Cutaneous and mixed nerve silent periods in syringomyelia

Objectives and methods: We studied cutaneous and mixed nerve silent periods (CSP, MNSP) in 4 patients with cervical syringomyelia documented by magnetic resonance imaging who on clinical examination presented with unilateral hypalgesia and hypothermesthesia. In addition, we recorded upper and lower extremity somatosensory and motor evoked potentials (SEP, MEP), and cortical silent periods. Results: In all patients, CSP and the later portion of MNSP were absent or shortened on their affected side, while both were normal on their unaffected side. In all patients, SEP latencies were normal following both median and tibial nerve stimulation. In two patients, the amplitude N13 (median nerve SEP), and in one patient each the amplitudes N20 (median nerve SEP) and P37 (tibial nerve SEP) were reduced. Central motor conduction time was prolonged to abductor digiti minimi muscle in one patient on the affected side, but was normal to tibialis anterior muscle in all patients. Cortical silent periods where present bilaterally in spite of unilateral complete absence of CSP and MNSP in two subjects tested. Loss of CSP and MNSP were a sensitive parameter of spinal cord dysfunction in syringomyelia. The cervical median nerve SEP response N13 reflected gray matter involvement, while corticospinal tract dysfunction was less frequently observed. Conclusion: Our data suggest that CSP and later portion of MNSP are generated at the spinal level by the same small myelinated A-δ fibers, and that their central network is distinct from large diameter fiber afferents and efferents.