white matter was restored to approximately 60% of the pretrauma value 5 minutes after the compression (Figures 6 C and 7 C). It was still significantly lower than the pretrauma
358
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value, however. The flow in the noncompressed adjacent few millimeters of spinal cord was also decreased. In comparison with the 9 g group, the animals subjected to compression at 35 g showed more pronounced impairment in gray and white matter SCBF, both in the compressed and in the adjacent areas, 60 minutes after decompression (Figures 6 D and 7 D). At that time SCBF in the compressed area was restored to approximately 30% of the pretrauma value. There was no further improvement in SCBF in either the 9 g or 35 g group 60 minutes after decompression, either in the compressed area or in the adjacent levels.
B Discussion
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Figure 8. Autoradiographs of the central area of compression in representative animals subjected to different grades of trauma. (A) Control animal, (B) 9 g compression, (C) 35 g compression, and (D) 55 g compression.
In this study we have shown that exposure of the rat spinal cord to three increasing compression loads was associated with increasingly pronounced derangements of SCBF, both in the compressed and in the adjacent noncompressed spinal cord. At a 9 g load for 5 minutes causing myelographic blockage, the gray matter SCBF was reduced to about 25% of normal. An extension of the compression time to 60 minutes did not have any impact on SCBF in the compressed cord, but there was a further reduction in the gray matter SCBF in the noncompressed adjacent spinal cord, indicating that the duration of compression influences the flow to the noncompressed parts of the spinal cord. After decompression, SCBF reached 60% of the control flow value in the compressed area within 5 minutes and remained unchanged at the 60-minute measurement. As the experimental setup did not allow an evaluation of SCBF at later times, we have not determined how long this depression lasts. In the 35 g compression group, where SCBF during compression was less than 10% of the control value, a transient paraparesis followed, and although there was a marked impact on the amplitude and conduction time for the SEP, it could still be evoked (Figure 4). This is in accordance with the findings of Kobrine et al [11], who observed that, as long as there was a significant flow through the spinal cord, SEP remained. Our result also suggests that at this load the compression as such is not severe enough to extinguish signal conduction over the damaged area, although a paraparesis later follows. The first hour after decompression the flow restitution was limited in the 35 g group and at both 5 and 60 minutes SCBF was only about 30% of the control flow. This value is better than that reported by Rivlin and Tator [14], who observed a recovery of gray matter SCBF to less than 10% at 2 hours. In their study, however, they used a %minute clip compression which resulted in very
Spinal Cord Compression in the Rat
severe neurological damage exceeding that obtained by 50 g of compression in a previous study in our laboratory [12]. Although the blocking weight-technique has been used by several authors to obtain a standardized spinal cord trauma, the effects on SCBF have not been evaluated. Data on SCBF are available, however, f r o m studies with the extradural cuff/balloon technique, which has similarities to the blocking weight-method [10]. Sandier and Tator [16] evaluated the former technique in primates and used two degrees of damage. After a moderate trauma which caused paraplegia in 50% of the animals and paraparesis in the remainder, they observed a 50% decrease in both gray and white matter SCBF 1 hour after the compression. These are figures close to those of the present study. After severe trauma, producing paraplegia in almost all animals, they observed that SCBF was reduced to less than 10% of that in the controls at the 1-hour period. In the present study a 55 g load for 5 minutes caused SCBF to fall very close to zero. This was sufficient to result in a virtually extinguished SEP at the end of compression and paraplegia for at least 3 weeks. In the animals exposed to a 35 g load, SEP persisted but was depressed, and SCBF was low but not totally eliminated during compression. As these animals recovered almost completely with time, a persistent SEP, even if very weak, could indicate that a good functional recovery is possible. In contrast to the above results by Sandier and Tator [16], and those of the present study, Senter and Venes [ 17], using the weight drop-technique, found that SCBF was not affected until 1 hour after trauma, when ischemia developed. This suggests that a different type of vascular damage is involved in the SCBF disturbance seen after a kinetic trauma than after a static compression. As the decrease in SCBF is related to the load applied and also to the dysfunction obtained, it may be suggested that an ischemic component contributes to the final damage. It seems unlikely, however, that the ischemia achieved only during the 5-minute compression is responsible for the damage. After such an ischemic insult to the brain there is a profound postischemic hyperperfusion [9]. This reflow pattern is distinctly different from that seen after the spinal cord compression in the present model, which is characterized by a lack of postischemic hyperperfusion and a rapid but limited restoration of flow as early as within 5 minutes, whereafter no further improvement occurs during the first hour. As SCBF during the first hour after 35 g compression was only 30% of the precompression value, it is possible that a long period of relative ischemia follows the compression. The absence
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of postischemic hyperperfusion after only 5 minutes of subtotal ischemia further indicates that the spinal cord vascular bed in itself has suffered from the compression trauma. Such damage, by its hampering effects on postcompression circulation, might contribute to the pathophysiology of the final insult. The immediate and transient increase in blood pressure on spinal cord compression is mediated by a peripheral c~-adrenergic stimulation [6]. In the present experiments the blood pressure response was very transient and phasic in nature and was more pronounced at the two heavier loads. It has been suggested that this response is not depending on efferent signals through the dorsal column, but rather arises from the compression of the cord itself [1]. During compression at 35 and 50 g there was a significant increase in arterial PaCO2. The mechanism for this is not clear. The animal is mechanically fixed in such a position that no impact of the spinal cord load on the respiration is possible. It might, however, be secondary to an increased peripheral metabolism and washout of COz from the tissues due to the profound and rapid sympathetic stimulation that occurs when the spinal cord is compressed. In reference to the clinical situation, it is interesting to note that a degree of compression that is barely sufficient to obstruct the passage of contrast medium at myelography reduces SCBF to about 25% of normal. After a 5-minute compression at this load there is no permanent neurological damage [12]. It is not known, however, what damage is caused by longer exposure. The time limit beyond which irreversible neurological damage occurs in humans following a similar compression is not known. Furthermore, the compression force causing an obstructed passage of contrast medium in an individual patient is not measurable. In the most favorable situation the patient might have suffered from only a minor initial contusion and subsequent compression, just enough to block contrast passage. Emergency myelography or computed tomography myelograpky, with surgical decompression in cases of contrast blockage, is therefore a reasonable clinical measure [8]. We wish to thank Ulla Karlsson and Inger Fogelberg for skillful technical assistance, and the Department of Neurophysiologyfor the SEP evaluations. We are also grateful to Mr. Mats Linder for the excellent artistic illustrations.
References 1. Alexander S, Kerr FWL. Blood pressure responses in acute compression of the spinal cord. J Neurosurg 1964;21:485-91. 2. Allen AR. Surgery of experimental lesion of spinal cord equivalent to crush injury of fracture dislocation of spinal column. JAMA 1911;57:878-80.
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3. Abdul-Rahman A, Dahlgren N, Ingvar M, Rehncrona S, Siesj6 BK. Local versus regional cerebral blood flow in the rat at high (hypoxia) and low (phenobarbital anesthesia) flow rates. Acta Physiol Scand 1979;106:53-60. 4. Croft TJ, BrodkeyJS, Nulsen FE. Reversible spinal cord trauma: a model for electrical monitoring of spinal cord function. J Neurosurg 1972;36:402-6. 5. Dolan EJ, Tator CH, Endrenyi L. The value of decompression for acute experimental spinal cord compression injury. J Neurosurg 1980;53:749-55. 6. Eidelberg EE. Cardiovascular response to experimental spinal cord compression. J Neurosurg 1973;38:326-31. 7. Holtz A, Nystr6m B, Gerdin B. Regulation of spinal cord blood flow in the rat as measured by quantitative autoradiography. Acta Physiol Scand 1988;133:485-93. 8. Jelsma RK, Rice JF, Jelsma LF, Kirsch PT. The demonstration and significance of neural compression after spinal injury. Surg Neurol 1982;18:79-92. 9. Kalgstr6mE, Smith ML, Siesj6 BK. Recirculation in the rat brain following incomplete ischemia. J Cereb Blood Flow Metabol 1983;3:183-92. 10. Khan M, Griebel R. Acute spinal cord injury in the rat: comparison of three experimental techniques. Can J Neurol Sci 1983;10:161-5.
Holtz et al
11. Kobrine AI, Evans DE, Rizzoli H. Correlation of spinal cord blood flow and function in experimental compression. Surg Neurol 1978;10:54-9. 12. Nystr6m B, Berglund J°E. Spinal cord restitution following compression injuries in rats. Acta Neurol Scand 1988;78: 467-72. 13. Palleske H, Kivelitz R, Loew F. Experimental investigation on the control of spinal cord circulation. Acta Neurochir 1970;22: 29-41. 14. Rivlin AS, Tator CH. Effect of duration of acute spinal cord compression in a new acute cord injury model in the rat. Surg Neurol 1978;10:39-43. 15. Rivlin AS, Tator CH. Objective clinical assessment of motor function after experimental spinal cord injury in the rat. J Neurosurg 1977;47:577-81. 16. Sandier AN, Tator CH. Effect of acute spinal cord compression injury on regional spinal cord blood flow in primates. J Neurosurg 1976;45:660-76. 17. Senter HJ, Venes JL. Loss of autoregulation and posttraumatic ischemia following experimental spinal cord trauma. J Neurosurg 1979;50:198-206. 18. Tarvlov IM, Klinger H, Vitale S. Spinal cord compression studies. I. Experimental techniques to produce acute and gradual compression. Arch Neurol Psychiat 1953;70:813-9.
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Spinal Cord Blood Flow Measured by 14C-Iodoantipyrine Autoradiography During and After Graded Spinal Cord Compression in Rats* Anders Holtz, M.D., Bo Nystr/Sm, M.D., and Bengt Gerdin, M.D. Departments of Neurosurgery and Surgery, University Hospital, and Department of Experimental Medicine, Pharmacia AB, Uppsala, Sweden
Holtz A, Nystr6m B, Gerdin B. Spinal cord blood flow measured by z4C-iodoantipyrineautoradiography during and after graded spinal cord compression in rats. Surg Neurol 1989;31:350-60. The relations between degree of thoracic spinal cord compression causing myelographic block, reversible paraparesis, and extinction of the sensory evoked potential on one hand, and spinal cord blood flow on the other, were investigated. This was done in rats using the blocking weight-technique and ~4C-iodoantipyrine autoradiography. A load of 9 g caused myelographic block. Five minutes of compression with that load caused a reduction of spinal cord blood flow to about 25%, but 5 and 60 minutes after the compression spinal cord blood flow was restored to 60% of the pretrauma value. A load of 35 g for 5 minutes caused transient paraparesis. Recovery to about 30% was observed 5 and 60 minutes thereafter. During compression at a load of 55 g, which caused almost total extinction of sensory evoked potential and irreversible paraplegia, spinal cord blood flow under the load ceased. The results indicate that myelographic block occurs at a load which does not cause irreversible paraparesis and that a load which permits sensory evoked potential to be elicited results in potentially salvageable damage. WORDS: Autoradiography; 14C-iodoantipyrine; Myelography; Rats; Spinal cord blood flow; Spinal cord injury KEY
Introduction T h e introduction o f the weight drop-technique by Allen in 1911 [2] was the beginning o f the modern era of spinal cord trauma research. The kinetic compression
model used, in which the spinal cord is struck by a falling weight, has since been the most frequently employed trauma model in research in this field. It has great similarities to spinal cord trauma in humans, which has been a strong argument for its use. This model was later followed by others mimicking various factors considered to be involved in the pathophysiology o f spinal cord damage. Compression injuries have been studied with the balloon compression technique [11,16,18], the clip compression model [14], and the method used in the present s t u d y - - t h e blocking weight-technique [4]. The impact o f compression on the spinal cord blood flow (SCBF) has been evaluated in a few studies. Rivlin and Tator [14] observed generalized hypoperfusion in the compressed area after clip compression in the rat, and Dolan et al [5] reported a decrease in SCBF rostral and caudal to the damage. Palleske et al [13] claimed that the caudal SCBF was decreased after balloon cuff trauma, and Sandler and Tator [16] found that there was a relation between the posttraumatic SCBF and neurologic deficit. There ap° pear to have been no studies hitherto, however, on the relation between different degrees of compression trauma, the extent o f SCBF deterioration during compression, and the subsequent neurological damage. In the present study we have investigated the effects o f three different intensities o f spinal cord compression, each with its defined clinical outcome, on SCBF in the rat. The blocking weight-technique o f Croft et al [4] was used and SCBF was measured during and after compression trauma by means o f the 14C-iodoantipyrine (*4C-IAP) quantitative autoradiography technique.
Materials and Methods * The experimentalprotocol was approved by the EthicalCommittee of Uppsala University. Address reprint requests to: Anders Holtz, M.D., Department of Neurosurgery, University Hospital, S-751 85 Uppsala, Sweden. Received May 31, 1988; accepted November 25, 1988. © 1989by ElsevierSciencePublishingCo., Inc.
Animals
Male Sprague Dawley rats (ALAB, Sollentuna, Sweden) were used. T h e animals were fasted over night but had free access to tap water. 0090-3019/89/$3.50
Spinal Cord Compression in the Rat
Anesthesia and Surgery The animals were anesthetized with 3% halothane in 70/30 N20/O2, orally intubated with a polyethylene tube (PE 160) and artificially ventilated in a small-animal respirator (Braun-Melsungen, FRG). The halothane concentration was thereafter decreased to 0.5% and muscular relaxation was achieved by repeated doses of suxamethonium. A P E 50 catheter was inserted into the tail artery for repeated blood sampling and for measurement of mean arterial blood pressure (MABP). PE 50 catheters were also inserted into the carotid artery and jugular vein for blood sampling and 14C-IAP infusion. A midline longitudinal incision extending from Th4 to T h l 0 was made and a dorsal laminectomy of Th7 and Th8 was performed under microscopic guidance, leaving the dura intact. In animals subjected to methylene blue myelography (see below), the laminectomy was extended to Th9 and Thl0. The rats were placed prone on a heating pad and the body temperature was kept close to 37°C with the aid of a rectal thermistor. The halothane was now discontinued and before proceeding with the compression injury a steady-state period of 30 minutes was allowed, during which the animals were anesthetized with a 70/30 N20/O2 mixture. The acid-base status was measured at least twice and the ventilation was adjusted to obtain values within the following ranges: pH 7.35-7.45; PCO2 4.5-5.5 kPa; PO2 > 9.5 kPa; and base excess -+ 3 mmol/1 L (pH blood gas analyzer, IL 1302). Animals not fulfilling these criteria at the end of the steady-state period or showing signs of surgical injury to the spinal cord were excluded. There was concern throughout the experiment that there would be no suffering by the animals, and the animals were on anesthesia during the steady-state period when the halothane was discontinued. There are several reasons to think that the animals did not suffer pain during that period. First, during the steady-state period the mean arterial pressure was unaltered, indicating the N20/O2 anesthesia is sufficient to maintain the animals unconscious and without stress. During the steady state, the animals did not respond with an increase in blood pressure when stimulated by pain. Second, in later experiments yet to be published, spinal cord compression was carried out under combined chloral hydrate/barbiturate anesthesia, and in those animals the spinal cord compression caused a similar increase in MABP to that seen in the present study. In conclusion, there is no evidence that the animals perceived pain during the spinal cord compression.
Surg Neurol 1989;31:350-60
35 l
Spinal Cord Injury An apparatus for applying a predetermined load on the dorsal surface of the spinal cord was designed (Figure 1). An adjustable aluminum rod (A) is mounted horizontally on the main upright (B). A short weight-bearing rod (C) passes freely through the end fixture (D) on the horizontal rod. A slightly curved rectangular baseplate (E), 2.2 × 5.0 mm, area 11.0 mm 2, is fitted at the lower end of the rod and constitutes the contact area against the dura. A small plate is fixed to the upper end of the rod (F) and, by loading this plate with given weights, a defined compression force can be applied to the spinal cord. The spine was stabilized on a fixed position by clamping the spinous process of Th6 with a specially designed tissue forceps mounted on the framework. A screw (G) on the framework permits this position to be varied. At the time of injury the spine is adjusted horizontally and a predetermined weight is applied on the plate of the weight-bearing rod (F).
Methylene Blue Myelography After laminectomy a small incision was made in the dura at the T h l 0 level and a PE 50 catheter was inserted in the subarachnoid space and directed rostrally a few millimeters, whereafter the dural incision was closed by cyanoacrylate glue. The spinal cord was then exposed to a load of 20 g at the usual Th7-8 level and methylene blue (1 mg/mL in physiological saline) was infused at a pressure of 20 cm H20. The compression was then gradually reduced and the passage of contrast medium was observed and photographed.
Measurement of Sensory Evoked Potentials After laminectomy a midline scalp incision was made and recording and reference electrodes were placed frontally in the midline. Bipolar stimulating electrodes were placed in the tail. Stimulation was performed with a pulse duration of 0.1 msec and a repetition rate of 2 Hz. The stimulus intensity was gradually increased until muscle twitches were noted. The cortical activity was monitored on a Disa Averager (Type 1 4 G l l , Dantec, Skovlunde, Denmark) and Disa Electroencephalograph (Type 14All, Dantec, Skovlunde, Denmark). Cortically evoked responses before and after laminectomy served as controls.
Determination of Spinal Cord Blood Flow Spinal cord blood flow was measured by the 14C-IAP autoradiography method according to the technique described by Abdul-Rahman et al [31 and presented in detail previously [7]. Briefly, 40 IzCi of ~4C-IAP (Du-
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imml
F
Figure 1. An artist's view of the weight-holding equipment. The construction makes it possible to expose the spinal cord parallel to the table and to apply the weight perpendicularly. For an explanation of the letters, see Materials and Methods.
Pont, N E N Products, Boston, MA) was infused intravenously in 45 seconds. Repeated blood samples were obtained and the blood sampling times were recorded and used together with the isotope concentration of each blood sample to describe the temporal profile of the arterial 14C-IAP concentration. At the end of the infusion access to the spinal cord was obtained with a guillotine procedure using a double-edged chisel. The part o f the spinal cord located between the two incisions, including at least the compressed area and 5 mm above and below this area, was removed and immediately frozen in isopentane chilled to -50°C. Autoradiographs were obtained from 20 izm thick cryostat sections and from standards of known activity. Local tissue 14C activities were then determined from the optical densities of areas representing gray and white matter, using a densitometer with an aperture of 0.1 mm (Microdensitometer 3 CS, Joyce Loebel, Gateshead, England). In both gray and white matter 10 to 12 determinations were made in two to three adjacent sections and the mean of these values was recorded for each rat. Determinations were made at three levels in the compressed area: in the center of the 5.0 mm compression length, and 2 mm rostral and caudal to that point. Rostral and caudal to the compression area determinations were made 1, 3, and 5 mm from the edge o f the compression plate. Totally, nine different levels were evaluated.
Statistical Methods A multifactorial analysis of variance with multiple range testing, according to the method of least significant differences, was employed. A difference at the 5 % level was considered significant and is denoted by an asterisk. Values in Table 1 and Figures 6 and 7 are given as mean - SEM.
Experiments and R e s u l t s The experiments were divided into two main sessions. First, it was determined which three degrees of spinal cord compression would cause myelographic blockage, transient paraparesis with recovery over weeks, and extinction of sensory evoked potentials, respectively. Subsequently, the relation between these three degrees of spinal cord compression and SCBF was studied. Determination of the Graded Spinal Cord Compression
Evaluation of the Compression Requiredfor Myelographic Block Four rats were investigated with myelography. In none of the animals was there any passage of contrast medium over the compressed spinal cord area at weight loads of 8 g or higher (Figure 2). Reduction of the weight to 7 g
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353
Table 1. PhysiologicalVariablesat SteadyState After 5 and 60 Minutes of Compressionand 5 and 60 Minutes after Decompression Group
Treatment
Arterial pH
1
Controls
SS
2
9g5'C
3
9 g 5'DC
4
9 g 60'DC
5
9 g 60'C
SS 5'C SS 5'DC SS 5'DC 60'DC SS 5'C 60'C
6
35 g 5'C
7
35 g 5'DC
8
35 g 6 0 ' D C
9
55 g 5'C
Arterial PCO2 (kPA)
7.39 -+ 0.02 5.0 + 0.4 Myelographic block compression (9 g) 7.40 + 0.02 5.0 -+ 0,2 7.37 -+ 0.03 5.3 -+ 0.4 7.40 -+ 0.02 5.0 -+ 0.4 7.38 -+ 0.04 5.3 -+ 0.6 7.39 +- 0.01 5.3 -+ 0.4 7.40 -+ 0.03 5.3 +- 0.3 7.40 + 0.03 5.2 -+ 0.3 7.41 -+ 0.03 4.9 + 0.2 7.33 -+ 0.03 5.3 + 0.3 7.39 -+ 0.0 5.2 + 0.3 Transient paraparetic compression (35 g)
SS 5'C SS 5'DC SS 5'DC 60'DC
7.38 7.29 7.41 7.35 7.43 7.42 7.38
± 0.02 -+ 0.1 +_ 0.04 -+ 0.10 -+ 0.01 -+ 0.02 + 0.02 Extinguished SEP
SS 5'C
7.40 +- 0.03 7.30 -+ 0.06
Arterial PO2 (kPa) 18.0 -+ 2.9
Base excess (ram) - 0 . 7 -+ 1.8
16.9 14.3 17.2 17.0 14.2 15.9 16.6 16.6 16.8 17.2
-+ 2.9 -+ 4,3 -+ 3,9 -+ 3,9 -+ 2,3 -+ 2,6 -+ 1,7 -+ 2.0 +- 1.7 -+ 3,8
-1.0 -1.2 -0.4 -0.4 0.0 0.6 0.2 -0.3 -1.1 0.0
-+ -+ -+ +-+ -+ -+ -+ + ---
1.8 2.2 1.9 1.9 1.9 1.4 1.9 1.5 1.3 1.0
4.9 -+ 0.3 5.9 -+ 0.7 5.1 -+ 0.3 5.8 -+ 0.8 4.9 -+ 0.3 4.8 -+ 0.5 5.6 -+ 0.2 (55 g)
17.9 11.6 15.1 11.1 16.7 15.3 14.0
++-+ -+ -+ -+ -+
-2.1 -4.7 0.8 0 1.3 -0.1 0.2
+-+ ++ + -+ +-
0.9 2.0 2.8 2.9 1.4 2.2 0.8
4.9 + 0.3 6.3 -+ 0.8
17.9 -+ 2.0 12.8 -+ 4.5
1.6 5.1 2.7 5.2 2.9 3.1 3.4
- 1 . 1 -+ 1 - 3 . 2 -+ 2.3
Abbreviations: SS, steady state; 5'C, 5 minutes of compression; 5'DC, 5 minutes after decompression; 60'DC, 60 minutes after decompression; 60'C, 60 minutes of compression.
resulted in minor but visible passage of contrast medium bilaterally to the compression plate, and at a load of 6 g the contrast flow was unlimited. To be sure of a reproducible myelographic block in the subsequent experiments, the 9 g load was chosen.
Evaluation of the Compression Required for Transient Paraparesis The relation between different loads, duration of compression, and the resulting motor disturbance has previously been determined in our laboratory [12]. The evaluations of the motor performance were made on the inclined plane as described by Rivlin and Tator [15]. Figure 3 shows the motor performance after compression of the spinal cord for 5 minutes with loads of 20, 35, and 50 g [12]. Following the 20 g compression the rats showed only a minor disability during the first postoperative days, whereas compression at a load of 35 g caused transient paraparesis with a maximum after 24-48 hours followed by almost complete restitution in 10-14 days. On compression at 50 g the animals became paraplegic, with only slight restitution during the 3 postoperative weeks investigated. In subsequent experiments the group subjected to the 35 g load was therefore selected as the transient paraparesis group.
Evaluation of the Compression Required for Extinction of Sensory Evoked Potentials The effect of increasing compression weights on sensory evoked potentials (SEP) was evaluated in four animals. As shown in Figure 4, increasing loads from 15-45 g resulted in a progressive weakening of SEP and an increase in conduction time. At 50 g SEP was virtually extinguished in all four rats. To ensure that a reproducible extinction of SEP would have occurred in the subsequent studies, a load of 55 g was chosen. Relationship B e t w e e n Spinal Cord C o m p r e s s i o n a n d S p i n a l Cord Blood F l o w The animals were divided into nine experimental groups.
Controls Group 1. Spinal cord blood flow was determined at the end of a 30-minute steady-state recovery period.
Myelographic Block Compression (9 g) Group 2. At the end of the steady-state period a compression of 9 g was applied. After 5 minutes of
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A
Figure 2. Representative photographs of methylene blue myelography at various loads. Contrast medium is injected from the right (single arrow). (A) A t 8 g no contrast medium has passed under the plate and the left part of the spinal cord is white (double arrow). (B) A t 7 g there is a minor contrast medium passage (double arrow) and (C)at 6 g the contrast medium flow is unlimited (double arrow).
C
compression, SCBF was measured without removal of the load. Group 3. As in group 2, but SCBF was determined 5 minutes after decompression. Group 4. As in group 2, but SCBF was determined 60 minutes after decompression. Group 5. As in group 2, but SCBF was measured
after 60 minutes of compression without removing the
load.
Transient Paraparesis Compression (35 g) Group 6. As in group 2, but the compression weight was 35 g.
Spinal Cord Compression in the Rat
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steady-state pressure was significantly higher in the 35 and 55 g compression groups than in the 9 g group (Figure 5 B). At the end of compression M A B P had returned to the steady-state level and it remained so until sacrifice in all the groups• Simultaneously, with the normalization of blood pressure the bradycardia and ectopic beats disappeared.
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Physiological Variables T h e values for the physiological variables before, during, and after the different grades o f compression are given in Table 1. T h e r e were no differences in body weight, duration o f operation, p r e - s t e a d y - s t a t e blood glucose concentration, or body temperature between the groups (data not shown). During compression there was a rise in PCO2 and a decrease in p H and base excess. This decrease was significantly m o r e pronounced at loads of 35 and 55 g than at 9 g. These alterations were transient, however, and were fully normalized 5 minutes after decompression.
_
35g
45g Cardiovascular Response A rapid increase in both systolic and diastolic pressure was noted within a few seconds after the onset of compression. T h e pressure response was followed by a bradycardia and ectopic beats somewhat after the initial pressure peak, which as a rule did not exceed 60 seconds (Figure 5 A). The rise in M A B P measured as the difference between peak pressure and preload
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During compression at all loads used there was a significant reduction of SCBF in both gray and white matter. Figure 8 shows the appearance of autoradiographs from animals exposed to the three different loads. After 5 minutes of compression with a 9 g load there was a reduction in the gray matter SCBF in the central compression area from a control value of 85 -+ 11 to 19 -+ 11 mL/min/100 g, and in the white matter SCBF from 16 to 4 mL/min/100 g, i.e., to 22% and 25% of the precompression values, respectively (Figures 6 A and 7 A). There was also a less pronounced decrease in SCBF in the noncompressed spinal cord 1 and 3 mm from the edge of the compression plate. An extension of the compression time to 60 minutes was not followed by any further reduction of SCBF (Figures 6 B and 7 B). At the end of the 60-minute compression, however, SCBF was more deteriorated in the noncom-
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pressed gray matter, while the white matter SCBF was similar after 5 and 60 minutes of compression. Compression weights of 35 and 55 g resulted in blood flow values in both gray and white matter that were significantly lower than in the 9 g group (to mean values of 9 -+ 5 and 0.4 -+ 1 mL/min/100 g for 35 g and 4 - 5 and 0 - 0 for 50 g, respectively) (Figures 6 A and 7 A). The 35 g load was followed by a decrease in SCBF, which on an average was between that seen with loads of 9 and 55 g. However, there was no difference in SCBF of the compressed area between 35 and 55 g loads, while SCBF was more affected in the adjacent spinal cord during compression with a 55 g than with a 35 g load.
Spinal Cord Blood Flow After Trauma In the 9 g group, SCBF in both gray and white matter was restored to approximately 60% of the pretrauma value 5 minutes after the compression (Figures 6 C and 7 C). It was still significantly lower than the pretrauma
358
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value, however. The flow in the noncompressed adjacent few millimeters of spinal cord was also decreased. In comparison with the 9 g group, the animals subjected to compression at 35 g showed more pronounced impairment in gray and white matter SCBF, both in the compressed and in the adjacent areas, 60 minutes after decompression (Figures 6 D and 7 D). At that time SCBF in the compressed area was restored to approximately 30% of the pretrauma value. There was no further improvement in SCBF in either the 9 g or 35 g group 60 minutes after decompression, either in the compressed area or in the adjacent levels.
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Figure 8. Autoradiographs of the central area of compression in representative animals subjected to different grades of trauma. (A) Control animal, (B) 9 g compression, (C) 35 g compression, and (D) 55 g compression.
In this study we have shown that exposure of the rat spinal cord to three increasing compression loads was associated with increasingly pronounced derangements of SCBF, both in the compressed and in the adjacent noncompressed spinal cord. At a 9 g load for 5 minutes causing myelographic blockage, the gray matter SCBF was reduced to about 25% of normal. An extension of the compression time to 60 minutes did not have any impact on SCBF in the compressed cord, but there was a further reduction in the gray matter SCBF in the noncompressed adjacent spinal cord, indicating that the duration of compression influences the flow to the noncompressed parts of the spinal cord. After decompression, SCBF reached 60% of the control flow value in the compressed area within 5 minutes and remained unchanged at the 60-minute measurement. As the experimental setup did not allow an evaluation of SCBF at later times, we have not determined how long this depression lasts. In the 35 g compression group, where SCBF during compression was less than 10% of the control value, a transient paraparesis followed, and although there was a marked impact on the amplitude and conduction time for the SEP, it could still be evoked (Figure 4). This is in accordance with the findings of Kobrine et al [11], who observed that, as long as there was a significant flow through the spinal cord, SEP remained. Our result also suggests that at this load the compression as such is not severe enough to extinguish signal conduction over the damaged area, although a paraparesis later follows. The first hour after decompression the flow restitution was limited in the 35 g group and at both 5 and 60 minutes SCBF was only about 30% of the control flow. This value is better than that reported by Rivlin and Tator [14], who observed a recovery of gray matter SCBF to less than 10% at 2 hours. In their study, however, they used a %minute clip compression which resulted in very
Spinal Cord Compression in the Rat
severe neurological damage exceeding that obtained by 50 g of compression in a previous study in our laboratory [12]. Although the blocking weight-technique has been used by several authors to obtain a standardized spinal cord trauma, the effects on SCBF have not been evaluated. Data on SCBF are available, however, f r o m studies with the extradural cuff/balloon technique, which has similarities to the blocking weight-method [10]. Sandier and Tator [16] evaluated the former technique in primates and used two degrees of damage. After a moderate trauma which caused paraplegia in 50% of the animals and paraparesis in the remainder, they observed a 50% decrease in both gray and white matter SCBF 1 hour after the compression. These are figures close to those of the present study. After severe trauma, producing paraplegia in almost all animals, they observed that SCBF was reduced to less than 10% of that in the controls at the 1-hour period. In the present study a 55 g load for 5 minutes caused SCBF to fall very close to zero. This was sufficient to result in a virtually extinguished SEP at the end of compression and paraplegia for at least 3 weeks. In the animals exposed to a 35 g load, SEP persisted but was depressed, and SCBF was low but not totally eliminated during compression. As these animals recovered almost completely with time, a persistent SEP, even if very weak, could indicate that a good functional recovery is possible. In contrast to the above results by Sandier and Tator [16], and those of the present study, Senter and Venes [ 17], using the weight drop-technique, found that SCBF was not affected until 1 hour after trauma, when ischemia developed. This suggests that a different type of vascular damage is involved in the SCBF disturbance seen after a kinetic trauma than after a static compression. As the decrease in SCBF is related to the load applied and also to the dysfunction obtained, it may be suggested that an ischemic component contributes to the final damage. It seems unlikely, however, that the ischemia achieved only during the 5-minute compression is responsible for the damage. After such an ischemic insult to the brain there is a profound postischemic hyperperfusion [9]. This reflow pattern is distinctly different from that seen after the spinal cord compression in the present model, which is characterized by a lack of postischemic hyperperfusion and a rapid but limited restoration of flow as early as within 5 minutes, whereafter no further improvement occurs during the first hour. As SCBF during the first hour after 35 g compression was only 30% of the precompression value, it is possible that a long period of relative ischemia follows the compression. The absence
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of postischemic hyperperfusion after only 5 minutes of subtotal ischemia further indicates that the spinal cord vascular bed in itself has suffered from the compression trauma. Such damage, by its hampering effects on postcompression circulation, might contribute to the pathophysiology of the final insult. The immediate and transient increase in blood pressure on spinal cord compression is mediated by a peripheral c~-adrenergic stimulation [6]. In the present experiments the blood pressure response was very transient and phasic in nature and was more pronounced at the two heavier loads. It has been suggested that this response is not depending on efferent signals through the dorsal column, but rather arises from the compression of the cord itself [1]. During compression at 35 and 50 g there was a significant increase in arterial PaCO2. The mechanism for this is not clear. The animal is mechanically fixed in such a position that no impact of the spinal cord load on the respiration is possible. It might, however, be secondary to an increased peripheral metabolism and washout of COz from the tissues due to the profound and rapid sympathetic stimulation that occurs when the spinal cord is compressed. In reference to the clinical situation, it is interesting to note that a degree of compression that is barely sufficient to obstruct the passage of contrast medium at myelography reduces SCBF to about 25% of normal. After a 5-minute compression at this load there is no permanent neurological damage [12]. It is not known, however, what damage is caused by longer exposure. The time limit beyond which irreversible neurological damage occurs in humans following a similar compression is not known. Furthermore, the compression force causing an obstructed passage of contrast medium in an individual patient is not measurable. In the most favorable situation the patient might have suffered from only a minor initial contusion and subsequent compression, just enough to block contrast passage. Emergency myelography or computed tomography myelograpky, with surgical decompression in cases of contrast blockage, is therefore a reasonable clinical measure [8]. We wish to thank Ulla Karlsson and Inger Fogelberg for skillful technical assistance, and the Department of Neurophysiologyfor the SEP evaluations. We are also grateful to Mr. Mats Linder for the excellent artistic illustrations.
References 1. Alexander S, Kerr FWL. Blood pressure responses in acute compression of the spinal cord. J Neurosurg 1964;21:485-91. 2. Allen AR. Surgery of experimental lesion of spinal cord equivalent to crush injury of fracture dislocation of spinal column. JAMA 1911;57:878-80.
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3. Abdul-Rahman A, Dahlgren N, Ingvar M, Rehncrona S, Siesj6 BK. Local versus regional cerebral blood flow in the rat at high (hypoxia) and low (phenobarbital anesthesia) flow rates. Acta Physiol Scand 1979;106:53-60. 4. Croft TJ, BrodkeyJS, Nulsen FE. Reversible spinal cord trauma: a model for electrical monitoring of spinal cord function. J Neurosurg 1972;36:402-6. 5. Dolan EJ, Tator CH, Endrenyi L. The value of decompression for acute experimental spinal cord compression injury. J Neurosurg 1980;53:749-55. 6. Eidelberg EE. Cardiovascular response to experimental spinal cord compression. J Neurosurg 1973;38:326-31. 7. Holtz A, Nystr6m B, Gerdin B. Regulation of spinal cord blood flow in the rat as measured by quantitative autoradiography. Acta Physiol Scand 1988;133:485-93. 8. Jelsma RK, Rice JF, Jelsma LF, Kirsch PT. The demonstration and significance of neural compression after spinal injury. Surg Neurol 1982;18:79-92. 9. Kalgstr6mE, Smith ML, Siesj6 BK. Recirculation in the rat brain following incomplete ischemia. J Cereb Blood Flow Metabol 1983;3:183-92. 10. Khan M, Griebel R. Acute spinal cord injury in the rat: comparison of three experimental techniques. Can J Neurol Sci 1983;10:161-5.
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11. Kobrine AI, Evans DE, Rizzoli H. Correlation of spinal cord blood flow and function in experimental compression. Surg Neurol 1978;10:54-9. 12. Nystr6m B, Berglund J°E. Spinal cord restitution following compression injuries in rats. Acta Neurol Scand 1988;78: 467-72. 13. Palleske H, Kivelitz R, Loew F. Experimental investigation on the control of spinal cord circulation. Acta Neurochir 1970;22: 29-41. 14. Rivlin AS, Tator CH. Effect of duration of acute spinal cord compression in a new acute cord injury model in the rat. Surg Neurol 1978;10:39-43. 15. Rivlin AS, Tator CH. Objective clinical assessment of motor function after experimental spinal cord injury in the rat. J Neurosurg 1977;47:577-81. 16. Sandier AN, Tator CH. Effect of acute spinal cord compression injury on regional spinal cord blood flow in primates. J Neurosurg 1976;45:660-76. 17. Senter HJ, Venes JL. Loss of autoregulation and posttraumatic ischemia following experimental spinal cord trauma. J Neurosurg 1979;50:198-206. 18. Tarvlov IM, Klinger H, Vitale S. Spinal cord compression studies. I. Experimental techniques to produce acute and gradual compression. Arch Neurol Psychiat 1953;70:813-9.