Periodic alternating nystagmus as a diagnostic clue in MRI-negative Wernicke encephalopathy mimicking multiple system atrophy: a case with complex heterogeneous dizziness.

Dizziness in elderly patients is heterogeneous and nonspecific, leading to frequent misdiagnosis. Wernicke encephalopathy (WE) is a severe but reversible nutritional disorder responsive to timely thiamine supplementation; it often lacks typical MRI changes and may mimic neurodegenerative diseases such as multiple system atrophy (MSA). A 62-year-old male with chronic heavy alcohol use presented with 1 year of recurrent heterogeneous dizziness, vertigo, unsteadiness, presyncope, falls, and nocturnal visual hallucinations. Severe orthostatic hypotension and pyramidal signs initially suggested MSA. Peripheral neuropathy was attributed to chronic alcohol intake. Brain MRI was normal without typical WE lesions. Videonystagmography revealed periodic alternating nystagmus (PAN), saccadic intrusions, and impaired smooth pursuit, indicating central vestibular dysfunction. These ocular abnormalities resolved rapidly after thiamine replacement, excluding MSA, confirming MRI-negative WE. With high-dose thiamine, cobalamin, and alcohol cessation, PAN disappeared, dizziness and hypotension improved markedly, and hallucinations resolved. Full functional recovery was achieved at 3-month follow-up. This case shows that WE may manifest with complex dizziness and normal MRI, mimicking MSA clinically. PAN is a key objective marker of central vestibular involvement. In alcoholic patients with unexplained dizziness, orthostatic intolerance and central ocular signs, reversible WE should be excluded before diagnosing progressive neurodegeneration.