11th August 2024

Mimics of dementia

Depression (pseudodementia)

Cognitive deficits are common in psychiatric disorders, as is the coexistence of depression with dementia. It can be difficult to explicitly distinguish the degree of contribution of each factor, and occasionally, a trial of an antidepressant and/or referral for further psychiatric testing is considered. Typically, individuals present with distractibility, poor concentration and short term memory in the context of low mood (although not always experienced) and may also include biological symptoms of depression such as poor sleep and loss of libido and appetite. Cognitive screening tests often show reduced engagement with testing and inconsistent results with impaired attention and variably poor performance in memory and behavioural domains.

Delirium

Delirium is an acute state of rapidly fluctuating consciousness impacting orientation and attention, that is usually reversible (to a degree), precipitated by a homeostatic or metabolic disturbance. The diagnosis of dementia must exclude cognitive impairment that is exclusively in the context of a delirium. Management is driven by identification and treatment of the precipitant with supportive care to allow for delirium to resolve.

Hypothyroidism

Severe hypothyroidism can cause symptoms of apathy, slowing of cognition and speech, overlapping with the symptoms of dementia and depression.

Mild cognitive impairment

Mild cognitive impairment (MCI) is an acquired cognitive decline beyond that expected for age or level of education, however, which does not cause significant functional impairment. It is a differential to consider if dementia is suspected but the criteria are not met, or if it is of low severity. Mild cognitive impairment can be divided into amnestic (affecting memory) or non-amnestic (affecting cognitive domains such as language, executive function or visuospatial skills, but not memory). There is an increased incidence of progression of amnestic MCI to Alzheimer’s disease compared to the general population.

Encephalitis and infectious causes of dementia

HIV

Progressive cognitive impairment in HIV-infected individuals requires evaluation to exclude systemic illness or opportunistic infections of the central nervous system (and their long-term sequelae). Key factors to consider are an individual’s CD4 count and duration of/concordance with antiretrovirals. HIV-associated neurocognitive disorder (HAND) represents a broad spectrum of cognitive impairment from asymptomatic/mild cognitive impairment to HIV-associated dementia that meets criteria of affecting at least two cognitive domains with significant effect on activities of daily living.

Opportunistic cerebral infections in HIV are prevalent when the CD4 count is <100 cells/microlitre. Toxoplasma, herpes simplex and varicella zoster encephalitis, cryptococcal or tuberculous meningitis, progressive multifocal leukoencephalopathy secondary to JC (John Cunningham) virus infection and EBV infection with primary CNS lymphoma.

There can also be cognitive and neuropsychiatric side effects to certain antiretrovirals (commonly efavirenz in early treatment), however immune reconstitution inflammatory syndrome (IRIS) as a cause of progressive cognitive impairment in the context of recent initiation on antiretrovirals should be considered.

HIV-associated dementia (HAD) presents with logopaenic speech, slow and poor thought processes, with flat or labile affect in the absence of sadness. There can often be a degree of overlap with psychiatric morbidity. There are no specific neuroimaging changes seen in HAD and neurological examination can show gait disturbance, frontal release and pyramidal signs.

Neurosyphilis

Infection with syphilis that has spread to the meninges and cranial nerves causes the neurological signs of absent reflexes, Argyll-Robertson pupils, ataxia, peripheral motor/sensory deficits and associated cognitive impairment.

Nutritional causes of dementia

Wernicke’s encephalopathy

Typically presents as the triad of a complex ophthalmoplegia, delirium and ataxia due to severe B1 deficiency, most commonly seen in alcoholics. Needs urgent treatment with Vitamin B complex replacement to prevent permanent cognitive impairment.

B12 deficiency

Other features, in addition to cognitive impairment, include macrocytic anaemia, neurological signs (subacute combined degeneration of the cord) such as ataxia, and reduced proprioceptive and vibration sense in the lower limbs.

B3 deficiency – pellagra

Pellagra is a systemic disease best remembered using the 3 Ds: dermatitis, diarrhoea and dementia. It is caused by deficiency in niacin, most commonly due to diet in developing countries. It usually presents initially with characteristic skin changes and non-specific abdominal symptoms. Neuropsychiatric involvement with cognitive impairment is a late sign in untreated disease.