29th April 2024

Dementia; subtypes & mimics

Background

Besides Alzheimer’s Disease, there are a number of other recognised dementia subtypes. These subtypes have their own distinct pathophysiology. As a result, each dementia subtype has identifying features that are characteristic to them in terms of presenting features, symptomology or disease progression. Identification of dementia subtypes is essential as management can vary depending on the subtype.

Dementia subtypes

Dementia with Lewy bodies and Parkinson’s disease dementia

Dementia with Lewy bodies (DLB) and Parkinson’s disease dementia (PDD) share many features, with the distinction between them being the onset of symptomatology. In DLB, there is progressive cognitive decline, similar to that of Alzheimer’s dementia, with subsequent onset of motor features in the form of parkinsonism. In PDD, the motor feature precedes the onset of the cognitive symptoms by at least one year. These dementia subtypes are characterised by Visual hallucinations, sleep disorders (especially REM sleep behaviour disorders) and depression.

Both DLB and PDD are characterised by the presence of intraneuronal inclusions containing synuclein and ubiquitin in the brain stem, limbic regions and forebrain, known as Lewy bodies. The diagnosis of DLB and PDD should be made in a specialist dementia diagnostic service, however if diagnosis is uncertain, I-FP-CIT SPECT (sometimes known as a DaTscan) should be utilised.  I-MIG cardiac scintigraphy can be used if I-FP-CIT SPECT is not available.

Vascular dementia

Vascular dementia (VD) is the second most common subtype of dementia after Alzheimer’s Disease and can co-exist with Alzheimer’s Disease. VD is caused by thromboembolic or hypertensive infarction of small and medium size vasculature in the brain. Onset and clinical features can vary. A Cognitive deficit can present suddenly following a single vascular event, or multiple events can lead to stepwise cognitive deficit with periods of stable disease between events. The latter is sometimes referred to as multiple-infarct dementia. Finally, there can be progressive small vessel disease caused by multiple micro-vessel infarcts; this can present in a slowly progressive cognitive decline.

Diagnosis can be made in a specialist dementia diagnostic service aided by MRI imaging where needed. CT imaging can be used if MRI imaging is not possible.

Management of VD revolves around managing vascular risk factors. This can include optimisation of hypertension, cholesterol, diabetes and using antiplatelet/anticoagulation where appropriate.  Acetylcholinesterase inhibitors and memantine should only be used if co-existent AD is thought to be present.

Frontotemporal lobe dementia

Frontotemporal lobe dementia (FTD) is a spectrum of neurodegenerative syndromes affecting the frontal and temporal lobes of the brain. FTD is typically of gradual onset but is progressive over time. Frontal lobe symptoms include personality and behavioural change, emotional blunting and disinhibition. Temporal lobe symptoms include a progressive decline in speech function. This can include progressive word finding difficulties and challenges with language fluency. Memory normally remains intact.

The onset of FTD is often younger, involving patients between the ages of 45 and 65. However, it is the third most common dementia in individuals over 65 years of age. FTD is caused by abnormal deposition of intracellular proteins in the frontal and temporal lobe that trigger the degeneration of neurons. Different protein aggregates have been identified for different FTD syndromes.

CT or MRI imaging normally shows bilateral asymmetrical abnormalities within the frontal and temporal lobes. If diagnosis is uncertain, Fluorodeoxyglucose (FDG) PET or perfusion SPECT can be used.

There are no known effective pharmacological interventions for FTD.

Dementia Mimics

A number of conditions can mimic dementia. The most commonly encountered in the inpatient hospital environment is delirium, however depression and mild cognitive impairment can also be confused for dementia. It is important to have an understanding of the distinctions and how they would apply to clinical practice. Mild cognitive impairment is cognitive decline beyond what would be expected for someone’s age and can be seen as a pre-dementia state. Sporadic Creutzfeldt-Jakob Disease, pellagra, Korsakoff’s syndrome and normal pressure hydrocephalus are other conditions which may be confused with, or overlap dementia.

References

• National Institute for Health and Care Excellence (UK). Dementia: assessment, management and support for people living with dementia and their carers. London; 2018 June. (NICE Guideline, No 97. Available from https://www.nice.org.uk/guidance/ng97
• World Health Organization (2019). International Statistical Classification of Diseases and Related Health Problems (11th ed.). https://icd.who.int/