Delusional parasitosis with shared psychotic features secondary to vitamin B12 deficiency and mild cognitive impairment: a case report

BACKGROUND: Visual illusions are misperceptions of real external stimuli and, when persistent, may indicate underlying neurological or metabolic disorders. Delusional parasitosis is characterized by a fixed belief of infestation without medical evidence and may occur secondary to medical conditions. However, the relationship between visual illusion-related perceptual abnormalities and delusion formation, particularly in reversible metabolic states such as vitamin B12 deficiency, remains unclear. CASE PRESENTATION: An 80-year-old man presented with a persistent belief that mites infested his body and environment. His delusion was preceded by misinterpretation of real stimuli, including lint and skin debris, consistent with visual illusions. Cognitive assessment indicated mild cognitive impairment. His wife gradually came to share his delusional beliefs. Laboratory findings revealed marked vitamin B12 deficiency. Brain perfusion imaging demonstrated hypoperfusion in the medial occipital and parietal association cortices. Treatment with low-dose quetiapine and mecobalamin resulted in rapid resolution of visual illusions and subsequent disappearance of delusional parasitosis. Quetiapine was discontinued without relapse, and follow-up imaging showed improvement in cerebral perfusion. The wife's shared beliefs resolved following clinical improvement and separation. CONCLUSIONS: This case suggests that visual illusions due to reversible metabolic disturbances may precipitate delusional parasitosis in cognitively vulnerable individuals and highlights the importance of evaluating all suspected patients and closely associated individuals.