A twenty-one-year-old female known to suffer from bipolar type I disorder d
eveloped features of a pseudodementia. Following prompt initial response to
treatment with antidepressants, there was an early recurrence of cognitive
impairment. Blood investigations confirmed a macrocytic anaemia and vitami
n B12 and folate deficiencies. There was dramatic resolution of cognitive i
mpairment after vitamin replacement. This suggested the occurrence of a rev
ersible nutritional dementia and reinforced the need to rule out secondary
organic causes of psychiatric symptoms even inpatients previously diagnosed
with a primary psychiatric disorder.