Mild cognitive impairment (MCI). Thought to affect 10–20% of those over 65 years [11, 12], MCI has been defined as an intermediate state of cognitive function between the changes seen in aging and those fulfilling the criteria for dementia [13]. It presents as concern from a patient or informant about cognitive decline and objective impairment in one or more cognitive domains, including memory, executive function, attention, language or visuospatial skills. The key aspect is that there is no functional impairment [14]. Subtypes have been described, denoting those more likely to go on to Alzheimer’s disease (amnestic MCI) and non-amnestic MCI. Between 12% and 20% of patients with MCI are recorded as converting to dementia annually [12], though other studies report the conversion rate as considerably less [15].

Alzheimer’s disease (AD) is the commonest cause of dementia, accounting for 50–56% cases in a clinical series at autopsy [16]. It is postulated to be caused by accumulated protein abnormalities: beta amyloid plaques and neurofibrillary tangles (abnormally phosphorylated tau proteins). AD is the prototypical cortical dementia, as the cerebral cortex is vulnerable, with temporal lobe (particularly hippocampal) deficits prominent in the early stages. Early symptoms include forgetfulness, repetitive conversation, word finding difficulty and language deficits, such as paraphrasias. The neurological examination may be normal. Cognitive testing may reveal poor orientation, poor object naming and some visuospatial deficits. The characteristic deficit, however, is rapid forgetting. A list of words can be registered well—increasing over a number of trials—and spontaneous recall impaired. However, cues do not assist the patient at delayed recall, suggesting the information was never encoded in the hippocampi. In fact, the individual may confidently suggest the wrong words. When cues do assist retrieval, this suggests processing difficulties (which can be reflected in requiring multiple registration attempts to learn the list of words). Vascular dementia (below) can present in this way, with apparent forgetfulness, without rapid forgetting. Routine pathology can be entirely normal in AD. MRI may be normal, but global atrophy and disproportionate hippocampal atrophy are common—the latter very suggestive of AD. Amyloid imaging using PET (positron emission tomography) is likely to be increasingly used, though availability is limited [17]. Cerebrospinal fluid analysis of beta amyloid levels can be valuable, particularly when the diagnosis is less certain [18].

Vascular dementia is a broad and controversial concept. Because of a lack of agreement about whether the cause extends from chronic subcortical ischaemic change to large cortical stroke, there are varying epidemiological figures provided in the literature. It is described consistently, however, as the second most common contributing pathology in dementia. A cortical stroke presenting with ‘stepwise’ cognitive decline is the classical description (multi-infarct dementia); however, it is apparent that subcortical vascular pathology accounts for more cases of dementia [19]. In the latter, symptoms accrue slowly with a gradual decline in processing speed, executive skills and sometimes personality change. Language is often preserved. The presentation can resemble a depressive episode, though this may be comorbid. Short-term memory concerns may be prominent but in the ‘purer’ vascular presentations, rapid forgetting is less likely to be present. Examination may show focal neurological signs, restricted affect and psychomotor slowing—so-called vascular Parkinsonism [20]. Brain MRI usually reveals extensive subcortical (or one or more cortical) vascular lesions; however, these are not always associated with dementia. Dementia appears associated specifically with multiple lacunes, strategic infarcts, substantial white matter lesions or a combination of these [21].

Dementia with Lewy bodies (DLB). McKeith et al. [22] proposed a new clinical subtype after 10–15% of patients with dementia at autopsy were demonstrated to have diffuse Lewy body disease. This pathological entity may be best conceptualized along a continuum of synucleinopathies, including idiopathic Parkinson’s disease (PD) and multiple systems atrophy. On history, patients with DLB will present with cognitive and functional decline with temporally related development of Parkinsonism. The close onset of the two syndromes (technically within 12 months) differentiates DLB from Parkinson’s disease dementia where the cognitive changes occur much later after PD diagnosis. The classic triad is fluctuation in symptoms, Parkinsonism and recurrent visual hallucinations. Supportive features for the diagnosis include REM sleep disorders, antipsychotic sensitivity, falls and autonomic dysfunction.

Frontotemporal dementia (FTD) is a clinically and pathologically heterogeneous group of dementias characterized collectively by relatively selective, progressive atrophy involving the frontal or temporal lobes, or both [23]. A disproportionately high number of those who present with a younger onset dementia have FTD. As a result, patients may be well served attending a specialist neuropsychiatry or behavioural neurology clinic for diagnosis and management. Clinical presentation includes those where behavioural changes predominate (such as disinhibition, emotionality or apathy) or where language deficits are prominent (primary progressive aphasia). The picture can mimic other frontal pathology, subcortical vascular pathology, psychiatric disorder or atypical AD. Aetiologies include cellular inclusions (both tau protein abnormalities and TDP-43: transitive response DNA-binding protein43) and significant genetic contributions. This includes both autosomal dominant inheritance and specific gene mutations [24].