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Somatosensory evoked potentials
Correspondence ESTIMATION OF GESTATIONAL AGE To the Editor:
The article by Drs. Scher and Barmada on the estimation of gestational age was read with great interest [ 1]. However, a misunderstanding has arisen in their discussion. They stated that the use of nerve conduction velocities to estimate gestational age had been questioned with respect to its expense and degree of prematurity, and recommended a delay of the first recording until after the first week of life. The authority given for these statements was our article on the method used in very low-birth weight and seriously ill neonates [2]. We, in fact, reported that the technique was extremely useful in the very premature and seriously ill neonate; in addition, it was not only valid in the first week of life (or indeed immediately postpartum), but also the results were reproducible after the first postnatal week. The expense of this technique was stated to be greater when compared to clinical methods. In comparison to the use of electroencephalography, it is less expensive in terms of time expended, cost of equipment, and personnel employed. Both electrographic and nerve conduction velocity methods can improve the accuracy of estimating gestational age in the very low-birth weight or extremely ill neonate. The latter is probably less expensive and is estimated to predict gestation with a standard deviation of + 1.14 weeks. Geoffrey Miller, MA, MD Princess Margaret Hospital for Children Perth, Western Australia References [1] Scher MS, Barmada MA. Estimation of gestational age by electrographic, clinical, and anatomic criteria. Pediatr Neurol 1987; 3:256-62. [2] Miller G. Heckmatt JZ, Dubowitz LMB, Dubowitz V. Use of nerve conduction velocity to estimate gestational age, in infants at risk and in very low birth weight infants. J Pediatr 1983;03:109-12. To the Editor:
The comments by Dr. Miller regarding our recently reported findings concerning gestational age estimates by electroencephalography, clinical, and anatomic criteria are greatly appreciated. We in no way meant to misinform readers on the usefulness of nerve conduction velocities to estimate gestational age. We specific ally men tioned that this method was not routine ly used in our neonatal intensive care unit, but acknowledged the findings of Miller et al. who reported its usefulness for the very premature and seriously ill newborn. We clearly stated that in our neonatal intensive care unit we have greater experience with the use of electroencephalography for gestational age assessment. Although Dr. Miller states that nerve conduction velocity studies are perhaps more cost effective, greater information regarding cerebral maturation can be obtained with routine electroencephalographic recordings, in addition to information regarding diffuse or focal encephalopathic abnormalities. This added information may certainly outweigh the concerns for added costs. In addition, Dr. Miller's estimate of gestational age maturity within a standard deviation of + 1.14 weeks is quite impressive. We expect a significant experimental error which can occur in the determination of the nerve conduction velocities, as previously described lbr older patients [ 1]. In addition, other reports of nerve conduction velocities in newborns report a wider scatter of velocities with greater standard deviations than reported by Miller et al. [2,3]. Both electroencephalographic and nerve conduction velocity methods can be used to assess gestational age accurately in the very low-birth weight infant and we in no way wish to deter others from using this method.
Mark S. Scher, MD Magee-Women's Hospital Pittsburgh, Pennsylvania
References [1] M a y n a r d FM, Stolov WC. Experimental error in determination of nerve conduction velocity. Arch Phys Med Rehabil 1972;53:362-72. [2] Schulte FJ, Michaelis R, Linke I, Nohe R. Motor nerve conduction velocity in term, preterm, and small-for- dates newborn infants. Pediatrics 1968;42:17-26. [3] Littman B. Peripheral nerve maturation in premature infants. Neuropaediatrie 1975;6:284-91.
SOMATOSENSORY EVOKED POTENTIALS To the Editor:
In the second of their review articles on somatosensory evoked potentials (SEPs), Drs. Fagan et al. discussed the usefulness of evoked potentials for intraoperative monitoring [1]. Although their review of this and other topics is quite excellent, they seem to mislead the reader on the issue of false-negative monitoring events. They cited a publication in which I am an author [2] as justification for their statement that "incidents of postoperative neurologic deficits were reported in patients whose intraoperative SEPs remained normal." I would like to point out that this is not true in the general sense conveyed by this statement. There has never yet been a documented case in which normal, stable SEPs were recorded at a time when the patient had a significant impairment along the monitored pathway. This has been reviewed in detail elsewhere [3]. There have been cases in which the intraoperative SEP showed some fluctuation, the patient awoke with normal function, and then developed neurologic complications several hours later. There has also been an instance in which the median nerve pathway was monitored but the patient developed a thoracic level impairment. Another patient developed a right hemicord syndrome while the monitoring team followed SEPs from a relatively intact left leg pathway. In another patient, the SEPs were substantially changed without disappearing completely; this change was well-documented by a monitoring team who failed to consider it as a warning of possible neurologic impairment. These and other similar instances make several lessons clear. First, although one pathway may be intact, it does not mean other pathways elsewhere will not be impaired. Second, normal intraoperative function cannot predict the occurrence of new complications several hours post-operatively. Third, substantial changes in SEPs can be a sign of impaired function (i.e., it is not necessary to completely lose the SEP before becoming alarmed about changes). Fourth, both halves of the spinal cord should be monitored. When interpreted in a knowledgeable way, intraoperative SEPs are excellent predictors of post-operative function. Drs. Fagan and colleagues have overestimated the uncertainty in interpreting these tests. Marc R. Nuwer, MD, PhD University of California, Los Angeles Reed Neurological Research Center References [1] Fagan ER, Taylor MJ, Logan WJ. Somatosensory evoked potentials, Part II: A review of the clinical applications in pediatric neurology. Pediatr Neuro11987;3:189-96. [2] Lesser RP, Raudzens P, LOders H, et al. Postoperative neurological deficits may occur despite unchanged intraoperative somatosensory evoked potentials. Ann Neurol 1986; 19:22-5. [3] Nuwer MR. Evoked potential monitoring in the operating room. New York: Raven Press, 1986;246. To the Editor:
We are in general agreement with Dr. Nuwer's comments, both in his letter and in the referenced article where he and his coinvestigators stated "SEP monitoring should be used with a clear awareness of its limitations" [ 1]. This finding is in concordance with our conclusion that "SEP recordings, while helpful in many patients, do have limitations which need to be
PEDIATRIC NEUROLOGY
Vol. 4 No. 2
129
The article by Drs. Scher and Barmada on the estimation of gestational age was read with great interest [ 1]. However, a misunderstanding has arisen in their discussion. They stated that the use of nerve conduction velocities to estimate gestational age had been questioned with respect to its expense and degree of prematurity, and recommended a delay of the first recording until after the first week of life. The authority given for these statements was our article on the method used in very low-birth weight and seriously ill neonates [2]. We, in fact, reported that the technique was extremely useful in the very premature and seriously ill neonate; in addition, it was not only valid in the first week of life (or indeed immediately postpartum), but also the results were reproducible after the first postnatal week. The expense of this technique was stated to be greater when compared to clinical methods. In comparison to the use of electroencephalography, it is less expensive in terms of time expended, cost of equipment, and personnel employed. Both electrographic and nerve conduction velocity methods can improve the accuracy of estimating gestational age in the very low-birth weight or extremely ill neonate. The latter is probably less expensive and is estimated to predict gestation with a standard deviation of + 1.14 weeks. Geoffrey Miller, MA, MD Princess Margaret Hospital for Children Perth, Western Australia References [1] Scher MS, Barmada MA. Estimation of gestational age by electrographic, clinical, and anatomic criteria. Pediatr Neurol 1987; 3:256-62. [2] Miller G. Heckmatt JZ, Dubowitz LMB, Dubowitz V. Use of nerve conduction velocity to estimate gestational age, in infants at risk and in very low birth weight infants. J Pediatr 1983;03:109-12. To the Editor:
The comments by Dr. Miller regarding our recently reported findings concerning gestational age estimates by electroencephalography, clinical, and anatomic criteria are greatly appreciated. We in no way meant to misinform readers on the usefulness of nerve conduction velocities to estimate gestational age. We specific ally men tioned that this method was not routine ly used in our neonatal intensive care unit, but acknowledged the findings of Miller et al. who reported its usefulness for the very premature and seriously ill newborn. We clearly stated that in our neonatal intensive care unit we have greater experience with the use of electroencephalography for gestational age assessment. Although Dr. Miller states that nerve conduction velocity studies are perhaps more cost effective, greater information regarding cerebral maturation can be obtained with routine electroencephalographic recordings, in addition to information regarding diffuse or focal encephalopathic abnormalities. This added information may certainly outweigh the concerns for added costs. In addition, Dr. Miller's estimate of gestational age maturity within a standard deviation of + 1.14 weeks is quite impressive. We expect a significant experimental error which can occur in the determination of the nerve conduction velocities, as previously described lbr older patients [ 1]. In addition, other reports of nerve conduction velocities in newborns report a wider scatter of velocities with greater standard deviations than reported by Miller et al. [2,3]. Both electroencephalographic and nerve conduction velocity methods can be used to assess gestational age accurately in the very low-birth weight infant and we in no way wish to deter others from using this method.
Mark S. Scher, MD Magee-Women's Hospital Pittsburgh, Pennsylvania
References [1] M a y n a r d FM, Stolov WC. Experimental error in determination of nerve conduction velocity. Arch Phys Med Rehabil 1972;53:362-72. [2] Schulte FJ, Michaelis R, Linke I, Nohe R. Motor nerve conduction velocity in term, preterm, and small-for- dates newborn infants. Pediatrics 1968;42:17-26. [3] Littman B. Peripheral nerve maturation in premature infants. Neuropaediatrie 1975;6:284-91.
SOMATOSENSORY EVOKED POTENTIALS To the Editor:
In the second of their review articles on somatosensory evoked potentials (SEPs), Drs. Fagan et al. discussed the usefulness of evoked potentials for intraoperative monitoring [1]. Although their review of this and other topics is quite excellent, they seem to mislead the reader on the issue of false-negative monitoring events. They cited a publication in which I am an author [2] as justification for their statement that "incidents of postoperative neurologic deficits were reported in patients whose intraoperative SEPs remained normal." I would like to point out that this is not true in the general sense conveyed by this statement. There has never yet been a documented case in which normal, stable SEPs were recorded at a time when the patient had a significant impairment along the monitored pathway. This has been reviewed in detail elsewhere [3]. There have been cases in which the intraoperative SEP showed some fluctuation, the patient awoke with normal function, and then developed neurologic complications several hours later. There has also been an instance in which the median nerve pathway was monitored but the patient developed a thoracic level impairment. Another patient developed a right hemicord syndrome while the monitoring team followed SEPs from a relatively intact left leg pathway. In another patient, the SEPs were substantially changed without disappearing completely; this change was well-documented by a monitoring team who failed to consider it as a warning of possible neurologic impairment. These and other similar instances make several lessons clear. First, although one pathway may be intact, it does not mean other pathways elsewhere will not be impaired. Second, normal intraoperative function cannot predict the occurrence of new complications several hours post-operatively. Third, substantial changes in SEPs can be a sign of impaired function (i.e., it is not necessary to completely lose the SEP before becoming alarmed about changes). Fourth, both halves of the spinal cord should be monitored. When interpreted in a knowledgeable way, intraoperative SEPs are excellent predictors of post-operative function. Drs. Fagan and colleagues have overestimated the uncertainty in interpreting these tests. Marc R. Nuwer, MD, PhD University of California, Los Angeles Reed Neurological Research Center References [1] Fagan ER, Taylor MJ, Logan WJ. Somatosensory evoked potentials, Part II: A review of the clinical applications in pediatric neurology. Pediatr Neuro11987;3:189-96. [2] Lesser RP, Raudzens P, LOders H, et al. Postoperative neurological deficits may occur despite unchanged intraoperative somatosensory evoked potentials. Ann Neurol 1986; 19:22-5. [3] Nuwer MR. Evoked potential monitoring in the operating room. New York: Raven Press, 1986;246. To the Editor:
We are in general agreement with Dr. Nuwer's comments, both in his letter and in the referenced article where he and his coinvestigators stated "SEP monitoring should be used with a clear awareness of its limitations" [ 1]. This finding is in concordance with our conclusion that "SEP recordings, while helpful in many patients, do have limitations which need to be
PEDIATRIC NEUROLOGY
Vol. 4 No. 2
129