SummaryOver 95% of cases of dementia are attributable to Alzheimer's disease, vascular, Lewy body and frontotemporal dementia, and a few other common causes. In this article we consider some of the rare and unusual causes that account for the remaining 5%. Categorising them according to aetiological group (degenerative, vascular and infectious causes, and human prion diseases), we discuss the presentation of these forms and reasons for variations in estimated prevalence rates in the general population.

SummARyIn the second of two articles on rare causes of dementia, the authors describe toxic, iatrogenic, nutritional, traumatic, metabolic, neoplastic and autoimmune causes of dementia. Disorders are graded according to their prevalence, to give an idea of the likelihood of their presentation. Guidance is given on the investigation of uncommon cognitive impairment and dementia, especially in early-onset illness. DECLARATIOn OF InTERESTNone. Susham Gupta

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