The prognostic value of intra-operative observations during thalamotomy for parkinsonian tremor

25

CliniculNeurologycmdNeurosurgery.94 (I 992) 25-30 0 1992Elsevier Science Publishers B.V. All rights reserved 0303-8467/92/$05.00

CLINEU

00179

The prognostic value of intra-operative observations during thalamotomy for parkinsonian tremor K. Wester” and E. Hauglie-Hanssenb Departments of Neurosurger.v, “Haukeland Sykehus, University of Bergen Medical School, Bergen, Norway, and bRikshospitalet, University of Oslo Medical School, Oslo, Norway (Received (Revised,

(Accepted

Key words:

Brain stimulation;

Parkinson’s

disease;

19 March,

1991)

received 29 July, 1991) 26 August,

Stereotaxic

1991)

techniques;

Thalamotomy;

Tremor

Summary Data from 27 thalamotomies observations and the long-term electrodes

in the target

threshold

intensities

were analyzed with respect to possible correlations between certain intra-operative effect on parkinsonian tremor. Tremor reduction caused by mechanical impact of the

area was not correlated

during

the intra-operative

with the long-term electrical

stimulation

effect on the tremor. of the target

The same was true for the

area,

stimulation

that

facilitates

and/or inhibits the tremor. In a minority of the patients, all with good long-term results, a combination of a pronounced tremor inhibition from the electrode insertion and a low threshold intensity was observed. Variations in other lesion parameters were not correlated with the outcome. The results are discussed within the framework of a “tremor” vs. a “tonus” mechanism underlying the thalamotomy effect.

Introduction

most effective target area in a possible tremorogenic tremor-mediating part of the diencephalon.

Stereotaxic thalamotomy is a well established symptomatic treatment for parkinsonian tremor and other

An acute oedema surrounding the lesion may attribute to the immediate inhibitory effect on tremor by tempo-

movement

rarily blocking

disorders.

The majority

of patients

resistant parkinsonian tremor benefit tion [l-13]. In a few cases, however,

with drug-

from this operathe tremor may

neural elements

gradually be reactivated, tremor. The postoperative

permanent tremor relief. The cause of the tremor

1). Provided

is not

known. However, it seems reasonable to suspect the lesion to have been too small and/or placed outside the

Correspondenceto: Dr. Knut Wester, Department Haukeland

Sykehus,

N-5021 Bergen. Norway.

of Neurosurgery,

in tremor

mech-

anisms. As the oedema subsides and natural repair processes take place, parts of the tremor mechanisms may

eventually reappear. This may happen also after excellent immediate results, in spite of apparently identical operative procedures as those performed in cases with relapse in these patients

involved

or

sive compared

causing reappearance oedema can be rather

with the volume

the concept

of the lesion proper

of a tremorogenic

thalamic

of the exten(Fig. area

is of some validity, one would expect this to be reflected in intra-operative observations. Although seldom discussed in scientific reports, most neurosurgeons performing thalamotomy probably assume this to be the case. As few have access to refined neurophysiological recording techniques [ 14-201, peroperative stimulation studies is the most common method for physiological

26

Fig. 1. Horizontal CT scans showing the maximal extent of the thalamic lesion 1 day (left) and 4 months (right) postoperatively. Note

the oedematous zone surrounding the inner core of the lesion.

target confirmation. Electrical stimulation of the target area may have dramatic effects on tremor (Fig. 2), and the threshold intensity for eliciting these effects can be determined (Fig. 3). Low threshold intensities are logically considered to indicate proximity to the optimal target area, and should therefore predict a successful lesion. When the electrodes are inserted into the target area, reduction or arrest of tremor is often observed (Fig. 4). This is presumably due to mechanical impact on neural elements, and is regarded as another promising sign, indicating a good long-term result. In the present study, we have analyzed whether these effects or other peroperative factors could be attributed to any prognostic significance for the final outcome of the operation.

larger sample of patients described elsewhere [I 31. Only those who had been operated on under standardized intra-operative conditions were selected. One patient was operated twice due to reappearance of the tremor; thus the total number of operations included was 27. Surgery

A detailed account for the surgical procedures is given elsewhere [13]. The thalamic target was located 2 mm behind the mid-commissural plane, 3 mm above the inter-commissural line and, most commonly, 14 mm laterally from the mid-sagittal plane. Two electrodes were used, 6 mm apart, the active tips measuring 2 mm. The inter-electrode plane was approximately parallel to the internal capsule. With the pre-selected target area, stimulation and lesioning was assumed to affect the ventrooral (v.o.a. and v.0.p.) and the reticular thalamic nuclei

VI. Material and methods

The lesions were calculated to be of the same size as those reported by Hirai et al. [6].

Patients

Twenty-six patients with parkinsonian tremor (age 49-73 years, median 64) were selected from a slightly

Recording

The tremor was recorded continuously

by means of

27

Fig. 2. Polygraph recordings showing effects of stimulation in the left thalamus on tremor in the contralateral (right) index finger. Stimulation parameters: 1.O mA, 50 Hz, 1.O msec. Upper trace: Time in set with stimulus marker (horizontal bar). Middle trace: left index finger (no tremor). Lower trace: right index finger. Left: arrest of ongoing tremor at the onset of stimulation. of tremor when stimulating with the same parameters against a background of no tremor.

mechano-electrical transducers mounted of both index fingers (see Figs. Z-4). Stimdution Prior to lesioning,

on the dorsum

the target area was screened

larily from a constant-current

stimulator

or (4) absent. In the final evaluation,

were grouped together as “good” results.

as “poor”

(1) and (2)

results, and (3) and (4)

for ef-

fects on tremor as well as for unwanted side effects by electrical stimulation. The stimulus was delivered bipopulses, pulse width

present,

Right: initiation

(50 Hz biphasic

1.O msec).

Results The present

paper deals only with the effect of the tha-

lamotomy on the tremor per se. According to the criteria given above, 5 of the operations were found to have

Ewluation of long-term t@cts All the patients were re-admitted

to our department

given “poor”, and 22 “good” results. This corresponds exactly with our own evaluation at the post-operative

for a 3-month post-operative control. However, to avoid possible bias on the authors’ part, the final evaluation was left to the patients’ referring neurological depart-

control

3 months

IZfjxt

qf’elrctrode

ment

(median 17) months (see ref. 13 for details). Among other questions, they were asked to indicate

In 14 operations, insertion of the electrodes into the target area caused a pronounced reduction of the tremor, i.e. an amplitude reduction of 50% or more. lasting for

whether the tremor, at the most recent visit, was (1) prc~mt (It ,fklI strength. (2) nlodwate!)~ presrnt, (3) slight!\,

minutes (Fig. 4). In a few cases, the tremor was almost abolished for the rest of the operation. In the remaining

after

_- ,--.%._ W-_-S_

a post-operative

observation

time

of 3-31

-....-q~.----I_

._-.--

after surgery. insertion

-.

------__a..----.

Fig. 3. Example of determination of threshold intensity. Same patient and stimulation parameters as in Fig. 2. Horizontal bars indicate thalamic stimulation with the following intensities (in mA, from left to right): 0.60. 0.55. 0.60. 0.80. 0.90. Note also the decreasing latencies with increasing stimulus intensities.

*_., -- ._ ._ A

-

-A

Fig. 4. Marked reduction of tremor in the left index finger (lower trace) induced by mechanical insertion (between arrows) of the electrodes from the cortical surface into the target in the right thalamus. Due to higher amplitude of the left-sided tremor, the gain in this trace is reduced 5-fold.

13 operations,

this effect was not convincingly observed, or it was less pronounced. In the analyses below, this mechanical effect was accordingly classified as being “pronounced” or “weak or absent”. The 2 result groups (“good” and “poor”) did not differ with respect to the presence or absence of mechanical insertion effect (Table 1).

and “poor”) did not differ significantly. either when tested on a ranking scale (Mann-Whitney U-test). or when tested with the Fischer exact probability test (Table 2). The combination of mechanical insertion ejfect and loo. threshold intensity In a minority of the operations, a pronounced mechanical insertion effect and a low threshold were both observed. All these operations had good long-term results (Table 3). This correlation approaches the level of statistical significance (x2 = 3.25, P = 0.07).

Stimulation effkcts Electrical stimulation of the ventrolateral thalamus most often had a dichotomic effect on the contralateral tremor. With the hands resting comfortably, stimulation most often elicited a typical parkinsonian tremor. When stimulating against a background of positional tremor, obtained by asking the patient to elevate his arms, identical stimulation could cause a prompt arrest of tremor (Fig. 2). The threshold intensities for eliciting these effects were found to vary between 0.5 and 2.0 mA, with a median of 1.25 mA. Threshold intensities below or above this median were classified as being “low” or “high”, respectively. Threshold intensities for the 2 result groups (“good”

Other factors The long-term results did not correlate with parameters relating to the size of the lesion (maximal heating time, temperature or total extent of the lesioned area).

TABLE I

TABLE 2

TREMOR-REDUCING EFFECT OF ELECTRODE INSERTION IN THE TARGET AREA VS. LONG-TERM EFFECT OF THE OPERATION -

THRESHOLD INTENSITIES FOR STIMULATION EFFECTS ON TREMOR VS. LONG-TERM EFFECT OF THE OPERATION

Insertion effect

Threshold intensity

Long-term effect of the operation Good

Poor

Pronounced Weak or none

12 9

2 4

Total

21

6

Discussion Although not commonly mentioned in the literature, it

Long-term effect of the operation Good

Poor

Low High

12 9

2 4

Total

21

6

29 TABLE 3

lesions for a wide variety of dyskinesias

COMBINED FACTORS (EFFECT OF ELECTRODE SERTION AND LOW THRESHOLD INTENSITIES) LONG-TERM EFFECT OF THE OPERATION

INVS.

understand

targets

ful targets

21

6

although thalamic

mal movements

subdivision

neurosurgeons

perform-

ing thalamotomies that substantial tremor reduction, when the electrode is inserted into the target, is a good omen, as it is when electrical stimulation of the target affects tremor

even at low intensities.

these assumptions tremorogenic or

The logic behind

must be that there exists a restricted tremor-mediating diencephalic area,

and that the effect of the operation lesion is to this area.

to

throughout has yet been

clinical

targets.

can alleviate character

in different

abnor-

and etiol-

parts of this digood effects

These facts are difficult

of a restricted

on Use-

to ex-

and specific tremor

mechanism. The present

among

widely

seem to have equally

tremor.

plain by the existence

opinion

operations.

no attempt lesions

of different

on parkinsonian

,$ = 3.25. P = 0.01.

is a wide-spread

scattered

ogy, and vice versa: lesions encephalic

of symptoms. Laitinen’s [22] survey

the results of using different

identical

to the

in a specific tremor mechanism

are apparently

the diencephalon, Thus,

in addition

[I 31. It is difficult

for tremor-alleviating

made to compare 0 6

and rigidity

how lesions

preferred

Poor

Low threshold und pronounced insertion 8 effect 13 Other operations Total

tremor

should alleviate such a variety More doubt emerges from

Long-term effect of the operation Good

parkinsonian

is better the closer the

The concept of a restricted tremorogenic area receives support from the electrophysiological works of AlbeFessard [14], Jasper [16], Ohye [18,19] and others [17,20]. In his microelectrode studies, Ohye has demonstrated thalamic (Vim) cells that fire in a synchronous fashion

results, especially

those from the stimula-

tion studies, are similarly difficult to explain within the concept of such a mechanism. To us, the presently available data seem to favour some sort of “tonus” rather than a “tremor” mechanism theory. Such a tonus mechanism may possibly play on the muscular tone [23]. There is also evidence that this part of the thalamus may play some part in the attentional tone [24-291. The proximity of the target

area

such as the reticular this context.

to non-specific nucleus,

thalamic

is of particular

structures, interest

in

The prognostic value of the intra-operative observations described here seems to be of limited importance. One can relatively safely predict a good long-term outcome if a pronounced insertion effect and a low threshold intensity appear in combination. However, excellent re-

with the tremor. These cells are located in the same thalamic subdivision, or even within the same nucleus where

sults may occur without

lesions can alleviate tremor. It is therefore possible that these cells “are involved in the tremor-mediating system.

Acknowledgements

if not in the tremor genesis” [18]. However, such observations cannot be taken as evidence for a causal relation-

The authors wish to thank professor E.-O. Backlund for his invaluable criticism and comments, and Dr. M.

ship between

Lund-Johansen

the two phenomena,

with the thalamic

unit

firing being the primary event. On the contrary, it may well be that the thalamic cell activity is merely reflecting the peripheral muscle activity associated with the tremor, thus being a secondary event. Ohye has himself demonstrated that such units can be driven by stretching muscles

involved

in the tremor

[18].

There are also intriguing facts that undermine the notion of a restricted tremorogenic area in the thalamus. Lesions in the same area can alleviate a number of different movement disorders. also when tremor is not involved [20]. We have ourselves used identical thalamic

this combination.

for technical

assistance.

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