Somatosensory evoked cervical and lumbosacral dermatomes

potentials from

Liguori R, Taher G, Trojaborg W. Somatosensory evoked potentials from cervical and lumbosacral dermatomes. Acta Neurol Scand 1991: 84: 161-166.

A method for recording the somatosensory evoked potentials after stimulation of the cervical and lumbosacral dermatomes is described. Normative values and their ranges are given for each dermatome including left-right differences. A significant correlation was found between latencies and conduction distance. Dermatomal SEPs may play a role in diagnosing cervical and lumbosacral radiculopathies.

Several electrophysiological methods can be used to evaluate nerve root function (1-9). The needle elec- tromyography aims to document which myotome is involved and to what extent. F wave latency mea- surements may reveal a slowing of proximal motor conduction. H reflex studies may indicate a lesion in the S 1 pathway of this reflex. These tests are useful but give information only about the efferent fibres except the H reflex which involves both afferent and efferent conduction. However, the majority of the patients with radiculopathy present with sensory symptoms prior to or in the absence of motor im- pairment. When motor signs are absent needle elec- tromyography and F wave studies may be normal. Somatosensory evoked potentials (SEPs) have been used to investigate the function of cervical and lum- bosacral sensory roots. Some authors elicited SEPs by stimulation of a mixed nerve (10-13). Whereas others stimulated sensory nerves such as the sural, saphenous, and superficial peroneal nerves (14-17). However, these methods have not been particularly helpful. The low diagnostic yield in single root le- sions found by these studies may be due to the mul- tisegmental activation caused by nerve stimulation. Therefore, a monoradiculopathy can be masked by normal responses mediated through unaffected roots. It might be useful then to elicit SEPs by stimulating an area of the skin representative of the affected single dermatome in the radiculopathy (18-21). The aim of this study was to determine normal values for the cervical dermatornes C6, C7 and C8 and lum- bosacral dermatomes L4, L5 and S1.

R. Liguori, G. Taher, W. Trojaborg Laboratory of Clinical Neurophysiology, Rigshospitalet, University Hospital, Copenhagen, Denmark

Key words: SEP; cervical derrnatomes; lumbosacral dermatomes; segmental innervation Rocco Liguori, M.D., Laboratory of Clinical Neurophysiology, Rigshospitalet, Blegdamsvej 9, 2 lOO-DK, Copenhagen N, Denmark

Accepted for publication February 8, 1991

Material and methods

Cortical recordings after cervical dermatome stimu- lation were made in 15 asymptomatic subjects (aged 18-65 years, mean & SD 44 15 years) and after stimulation of lumbosacral dermatomes in 20 sub- jects (aged 18-65 years, 42+ 14 years). During the recording session the subjects were in a supine po- sition and instructed to relax. The skin temperature was measured at the site of stimulation and kept above 32°C. For the cervical segments the site of stimulation for C6 was about 7 cm above the styloid process of the radius, for C7 between the second and third metacarpal bones and for C8 between the fifth and forth metacarpal bones (Fig. 1). For the lum- bosacral segments the stimulation site for L4 was about 6 cm above the medial malleolus along a line connecting the medial malleolus and medial epicon- dyle of the tibia1 bone. The stimulation site for L5 was at the medial side of the second metatarsal bone and for S1 at the lateral side of the fifth metatarsal bone (Fig. 1). The places of stimulation of der- matomes were chosen in accordance with the der- matome maps, clinically and anatomically consid- ered as the areas least contaminated by adjacent dermatomes (22-24). To evaluate whether the trans- mission of sensory impulses from the dorsum of the foot took place via the sural or the peroneal nerves or both, we stimulated four different areas of the skin in 7 subjects bilaterally (Fig. 2). Simultaneous re- cordings from the peroneal and sural nerves were performed via subcutaneous needle electrode placed at the midleg and distally to capitulum fibulae, re- spectively.

A bipolar surface electrode was used for stimula-

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Fig. 1. Stimulation sites of the cervical and lumbosacral dermatomes used to elicit SEPs

tion with an interelectrode distance of 2.5 cm. The stimulating cathode was placed proximally. Stimuli of 0.2 ms duration were delivered at a rate of 3 Hz. The sensory threshold for the electrical stim- ulus was determined and a tolerable and painless stimulus intensity was usually set 2.5 times above this level. Subcutaneous needle electrodes were placed at C3’ and C4’ (between C3 and P3 and C4 and P4 of the international EEG 10-20 system) when stimulating the arm and at Cz’ (between Cz and Pz) when stimulating the leg. The reference electrode was placed at Fz, the ground electrode at Fpz. The cortical responses were amplified, averaged and dis- played using an analysis time of 150 ins for the cer- vical segments and 200 ms for the lumbosacral seg- ments. Filter settings of 2 Hz to 1 KHz and 5 Hz to 500 Hz were utilized for the cervical and lumbo- sacral segments, respectively. The latter bandwith had no significant influence on peak latencies or amplitude and was chosen to facilitate the investi- gation (21). All procedures were performed using an %channel machine (Evomatic 8000, Dantec). Stim-

ulation was performed at least twice and traces werc superimposed to ensure reproducibility. In the upper limb distances were determined from the site of stim- ulation to Erb’s point with a tape measure; from there on to C7 and site of scalp recording using a caliper. Similarly, in the lower limb distances were measured from the site of stimulation to the gluteal crease with a tape measure; from there to S1 using a caliper. The distance from S1 to C7 was deter- mined with a tape measure. The total conduction distance was obtained by adding the relevant values.

Results

Evaluation of the contribution of sensory input from L5 and S 1 dermatomes by stimulation at the first metatarsal bone and the first interstice (sites A and B, Fig. 2) revealed a sensory action potential (SAP) over the peroneal nerve only. When the stimulus was applied between the second and third metatarsal bones (site C, Fig. 2) a SAP was present over the peroneal nerve in all subjects and over the sural

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Peroneal nerve Sural nerve Scalp C ~ F Z

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0 5 lOmsec 0 5 lOrnsec 0 25 50 75 100rnsec

Fig. 2. Sensory action potentials recorded simultaneously over the peroneal and sural nerves at the fibular head and mid-calf, respec- tively after stimulation at four different sites (A, B, C and D) on the dorsal aspect of the foot. Stimulation at A and B evoked a SAP in the peroneal nerve only, stimulation at D evoked a SAP in the sural nerve only. Stimulation at C evoked SAP'S in both peroneal and sural nerves. Subject Z.L., a 37-year-old healthy woman.

nerve in one only. Stimulation at the fifth metatar- sal bone corresponding to S 1 dermatome activated sensory fibres only in the surd nerve.

A cortical potential following dermatomal stimu- lation of upper (C6-C8) and lower (L4-S1) limbs, respectively, was present in all the normal subjects. The configuration of the evoked potentials was sim- ilar to that elicited after stimulation of the respective mixed nerves of upper and lower limbs. The first negative peak of the potential complex evoked by stimulation of C6-C8 dermatomes was used for la- tency determination (Fig. 3). The mean values are presented in the Table in which also the differences between latencies after left and right dermatome stimulation are given. A difference of more than 2.2, 1.9, or 1.7 ms (> 2.5 SD of mean value) between left and right C6, C7, or C8, respectively are considered abnormal. The individual latencies as a function of the conduction distances are shown in Fig. 5 . La- tencies increased with increasing distance between stimulation and recording sites (r = 0.74, 0.86 and 0.81 for C6, C7 and C8, respectively: P<O.OOl). Values above the upper 98% confidence limit illus- trated in the scattergram (Fig. 5 ) are considered ab- normal.

As to the dermatomal SEPs following stimulation

of L4-S 1, the first positive peak of the response was used as the point of reference (Fig. 4). The mean latency values are shown in the Table together with

0 50 100 150ms Fig. 3. Responses recorded over the scalp at C3' or C4' elec- trodes after stimulation of C6, C7 and C8 dermatomes.

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Fig. 4 . Responses recorded over the scalp at Cz' electrode after stimulation of the posterior tibia1 nerve, L4, L5 and S1 der- matomes. An upward deflection at the active electrode indicates negativity.

mean differences of latencies for potentials evoked by stimulation of left and right side dermatomes. A difference of more than 5.3, 5.9, or 6.3 ms (> 2.5 SD of mean value) between left and right L4, L5, or S 1, respectively are considered abnormal. In- dividual values for dermatomal SEPs following stim- ulation of the lower limbs are shown in Fig. 6 as a function of the conduction distance. As for the cer- vical dermatomal SEPs, latencies increased with in- creasing distance between stimulation and recording sites (r = 0.64, 0.72 and 0.69 for L4, L5, and S 1, respectively: P < 0.00 1). Values above the upper 98 % confidence limit illustrated in the scattergram (Fig. 6) are considered abnormal.

Discussion

Electrophysiological documentation of radiculopa- thies by means of EMG and conduction studies has been disappointing particularly when sensory symp- toms and signs or both are predominating. Except for the H reflex, standard electrodiagnostic tech- niques do not evaluate the sensory roots function. The method of stimulating peripheral sensory or mixed nerves (5, 10-12, 14, 15, 17) seems to have limited value, as these nerves generally contain fibres belonging to more than one root. However, segmen- tal specificity can be improved when a cutaneous nerve representative of an appropriate dermatome is stimulated to elicit SEPs (8, 19-21). In this study we present normal values for cortical evoked potentials after stimulation of the cervical and lumbosacral

C6-DERM ATOME

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Fig. 5. Relationship between latency of the cervical dermatomal SEPs from C6-C8 and conduction distance in normal subjects. The middle line represents the regression line, the outer lines the upper and lower 98% confidence limits.

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Cervical & lumbosacral dermatomes

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Fig. 6 Relationship between latency of the lumbosacral der- matomal SEPs from L4-S 1 and conduction distance in normal subjects. The middle line represents the regression line, the upper and lower lines 98% confidence limits.

dermatomes in healthly volunteers. Our findings con- cerning lumbosacral dermatomes are in accordance with those previously reported (18, 19, 21) regarding configuration and latency. The cortical evoked po- tentials elicited by stimulation of the cervical and lumbosacral dermatomes have the same shape as those evoked by stimulation of the median nerve at wrist and the tibial nerve at the ankle, respectively. The latency values for the cervical and lumbosacral dermatomes were longer than those obtained in this laboratory for the comparable component elicited by stimulation of the median nerve at wrist (mean 19 1.6 ms) and tibial nerve at the ankle (mean 41 2.0 ms). This latency difference is presumably related to the different conduction distance (25). Re- cording of SAP'S over the surd and peroneal nerves after dermatomal stimulation revealed in one subject dual innervation of a segment supposed to be inner- vated by L5 only. Thus, dermatome borders should not be represented by strict defined lines.

The criteria which can be used to define abnor- mality are: 1) absent dermatomal SEP, 2) latency deviating from the regression line by more than 2.5 SD, which represent the 98% confidence limits, 3 ) a latency difference between left and right exced- ing more than 2.5 SD.

The technique used to elicit dermatomal SEPs is simple and non-invasive and may be valuable in diagnosing cervical and lumbosacral root compres- sion when sensory symptoms and signs are domi- nating or the only evidence of radiculopathy. Our experience with dermatomal stimulation in patients with radiculopathies will be described in a separate communication. In patients with C6 or C7 root com- pression SEPs after digit 1 and digit 3 stimulation have proven useful to determine the site of cervical disc protrusion (9). Whether this method is superior to the one described here remain to be shown.

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