MEP and OCT
optic neuritis, ON, MS-ON
OPTIC NEURITIS
MEP, SEP, VEP and OCT were performed at baseline and at 6, 12 and 24 months. MEPs of the four limbs were obtained using a Cadwell MS10 magnetic stimulator with a round coil (outer diameter, 12 cm). The coil was placed tangentially to the scalp, with its center over the vertex. Patients were asked to slightly contract the target muscles (abductor pollicis brevis and abductor hallucis) at about 20% of the maximum voluntary effort to facilitate motor responses. Spinal roots were stimulated at C6–C7 and L4– L5 spaces while recording from the same muscles. Central motor conduction time was measured as the difference between total and peripheral motor conduction time. SEPs were obtained on electrical stimulation of the median nerve at the wrist for the upper limb and at the tibial nerve for the lower limb. Latencies of the main peripheral, spinal and cortical components were measured, and central conduction time was calculated as the difference between cortical and spinal latencies (for tibial nerve SEP, latencies were corrected by height). VEPs to reversal achromatic checks (each subtending 60, 30 and 15 minutes of visual angle (min arc)) were recorded over Oz of the 10–20 international EEG system, with Cz as the reference. Latency and amplitude of the P100 component were measured. For all evoked potential modalities, latencies and amplitudes were measured. OCT was performed using spectral domain Spectralis OCT (Heidelberg Engineering, software version 5.8). Standard pRNFL was measured on a standard 3.5-Ø-mm circle scan protocol centered on the optic disc by the scanning technician. Global pRNFL thickness was automatically measured through the pre-installed software provided by the manufacturer. pRNFL thinning and p100 latency increase may also occur as a result of MS-associated optic neuritis (MS-ON). Patients were categorized into those with prior history of MS-ON and those without MS-ON (non-MS-ON) and assessed accordingly. For patients with a history of MS-ON in one eye, the eye without prior MS-ON was studied. For patients with a history of MS-ON in both eyes or no optic neuritis (ON) at all, the pRNFL and P100 mean of both eyes was considered. MEP and SEP abnormalities were quantified according to a conventional four-point graded ordinal score (0 = normal; 1 = increased latency; 2 = increased latency plus morphological abnormalities of a major component; and 3 = absence of a major component). For each modality, the evoked potential score was the sum of the scores in the two sides and in upper and lower arms (from 0 to 12), with higher values representing a more severe evoked potential involvement