J7ournal of Neurology, Neurosurgery, and Psychiatry 1992;55:461-465 461 Value of somatosensory evoked potentials in J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.55.6.461 on 1 June 1992. Downloaded from saphenous entrapment neuropathy S Tranier, A Durey, B Chevallier, F Lot Abstract Primary entrapment of the saphenous nerve Neuralgia ofthe saphenous nerve (SN) is a (SN) is very uncommon,' but scar entrapment " rare clinical syndrome simulating a vas- may occur often after knee surgery,2 and has cular disorder of the lower extremities. In been the subject of some recent studies.46 It is four cases, the presenting complaint was commonly taken for a radicular (L3-L4), persistent pain on the medial aspect ofthe vascular, muscular or articular disorder and knee. Examination revealed tenderness clinical diagnosis is difficult. In many diseases over the site of exit of the SN from the of peripheral nerves, electromyography femoral canal. Femoral nerve motor con- (EMG) may be adequate for diagnosis, espe- duction, quadriceps H-reflex and EMG of cially if the sensory nerve action potential the leg muscles were normal. The sensory (SNAP) can be recorded. Unfortunately, nerve action potential of the SN in the leg SNAP cannot be recorded in some SN entrap- was not obtained in some patients, even in ment patients, even in the unaffected leg. This the unaffected leg. SEP were therefore method is therefore inconclusive. preferred for diagnosis and performed at The clinical application of somatosensory the infrapatellar and descending branches evoked potentials (SEP) has helped resolve the of the right and left SN and recordings diagnosis of both central and peripheral nerve from the Cz'-Fz electrode. Latency and diseases. In recent years, SEP techniques have amplitude differences were evaluated and been used to study the SN. compared with a control group of healthy subjects. An alteration in the SEP from one branch was observed on the painful Patients and methods side. Posterior tibial responses were nor- 1) Four patients with SN entrapment were mal. In one case, pain resolved immedi- observed (2 males, 2 females, mean (SD) age ately after neurolysis, confirming SN 37.5 (13-2) years, range 21 to 52, mean (SD) entrapment above the femoral canal, height 1-70 (009) metres). The clinical,radio- before its division. Pain resolved in two logical and electrophysiological data ofthe four other cases and persisted in the last after patients are shown in table 1. medical treatment. SEP studies are valu- 2) Twelve normal subjects (9 males, 3 females, able in the diagnosis of an isolated lesion mean (SD) age 34-3 (9 5) years, range 21 to http://jnnp.bmj.com/ of the SN. 52, mean (SD) height 1-79 (006) metres) also Table 1 Clinical, radiological and electrophysiological findings in 4 patients with saphenous nerve entrapment. Patient (No) 1 2 3 4 Age (years) 43 34 21 52 on September 29, 2021 by guest. Protected copyright. Sex F M M F Service Central Height (metres) 1-63 1-78 1-80 1-62 Affected side L R R L d'Explorations Duration of pain (years) 3 2 0 25 > 5 Fonctionnelles, External causes 0 shock knee (football) heavy hyper- Meniscectomy H6pital Ambroise extension (knee) Par6, Boulogne, Aggravating factors jogging, gymnastics, walks, stairs Hyperextension of walks, stairs France rubbing of bed linen, knee and thigh S Tranier heat, constrained B Chevallier flexion of knee F Lot Tinel and local anaesthetic test + + + + Skin P-R test (cellulalgia) + + - + Service de Facilitation tests + + + + R66ducation Hypo or anaesthesia - + - + Fonctionnnelle, X Ray (knee lumbar spine) N N N N H6pital Foch, Arthroscopy N N N Suresnes, France Meniscography N N A Durey Echography (muscles) N N N Correspondence to: Arteriography N Dr Tranier, H6pital Needle EMG N N N N Avicenne, Service H orT reflex/rectus femoris N N N N d'Explorations MCV femoral nerve MCV m/s 68-5 83-8 72-9 80-7 Fonctionnelles (physiologie), Saphenous nerve 125 route de Stalingrad, 0 0 0 93009 France L SAP (V) 0 Bobigny, R SCV (m/s) 38 - 40-2 - Received 13 February 1991 SAP (uV) 15 0 6-5 0 and in final revised form 22 July 1991. F = female; M = male; L = left; R = right; + = positive; - negative. Skin P-R test = skin pinching-rolling test; SAP = sensory action Accepted 30 July 1991 potential. MCV or SCV = motor, sensory conduction velocity; N = normal. 462 Tranier, Durey, Chevalier, Liot Lateral cutaneous nerve of J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.55.6.461 on 1 June 1992. Downloaded from the thigh tendon, just below the medial tibial condyle. The stimulation of these two branches was Antero-superior iliac Fascia iliaca spine based on anatomical findings.'0" - Femoral nervFe C) The terminal descending branch (TDB) in the ankle was stimulated two fingers above the - Inguinal ligarnent medial malleolus just medial to the tibia.78 - Proximal trurik of Both legs were stimulated sequentially with saphenous nierve bipolar cutaneous electrodes with the cathode positioned proximally. Square pulses (0-2 ms) of constant current were delivered at a fre- quency of 3 Hz and an intensity four times the sensory threshold. The current was adjusted until sensation radiated along the distribution of the PDB and TDB. The posterior tibial nerve (PTN) was also stimulated. The subjects were instructed to lie supine and relax in a quiet room at a temperature of 23-25°C. The recordings were made from the active electrode 2 cm -Gracilis posterior to the vertex of the Infrapatellar scalp (Cz') with respect to the reference elec- branch -Semitendinosus trode at Fz (10-20 system). The ground electrode was attached to the thigh (lower Sartorius third) on the stimulated side. Standard needle Proximal electrodes (DANTEC) were used for record- descending branch ing the responses. The amplifier bandpass was 10-500 Hz. The analysis time was 200 ms and 2 sequential runs of 500 sweeps were averaged to check the constancy of the response. The latencies and amplitudes of the initial negative (upward, Nl) and initial positive (downward, Terminal descending P1) peaks were measured. branch OTHER INVESTIGATIONS WERE PERFORMED IN THE PATIENTS ONLY Needle EMG and motor conduction velocity (CV) of the femoral nerve were studied. Sensory CV of the SN was studied in the leg according to the antidromic method ofWaina- pel et al.'2 Hoffmann (H) or tendon (T) reflexes of the rectus femoris muscle were Figure I Saphenous nerve and its 3 branches: sites of stimulation to record somatosensory evoked potentials. measured according to the technique of Gui- heneuc.' http://jnnp.bmj.com/ had bilateral SEP testing. All were memberrs of Results our laboratory and gave their informed cwfon- SEP IN NORMAL SUBJECTS sent. The SEP of the SN is a W-shaped wave. The 3) SEP early cortical responses are labelled NI, P1, The SN was stimulated at three points tty a N2. The onset is referred to as N1. The results slightly modified version of a procedure desscri- of measurement of P1 are shown in table 2: on September 29, 2021 by guest. Protected copyright. bed in previous studies,4 - (figl). side-to-side, there was no statistically sig- A) The infrapatellar branch (IPB) was stiimu- nificant difference in P1 latency. The ampli- lated between the lower inner edge of the tude of P1 was variable from one subject to patella and the sartorius tendon. another, and even from side-to-side. Never- B) The proximal descending branch (PDB1) in theless, P1 was the most reliable response and the leg was stimulated behind the sartoprius formed the basis for most of the analysis. Table 2 SEP after stimulation of 24 different branches of the saphenous nerve (SN) and posterior tibial nerve (PTN) in 12 normal subjects: study of Pl (mean values). Amplitude Side to side Ratios of amplitude Latency (ins) peak to peak (,uV (differences in latency) (ms) (smallest/largest) Nerves Mean (SD) Mean (SD) Mean (SD) (range) Mean (SD) (range) SN IPB 37-7 (2 95) 1-34 (1-03) 0-85 (0-18) 0 80 (0 10) (0-6-1-2) (0 65-0-92) PDB 36-37 (2 66) 1-61 (0 87) 0 55 (0-35) 0 75 (0 09) (0 1-1) (0 60-0*90) TDB 40 97 (2-13) 1-43 (0 72) 0-68 (0-2) 0-79 (0 11) (0 4-1) (0 65-0{96) PTsN 38-53 (1-9) 2 63 (2-17) 0-54 (0-26) 0-77 (0 05) (0 1-08) (0-71-088) IPB = infrapatellar branch; PDB = proximal descending branch; TDB = terminal descending branch. Value of somatosensory evoked potentials in saphenous entrapment neuropathy 463 Table 3 Data of Pl of SEPfrom saphenous nerve (SN) stimulation on the affected sides in 4 patients. J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.55.6.461 on 1 June 1992. Downloaded from Control group Number of cases (normal values) SN Mean (SD) No ) No 2 NVo 3 No 4 IPB Latency-difference (ms) 0-85 (0 10) 0 70 Absent 2-8 5-2 Amplitude Ratio (S/L) 0 80 (0-10) 0 70 Absent 0-22 0-72 PDB Latency-difference (ms) 0-55 (0 35) 0-6 Absent 0 70 3-2 Amplitude Ratio (S/L) 0-75 (0-09) 0-29 Absent 0-72 0 58 TDB Latency-difference (ms) 0-68 (0-21) 3-6 - 060 4-8 Amplitude Ratio (S/L) 0-79 (0 11) 0-25 - 044 0-69 - = not studied; IPB = infrapatellar branch; PDB = proximal descending branch; TDB = terminal descending branch; S/L = smallest/largest. SEP in patients may be considered abnormal stimulation of the proximal trunk of the SN if an interside difference in latency exceeds the just below the inguinal ligament (fig 1), and mean value by more than 3 standard devia- were almost normal after left IPB stimulation.