Diagnosis and differential diagnosis

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Patients with DLB may present to psychiatric services (cognitive impairment, psychosis, or behavioural disturbance), internal medicine (acute confusional states or syncope), or neurology (movement disorder or disturbed consciousness). The details of clinical assessment and differential diagnoses will, to a large extent, be shaped by these symptom and specialty biases.(!3» In all cases, a detailed history from the patient and reliable informants should document the time of onset of relevant key symptoms, the nature of their progression, and their effects on social, occupational, and personal function.

The recent consensus criteria for the clinical diagnosis of DLB are shown in Table ?. Particular emphasis needs to be given to recognizing the characteristic dementia syndrome. Attentional deficits and prominent frontosubcortical and visuospatial dysfunction are the main features—symptoms of persistent or prominent memory impairment are not always present early in the course of illness, although they are likely to develop in most patients with disease progression. Patients with DLB perform better than AD on tests of verbal recall, but relatively worse on tests of copying and drawing. 'l4» With the progression of dementia, the selective pattern of cognitive deficits may be lost, making differential diagnosis based on clinical examination difficult during the later stages.

Table 2 Consensus criteria for the clinical diagnosis of probable and possible dementia with Lewy bodies (DLB)

Probable DLB can be diagnosed if any two of the three key symptoms (fluctuation, visual hallucinations, spontaneous motor features of parkinsonism) are present. Fluctuation is undoubtedly the most difficult symptom to establish. Some patients identify the variable cognitive state themselves, but generally the most productive approach is to interview a reliable informant. Questions such as 'Are there episodes when his/her thinking seems quite clear and then becomes muddled?' may be useful probes. Substantial changes in mental state and behaviour may be seen within the duration of a single interview or between consecutive examinations. Parkinsonism and visual hallucinations pose fewer problems of identification.

Categories of disorder

There are four main categories of disorder that should be considered in the differential diagnosis of DLB ( Tab.l.§... , 3).

Table 3 Conditions to be considered in the differential diagnosis of dementia with Lewy bodies

Other causes of dementia

Of autopsy-confirmed DLB cases, 65 per cent meet the NINCDS-ADRDA clinical criteria for probable or possible AD, (15> and this is the most frequent clinical misdiagnosis of DLB patients presenting with a primary dementia syndrome. This suggests DLB should routinely be excluded when making the diagnosis of AD. Up to one-third of DLB cases are additionally misclassified as vascular dementia by the Hachinski Ischaemic Index, by virtue of items such as the fluctuating nature and course of illness/1.5 Pyramidal and focal neurological signs are, however, usually absent. The development of myoclonus in patients with a rapidly progressive form of DLB may lead the clinician to suspect Creutzfeldt-Jakob disease.(8)

Other causes of delirium

In patients with intermittent delirium, appropriate examination and laboratory tests should be performed during the acute phase to maximize the chances of detecting infective, metabolic, inflammatory, or other aetiological factors. Pharmacological causes are particularly common in elderly patients. Although the presence of any of these features makes a diagnosis of DLB less likely, comorbidity is not unusual in elderly patients and the diagnosis should not be excluded simply on this basis.

Other neurological syndromes

In patients with a prior diagnosis of Parkinson's disease, the onset of visual hallucinations and fluctuating cognitive impairment may be attributed to side-effects of antiparkinsonian medications, and this must be tested by dose reduction or withdrawal. Other atypical parkinsonian syndromes associated with poor levodopa response, cognitive impairment, and postural instability include progressive supranuclear palsy and multisystem atrophy. Syncopal episodes in DLB are often incorrectly attributed to transient ischaemic attacks, despite an absence of focal neurological signs. Recurrent disturbances in consciousness accompanied by complex visual hallucinations may suggest complex partial seizures (temporal lobe epilepsy), and vivid dreaming with violent movements during sleep may meet criteria for REM-sleep behaviour disorder. Both these conditions have been reported as uncommon presenting symptoms of autopsy-confirmed DLB. (16!Z)

Late-onset functional psychiatric disorder

DLB should be considered if a patient spontaneously develops parkinsonian features or cognitive decline (or shows excessive sensitivity to neuroleptic medication) in the course of late-onset delusional disorder, depressive psychosis, or mania. (!6,)

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