Dermatomal Somatosensory Evoked Potentials in Udateral Lumbosacral

Wculopathy Michael J. Aminoff, MD," Douglas S. Goodin, MD,* Nicholas M. Barbaro, MD,? Philip R. Weinstein, MD,?

and Mark L. Rosenblum, MDP

We examined scalp-recorded somatosensory evoked potentials (SSEPs) to electrical stimulation of the peroneal nerves and to stimulation in the L5 and S1 dermatomes in 19 patients with unilateral radiculopathies involving these seg- ments. For the dermatomal studies at least two trials of 512 responses were recorded from the vertex with reference to both the midfrontal and contralateral parietal electrodes, using an averaging technique. Findings on the symptomatic and asymptomatic sides were compared in each patient. We found that peroneal SSEPs were normal in all patients. Dermatomal SSEPs correctly identified the lesion in 5 patients. In 1 patient dermatomal SSEPs laterdized the lesion correctly but localized it to the adjacent root. In 10 cases dermatomal SSEPs gave misleading information, indicating an abnormality on the asymptomatic side in 1 patient and no abnormality in 9. In the remaining 3 patients, both SSEPs and radiological contrast studies failed to identify any lesion, although the radiculopathy was confirmed by electromy- ography. These findings raise doubt about the ultimate utility of these evoked potential techniques in the evaluation of patients with suspected radiculopathies.

Aminoff MJ, Goodin DS, Barbaro NM, Weinstein PR, Rosenblum M L Dermatomal somatosensory evoked potentials in unilateral lumbosacral radiculopathy. Ann Neurol 17: 17 1-176, 1985

A noninvasive electrophysiological technique for reli- ably evaluating nerve root function would help in selecting patients with suspected radiculopathies for potentially hazardous radiological and surgical proce- dures. Somatosensory evoked potential (SSEP) studies permit the sensory function of proximal portions of the peripheral nerves and their constituent roots to be assessed, and may therefore fulfill such a role. As the studies are generally performed, however, SSEPs are elicited by stimulation of a mixed nerve trunk contain- ing fibers from several different segments; the re- sponse elicited in functionally normal fibers traversing intact roots may then obscure any abnormality result- ing from the involved root. With this consideration in mind, we studied SSEPs to both peroneal nerve and dermatomal stimulation in the L5 and S 1 territories in patients with clinically definite radiculopathy. The elec- uophysiological, clinical, and radiological findings were compared to evaluate the utility of these evoked po- tential studies in patients with suspected radiculopathy.

Methods We studied 19 patients ranging in age from 24 to 63 years (mean 2 standard deviation, 36.8 ? 8) who had clinical

features of unilateral L5 andor S 1 radiculopathies. Patients with bilateral symptoms or signs were excluded, but 2 with just radiological evidence of spinal stenosis were included. Peroneal-derived SSEPs were obtained by electrical stimula- tion of the nerve at the knee using stimuli (5 Hz, 0.2 ms duration) that were just above the motor threshold. Re- sponses were recorded from the cauda equina with elec- trodes placed over the L3 and L1 spinous processes, and from the scalp between the vertex (CZ) and midfrontal (FZ) electrodes and between CZ and the contralateral (Cl'lC4') parietal electrode. Responses occurring within 100 ms were filtered (bandpass, 30 to 3,000 Hz), amplified, and averaged using a Grass 10 evoked potential system. Two or more separate trials, each consisting of 1,024 responses, were re- carded and superimposed. The peak latency of two compo- nents (negativity at L3 spine, N11; relative positivity at CZ with respect to FZ or the contralateral parietal scalp elec- trode, P30) was measured and compared with normal values from this laboratory. Our normal values, derived from stud- ies of forty-three normal nerves, show a close correlation with height for N l1 (r = 0.74; p < 0.001) and P30 (r = 0.68; p < 0.001). The regression line has a slope of 0.05 ms per centimeter for N11, with an intercept of 1.3 ms and standard error of 0.5 ms; for P30 the regression line has a slope of 0.09 ms per centimeter, with an intercept of 11.0 ms and standard error of 1.0 ms. An interside difference in latency is considered abnormal if it exceeds 3 standard devia-

From rhe Departments of 'Neurology and ?Neurosurgery, School of Medicine, University of California, San Francisco, CA 94143. Received Feb 7 , 1984, and in revised form June 11. Accepted for publication June 16, 1984,.

Address reprint requests to Dr Aminoff, Room 794M, Department of Neurology, University of California Medical Center, San Fran- cisco, CA 94143.

171

Right L5 Dermatome Stimulation in Left 15 Dermotome

Stimulation in Left S1 Dermatome Right S1 Dermatome

L I I I I I I I I I 1 0 l 1 ~ 1 i l I I I 1 1 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100

Time (msec)

tions of our normal interside difference, i.e., 0.6 ms for N11 and 3.0 ms for P30.

Dermatomal SSEPs were obtained by electrical stimula- tion in the L5 and S1 territories on each side in all cases. For L5 dermatomal stimulation two standard electroencepha- lographic disc electrodes were placed approximately 3 cm apart on the skin of the dorsal aspect of the foot in the web between the first and second toes; for S1 stimulation they were placed laterally at the level of the fifth metatar- sophalangeal joint, with the cathode proximal to the anode. Stimuli of 0.2 ms duration were delivered at 5 Hz. Stimulus intensity was gradually increased until it was regarded by the patient as strong but easily tolerable and painless; this inten- sity was usually about twice the sensory threshold. We found no consistent difference in sensory threshold on the symp- tomatic and asymptomatic sides. Responses were recorded between CZ and the FZ reference, and also between CZ and the contralateral C3’/C4‘ position. The responses were amplified, averaged, and displayed using the Grass 10 system (bandpass, 1 to 300 Hz) and an analysis time of 100 ms. In general, 512 responses were averaged for each trial, and at least two trials were recorded to ensure replicability of the findings.

We also studied the responses to dermatomal stimulation in 32 normal subjects, aged 2 1 to 62 years (mean ? standard deviation, 3 1 ? 8), in order to provide normative criteria for evaluating patient data.

Results The typical response to dermatomal stimulation, as re- corded between CZ and both FZ and C3’/C4’, con- sisted of a positive-negative-positive complex at the vertex (Fig 1). Only the initial positive wave was evalu- ated for the purpose of this study. Latency was mea- sured to the peak of this wave, and amplitude from the preceding baseline to the peak. In general, this wave was of similar latency, but better formed and of larger amplitude, in the CZ-C3’/C4‘ than in the CZ-FZ derivation, as can be seen in Table 1. The latency of the response elicited from S1 stimulation was sig- nificantly longer (p < 0.001) than that from L5 (see Table 1). Latency values showed a significant correla-

Time (msec)

F i g I. Responses recorded over the scalp between vertex (CZ) and contralateral C3‘ or C4’ electrodes to demzatomal stimulation in the L5 and Sl territories on either side (normal subject). In each case the average of 5 12 responses is shown for each triaI, and two trials have been superimposed to demonstrate replicabiIity of the findings. An upward dejection indicates positivity at the CZ electrode.

tion with height, and this fact was taken into account in the analysis of our data.

Based on our findings in normal subjects, the der- matomal SSEPs in patients were considered abnormal if there was an interside difference in latency of more than 3 standard deviations for that particular segmental level, or an absolute latency for the responses elicited from the L5 or S1 segment that deviated from our regression line by more than 3 standard errors (see Table 1). Because amplitude is such a variable mea- sure, we considered a response as abnormal on this basis only if the interside amplitude ratio was less than 2 5 % in both recording derivations following stimula- tion at a particular segmental level, or if the response was absent unilaterally. In none of our normal subjects was this interside amplitude ratio less than 25% in both recording derivations, but in 1 it was between 30 and 40% in both. In fact, in all patients whose der- matomal SSEPs were judged abnormal on amplitude criteria, the response was absent unilaterally. Thus, each patient served as his or her own control for both latency and amplitude criteria.

The SSEP findings in our 19 patients are shown in Table 2 , together with the clinical and radiological data. In every case the responses to peroneal nerve stimulation were normal. We found that the der- matomal SSEPs correctly localized the root lesion in 5 patients (Fig 2 ) . In one of these patients (Patient 2), the findings were of particular interest: whereas the clinical deficit was more suggestive of an L5 radiculopathy, at operation the S1 root was seen to be compressed, as

172 Annals of Neurology Vol 17 N o 2 February 1985

Table 1 . Somatosensory Evoked Potential Findings in Response t o Demzatomal Stimulation in 32 Normal Subjectsa

Amplitude Latency

Ingrside Interside Amplitude

Difference Ratio (small/ Slope of Mean Regression SE (L-W Mean large) (%)

Derivation (ms) (mspercm) (ms) Mean SD (CLW SD Mean SD

c z - c 3 'tC4' L5 stimulation 50.3 0.16 4.6 0.03 1.7 0.6 0.3 67 20 S1 stimulation 51.7b 0.22 3.4 - 1.05 2.2 0.6 0.3 70 18

CZ-FZ L5 stimulation 51.0 0.22 4.7 -0.55 2.1 0.6 0.4 62 26 S1 stimulation 52.3b 0.25 3.8 - 1.17 1.7 0.5 0.4 54 28

"Mean height of subjects, 171 cm. The correlation with height for L5 and S1 elicited response in the CZ-C3'/C4' derivation was 0.27 (e < 0.05) and 0.46 (p < 0.001) and in the CZ-FZ derivation was 0.36 (p < 0.01) and 0.47 (e < O.OOl), respectively. bSignificantly different from L5 value (p < 0.001).

SE = standard error; SD = standard deviation; L = left; R = right.

had been suggested by the dermatomal SSEPs. In 1 other patient the SSEP findings lateralized the lesion correctly but to the S1 rather than the L5 root. In 1 patient with spinal stenosis the abnormality was later- alized to the clinically asymptomatic side. The der- matomal SSEPs were normal in 12 patients, in 3 of whom radiological contrast studies (myelograms and computed tomographic scans) were also normal even though needle electromyography confirmed the pres- ence of a radiculopathy.

Among the 7 patients with abnormal dermatomal SSEPs, the latency was prolonged in 2 and in the re- mainder the response was absent. In every patient in whom the SSEPs correctly identified the side of the lesion (at either the correct or an adjacent segment), a sensory deficit was evident clinically. A sensory deficit was also present clinically in 8 patients in whom the SSEPs were normal.

Discussion There are several reasons for developing a simple, noninvasive, objective electrophysiological method of evaluating root function. First, clinical examination may be unrewarding or unreliable in patients with nonspecific symptoms of a radiculopathy, such as pain. Second, neither clinical examination nor myelography provides a reliable guide as to whether axonal degener- ation has occurred, and this may have prognostic im- portance. Third, myelographic contrast studies some- times fail to show any abnormality in patients with lumbosacral disc prolapse. Fourth, the relevance of a radiological abnormality to the presenting complaint may be uncertain, especially when changes are present bilaterally or at several different levels, and in patients who have had previous surgery.

Patients with suspected radiculopathies have been investigated with a number of different electrophysio- logical techniques. Needle electromyography may show denervation in a myotomal distribution; F wave latency measurements (in conjunction with nerve con- duction studies) may suggest a slowing of proximal motor conduction velocity; and H reflex studies may indicate a lesion in the S1 pathway of this reflex. Such electrophysiological abnormalities have been reported in 29 of 57 patients (5 1%) with a clinical diagnosis of lumbosacral root disease, a number of whom had nor- mal myelograms (81; others have reported that as many as 70 to 80% of patients with a radiculopathy have an abnormality evident on needle electromyog- raphy alone 13, 51.

Although pain or other sensory complaints are usu- ally conspicuous early in radiculopathies, an objective electrophysiological test of sensory function has be- come available only with the development of tech- niques for recording SSEPs. SSEPs are usually ob- tained by stimulation of a mixed nerve trunk, and this was the approach adopted by Feinsod and colleagues 141 in their electrophysiological evaluation of 77 pa- tients with myelographically proved lumbar disc le- sions. They found that the peroneal-derived SSEP was abnormal in all of their patients, with prolonged laten- cies, disappearance of wave components, or both. Ab- normalities were found regardless of any clinical sen- sory findings, and large myelographic defects were associated with SSEP changes from the clinically unaf- fected side. Evaluation of these findings is difficult, however, because latency values in normal subjects were not related to height or limb length, no data concerning normal interside differences in latency were provided, and criteria for abnormality were not

Aminoff et al: Dermatomal SSEPs in Radiculopathies 173

Table 2. Clinical, Radiological, and Electropbysiological Findings in 1 9 Patients with Unilateral Radiculopathy"

Distribution Patient No., Duration Computed Dermatomal Age (yr). of Root Sensory Motor Ankle Myelogram Tomographic Final SSEP Sex Pain Side Deficit Deficit Jerkb Findings Scan Findings Diagnosis Findings

I., 37, M

2., 34, M

6 mo

11 mo

L

R

s1

L5

s1

L5

0

+

Protruded disc, L5-S1, on L

Protruded disc, L4-5 and L5-S 1, on R

L SIC Normal

Protruded disc, L5- S1, on R

R L 5 and SIC (see text)

L L5 and s Id

LL5 and S1'

Absent R S1 response

Normal 3., 34, F

4., 40, M

1 Yr

8 mo

L

L

L5, s1

L4

L5

L5. s1

Normal Normal + i Bulging disc, L4-5

and L5-S1, mid- line

Protruded disc, L4- 5 , on R

Normal

5., 37, M 8 mo R L5 L5 Absent R S1 response

Normal

+ R L5'

6., 42, F

7., 31, M

Protruded disc, L5-

Root cutoff, L5, on S1, on L

R

L

R

s1

L5

None

L5

L S l r

R L5'

+ 0 3 yr, 6

rno Absent

R L5 response

Normal 8., 32, M 10 mo R None None Protruded disc, L5-

Protruded disc, L5-

Bulging disc, L4-5,

Normal Protruded disc, L5-

S1, on R

S1, midline

midline

S1, midline

Same as myelogram R S1' +

9., 26, F 3 YE L None None LL5 and SIC

L L5'

+ Normal

lo., 33, F 6 mo L None None Normal + . . .

ll., 63, M 12., 40, F

2 Yr 4 mo

L L

None s1

s1 None

0 +

Normal Protruded disc, L5-S1

and L4-5, midline

L Sld L S1'

Normal Absent

L s1 response

Normal 13.,47,F 6 mo L S1 L5 + Protruded disc, L5- S1, on L

14., 35, M 2 mo R S1 L5 + Protruded disc, L5- S 1, on R; bulging disc, L4-5, on R

15., 38, M . . . R L5 None + Bulging disc, L4-5 and L5-S1, rnid- line

16., 35, M 1 yr L s 1 None 0 . . .

Protruded disc, L4-5, on L

L L5 and S1'

R L5 and S1'

Same as myelogram Absent R S1 response

Delayed R L5 response

Normal

Bulging disc, L5-S1, midline

R L5'

Protruded disc, L5-S1, on L

Normal . . .

L S1'

17., 36, F 4 yr R L5 None + Normal 18., 35, M 1 Yr, 1 R s1 None 0 Spinal stenosis; pro-

mo truded disc, L5- S1, especially on R

R LSd R S1'

Normal Normal

L L5' Delayed R S1 response

19.,24, M 2 mo L None L5 + Protruded disc, L4- Spinal stenosis; pro- truded disc, L4-5, mainly on L

5 , on L

"Peroneal derived SSEPs were normal in all cases. bo = absent or diminished ankle jerk; + = normal ankle jerk. 'Verified at operation. dVerified solely by electrornyography. 'Verified by discography; no operation.

SSEP = somatosensory evoked potential; M = male; F = female; L = left; R = right.

174 Annals of Neurology Vol 17 No 2 February 1985

Stimulation in Left S 1 Derrnatome Right S 1 Dermatorne

L I I I I I I I I I J I l l l l l l l l l l

Time (msec) Time (rnsec)

0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100

A

Stimulation in Left S1 Dermatome Right S1 Dermatorne

‘1

L I I I I I I I I I ] l l l l l ~ l l ~ ~ l

Time (rnsec) Time (msec)

0 10 20 30 A0 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 1 0 0

B

Fig 2. Dermutomal somatosenso?y evoked potentials recorded be- tween vertex (CZ) and contralateral C3’ or C4‘ electrodes to stimulation in the SI dermatome on either side in 2 patients. Two or three trials, each the average ojfSl2 responses, have been superimposed. An upward dejZection indicates positivity at the CZ electrode. In A there is loss of the first positive component of the response to stimulation in the right S l dewnutome; in B a prolonged response latency is euident with right SI stimulation. These findings suggest a right S l radiculopathy in each case.

defined. Moreover, in patients with an isolated radicu- lopathy, it is difficult to understand how stimulation of a nerve derived from several segments would ever lead to an SSEP abnormality.

Eisen and co-workers 133 evaluated 28 of 36 patients with suspected cervical or lumbosacral radiculopathies by SSEPs elicited by stimulation of cutaneous nerves that were more representative of a single dermatome. SSEP abnormalities were found in only 16 of the 28 (57%) and were most commonly encountered in asso- ciation with a clinical sensory deficit. Abnormal mor- phology and reduced amplitude were the most com- mon abnormalities; a prolongation in latency occurred only three times, in one of which it was the sole abnor- mality. The rarity of any latency change was related to the “diluting” effect of a long length of normally con- ducting peripheral nerve fibers on the short segment of these fibers with slowed conduction.

In contrast, the diagnostic yield of dermatomal SSEPs was considerably greater in the experience of Scarff and colleagues 171, who found that among 38 patients with surgically verified root entrapment from

a herniated disc, 35 (92%) had SSEP abnormalities that correctly localized the lesion, 2 had normal findings, and 1 had contralateral SSEP abnormalities; myelograms were normal or nondiagnostic in 5 of these patients.

In view of the high diagnostic yield reported by others, we undertook our own study of peroneal and dermatomal SSEPs in patients with radiculopathies. Our dermatomal SSEP findings in normal subjects agree well with those in the literature [G, 71 regarding response latency and configuration. Our latency values are considerably longer than those obtained by us for the comparable component elicited by stimulation of the tibial nerve trunk at the ankle (mean, 37.3 ? 1.7 ms) or the peroneal nerve at the head of the fibula (27.2 t 1.4) in normal subjects, regardless of height. This difference is too great to be due solely to the more distal site used for dermatomal stimulation and presumably relates also to the type of nerve fi- bers stimulated. Dermatomal stimulation activates the cutaneous afferents rather than the faster-conducting group I muscle afferents excited by stimulation of a mixed nerve [2).

In order to prevent any ambiguity, we were selective in the type of patients studied. Only patients with a clinically unilateral L5 andor S1 radiculopathy and without radiological findings suggesting involvement of additional roots at the L5 or S1 level on the oppo- site side were studied. No patients with clinical or elec- trophysiological evidence of peripheral nerve disease were included. Unlike Feinsod and colleagues [4), we found normal evoked potentials over the scalp and

Aminoff et al: Dermatomal SSEPs in Radiculopathies 175

spine to peroneal nerve stimulation in every case. For reasons already given, we anticipated such a finding.

The dermatomal SSEPs correctly localized the lesion in 5 patients and lateralized it correctly in another. In 1 patient the dermatomal SSEPs localized the lesion to the incorrect side, however, possibly reflecting a sub- clinical radiculopathy related to the spinal stenosis present in this patient; in the remaining 12 patients, the findings were normal. The test thus appears to be relatively insensitive. Like Eisen and co-workers { 3 ] , we found that a latency abnormality was relatively un- common. Scarff and associates {7] used less stringent criteria for defining an abnormality than we did, taking an interside latency difference of more than 3 ms as evidence of abnormality, but the manner in which this value was derived is not clear from their report and they did not present their control data. If we had ap- plied their criteria (which represent approximately 1.5 standard deviations of our normal interside differ- ence), we would have identified as abnormal the der- matomal SSEPs in 5 of our patients previously judged to have normal responses; these results would have localized the lesion to the wrong side in three instances (Patients 4,8, and 9), correctly in one (Patient 18), and to the correct side but the wrong level in one (Patient 17). Thus, our stricter criteria prevented erroneous conclusions in four instances. In fact, our use of 3 standard deviations in calculating the maximal normal interside difference in latency is equivalent to approxi- mately the 99% tolerance limit for 95% of the popula- tion 113.

We conclude that the SSEP findings in response to peroneal or dermatomal stimulation have no reliable role in the evaluation of patients with suspected lum- bosacral disc protrusions. Although they provide data complementary to those obtained with other electro- physiological techniques, they add little information to that already obtained by competent clinical examina- tion.

References 1. American Electroencephalographic Society: Guidelines for clini-

cal evoked potential studies. J Clin Neurophysiol 1:3-53, 1984 2. Burke D, Skuse NF, Lethlean AK: Cutaneous and muscle affer-

ent components of the cerebral potential evoked by electrical stirnulation of human peripheral nerve. Electroencephalogr Clin Neurophysiol 51:579-588, 1981

3. Eisen A, Hoirch M, Moll A: Evaluation of radiculopathies by segmental stimulation and somatosensory evoked potentials. Can J Neurol Sci 10:178-182, 1983

4. Feinsod M, Blau D, Findler G, et al: Somatosensory evoked potential to peroneal nerve stimulation in patients with herniated lumbar discs. Neurosurgery 11:506-511, 1982

5. Knuttson B: Comparative value of electromyographic, myelo- graphic, and clinical-neurological examination in diagnosis of lumbar root compression syndrome. Acta Orthop Scand [Suppl)

6. Nicpon KJ, Sedgwick EM, Brice J, Docherty T: Dermatornal somatosensory evoked potentials. Electroencephalogr Clin Neurophysiol 56:72P, 1983

7. Scarff TB, Dallmann DE, Toleikis JR, Bunch WH: Dermatomal somatosensory evoked potentials in the diagnosis of lumbar root entrapment. Surg Forum 32:489-491, 1981

8. Tonzola RF, Ackil AA, Shahani BT, Young RR: Usefulness of electrophysiological studies in the diagnosis of lumbosacral root disease. Ann Neurol 9:305-308, 1981

49:1-135, 1961

176 Annals of Neurology Vol 17 No 2 February 1985