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

Background 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 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 symptoms were 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 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 illusions due to reversible metabolic disturbances may precipitate delusional parasitosis in cognitively vulnerable individuals and highlights the importance of evaluating both patients and closely associated individuals.