TO THE EDITOR:
We were interested to read "The Ankle Clonus Test for Assessment of the Integrity of the Spinal Cord during Operations for Scoliosis" (79-A: 208—212, Feb. 1997), by Hoppenfeld et al. A sensitive and specific test of the integrity of the spinal cord that is noninvasive and simple to perform has obvious attractions for anesthesiologists and surgeons who care for young patients who have scoliosis.
The test may prove to be a useful addition to the techniques of spinal cord monitoring, but we believe that a note of caution is warranted. It is well known that transient neurological abnormalities, including ankle clonus, occur at low concentrations of volatile anesthetic agents. The conclusions of Hoppenfeld et al. are based on their finding of only three false-positive results in 1006 study patients and no false-positive results in 115 control patients. These remarkable results are at variance with observations reported in previous studies of normal patients; in those studies, the prevalence of sustained ankle clonus on emergence from anesthesia was five of sixteen patients who received isoflurane1 and only one of eight patients who received halothane2.
We can speculate about the reasons for the differing results in different studies. Objective comparison among these studies, however, is hindered by a lack of information. Because the ankle clonus test depends on a reduction in the neuraxial concentration of volatile anesthetic agents, it would have been helpful to report the end-tidal concentration of the agent at which clonus occurred. We accept that this would not have been an easy task in 1976, when this study began, but it would have been regarded as routine monitoring for this procedure by 1994. Despite the finding that narcotic analgesics were associated with return of clonus, we were not told whether the patients in this study received narcotics intraoperatively.
All anesthetics are not the same. If the utility of a monitor depends on important changes in anesthetic technique, then the description of that technique should be sufficiently clear to allow other clinicians to perform an objective assessment by duplicating the method. The report by Hoppenfeld et al. does not permit us to do this with confidence. Accordingly, we must reserve judgment on whether the ankle clonus test is a reliable monitor of the integrity of the spinal cord during operations for scoliosis.
Alastair Ewen, M.B., Ch.B., F.R.C.A., F.R.C.P.(C); Bevin B. Bart, M.D., F.R.C.P.(C); Gerald V. Goresky, M.D.C.M., F.R.C.P.(C): Department of Anaesthesia, Alberta Children's Hospital at the University of Calgary, 1820 Richmond Road S.W., Calgary, Alberta T2T 5C7, Canada
Dr. Hoppenfeld, Dr. Gross, and Dr. Lonner reply:
We appreciate the interest of Mr. Ewen and his colleagues regarding the ankle clonus test. As we stated, we have found this test to be an important adjunct to the standard Stagnara wake-up test and to continuous monitoring of somatosensory evoked potentials during operations for scoliosis.
The utility of the ankle clonus test was discovered incidentally while the operating surgeon was testing a patient who was emerging from anesthesia. Further evaluation of patients who were having an operation for scoliosis revealed this transient finding to be universal.
It is not clear to us why our findings are at variance with those of studies that Ewen et al. cited1,2, although there are a number of possible explanations. First, it must be emphasized that the operating surgeon or his or her first assistant is responsible for performing the ankle clonus test, which must be done continuously as the patient emerges from anesthesia in order to ensure that the transient occurrence is not missed. Furthermore, it must be noted that one or two beats of clonus is sufficient for the test to be considered normal. The examiner notes a gradual increase in tone and then the transient occurrence of ankle clonus. In some patients there is a more sustained clonus, and in others there is only a very short-lived finding. In addition, our patients tend to be adolescents or middle-aged adults, and we have found this test to be quite reproducible in these groups of patients. Perhaps the test may not be as reproducible in an older age-group, although we do not have any hard data to support this.
The anesthetic techniques that were used were indicated in our paper and evolved to remain consistent with modern techniques. Narcotics were routinely used intraoperatively. It is important to use a short-acting agent and to avoid administration or reversal of the drug before the ankle clonus test is performed. Our group is quite comfortable utilizing the ankle clonus test as an adjunct to other monitoring methods. If Ewen et al. have not been able to obtain reproducible results, we urge them to continue to use the standard methods of the Stagnara wake-up test or electrophysiological monitoring, or both, and perhaps to develop their techniques for the use of the ankle clonus test over time.
Stanley Hoppenfeld, M.D.: Scoliosis Associates, 1180 Morris Park Avenue, Bronx, New York 10461
Alan Gross, M.D.: 3801 East Las Posas Road, Suite 106, Camarillo, California 93010
Baron Lonner, M.D.: Department of Orthopaedic Surgery, Long Island Jewish Medical Center, 1554 Northern Boulevard, Manhasset, New York 11030