Isolated Parosmia Caused by Vitamin B12 Deficiency.

Vitamin B12 deficiency classically presents with neurological or hematological abnormalities. Prior reports have shown that vitamin B12 deficiency also causes olfactory dysfunction when patients have concurrent neurological or hematologic findings. To our knowledge, this is the first case of isolated parosmia caused by vitamin B12 deficiency in the absence of neurological/hematological abnormalities or structural sinonasal disease. A 20-year-old man presented with a 3-month history of parosmic symptoms. Nasal endoscopy, brain MRI, and neurological examinations were normal. A laboratory evaluation demonstrated marked vitamin B12 deficiency without hematologic abnormalities, and oral methylcobalamin led to the complete resolution of parosmia within 3 months. Clinicians should consider vitamin B12 deficiency as a reversible cause of olfactory dysfunction.