MUSK and Nystagmus: NCS, nerve conduction study; IVIG, intravenous immunoglobulin; HINTS, head impulse, nystagmus, and test of skew; MRI, magnetic resonance imaging; DWI, diffusion-weighted imaging; ECG, electrocardiogram; BPPV, benign paroxysmal positional vertigo; vHIT, video head-impulse test; RNS, repetitive nerve stimulation; AChR, acetylcholine receptor; MuSK antibodies, muscle-specific kinase antibodies