The diagnostic value of somatosensory evoked potentials in the diseases of peripheral nervous system

The mean latency value of somatosensory evoked potentials (SEP) recorded at the Erb's point, spinal and cortical level, was assessed in 20 normal subjects by percutaneous stimulation of the peripheral nerves (median, ulnar, superficial radial, tibial and peroneal). At the Erb's point and spinal cervical level, we distinguished the classically described potentials (N9, N13, N14) and the cortical "far-field" potentials such as: P17, N20, P24, N35 and sometimes P45. The central conduction time was calculated by determination of the interpeak latency N9-N13 and N13-N20 (for the upper limb) and LP-P37 for the lower one. A study of 100 patients with peripheral nerve diseases: 30 polyneuropathies, 50 radiculopathies, 9 cases with carpal tunnel syndrome, 5 with brachial plexus injury, 6 with compressive or traumatic diseases of the peripheral nerves, demonstrates the value of the SEPs in the assessment of the nervous lesion site (central or peripheral). In polyneuropathies, a decrease in amplitude and delayed latency of the N9 potential as well as delayed latency of the early cortical potentials on stimulation of the median and tibial nerves occurred. Delayed N9 and low amplitude with delayed latency spinal potential (N13 and LP) were found in radiculopathies. In myelopathies, the central spinal conduction time (N9-N13) was delayed and there were also delayed latencies of the cortical SEPs on lower limb stimulation. The patients with brachial plexus injury had a change in the N9 to N13 amplitude ratio, with prognostic value. Cortical recordings of the SEPs are also of special prognostic value and may suggest the surgical exploration when the axonal functional continuity is lost in brachial plexus injury and compressive or traumatic lesions of the peripheral nerves.