PMC6586985_01
Cutaneous silent periods in right tibialis anterior muscle after stimulating the right sural nerve at the lateral malleolus with stimulus intensities of 50 mA (two traces superimposed) and 80 mA (four traces superimposed) during the first month of intramuscular vitamin B12 substitution (04/2016) and 21 months later (01/2018). Arrows indicate stimulus onset. Thick horizontal lines indicate CSP onset, end, and duration.
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CSPs in tibialis anterior muscle after stimulating the sural nerve at the lateral malleolus (0.5 ms stimulus duration, 50 and 80 mA intensity, delivered during 5 s periods of 50% of maximum voluntary contraction every 10 s, filters set at 30 and 10000 Hz, single rectified EMG traces recorded with 500 ms sweep duration) appeared delayed, with incomplete EMG suppression, but normal duration as compared to normative values obtained in 23 healthy subjects (age 20-64 years) (Fig. 1, Table 1). Likewise, repeat CSP studies revealed normal values and no longer showed dysfunction of thin-myelinated A-delta nerve fibers (Fig. 1, Table 1).
Tibial nerve somatosensory evoked potentials following right (left panel) and left side stimulation (right panel) at the ankle (square pulses of 0.2 ms duration, 3.7 Hz), obtained 21 months after initiation of intramuscular vitamin B12 substitution. Traces from below to above represent recordings (2 times 150 responses plus grand average superimposed) of the tibial nerve in popliteal fossa (N8, PF - K), of the conus medullaris (N22; twelfth thoracic vertebra versus thoracic reference: T12 - T6), and of leg area of the contralateral somatosensory cortex (P37; cephalic recordings versus frontal reference, Ci' - Fz, Cz' - Fz). The bars at the bottom indicate the central conduction time (CCT, i.e., interpeak latency N22-P37), which is prolonged on both sides (upper normal limit 20.1 ms). Note also the untypical configuration of the cortical responses.
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SEPs were unremarkable, but tibial nerve SEPs showed a prolonged central conduction time (Fig. 2), pointing to a possible role of dorsal columns contributing to the patient's sensory disturbance. Suspecting SCD of the spinal cord, we decided to perform spinal magnetic resonance imaging (MRI) (sagittal T1-weighted turbo spin echo, sagittal and axial T2 weighted turbo spin echo); the results, however, were unremarkable.