Gupta and colleagues’ description of rarer forms of dementia, which draws attention to estimates of prevalence rates, is helpful for clinicians (Reference Gupta, Fiertag and ThanulingamGupta 2012). As there are more than 75 diseases that cause signs and symptoms of dementia, the differential diagnosis can be extensive. A useful way to classify dementias is into one of three categories: reversible, non-progressive or progressive (Reference Rabins, Slavney, Lyketsos, Rabins and LipseyRabins 2008).

A number of potentially reversible causes of dementia have been identified. The most common are medication-induced cognitive decline, hypo-thyroidism, major depression, normal-pressure hydrocephalus and alcoholism. These reversible causes account for around 1–2% of cases presenting for evaluation. However, many potentially reversible causes do not improve after treatment. Non-progressive dementias include those that follow head trauma and stroke, while progressive dementias include those with common causes (for example, Alzheimer’s disease) and those with rarer causes (for example, Creutzfeldt-Jakob disease).

Another classifier, pseudodementia, used to be used to refer to the dementia syndrome sometimes seen in major depression (Reference Rabins, Slavney, Lyketsos, Rabins and LipseyRabins 2008). It was thought to be ‘pseudo’ because it was reversible, and at the time the definition of dementia included irreversibility. This criterion has since been removed from the major classification systems. The dementia of depression is a ‘real’ dementia in that it meets the current defining characteristics of the syndrome.

In an article on late-onset depressive disorder (Reference YadavYadav 2010), I draw clinicians’ attention to this fact and advise them to be wary. Although the cognitive impairment in major depressive disorder may initially be ‘pseudodementia’, i.e. reversible, in some cases it progresses to a true, i.e. irreversible, dementia. Aggressive treatment of the depression and timely referral, investigation and treatment of cognitive impairment in late-onset depressive disorder are of paramount importance (Reference Alexopoulos, Meyers and YoungAlexopoulos 1993).