Miscellaneous causes of dementia
Since cognition is so easily disrupted by diseases affecting the cortex and its subcortical connections, cognitive impairment or dementia is very common in neurological practice. In the majority of cases the other clinical features provide the diagnostic clues. These dementia plus syndromes are numerous and include Huntington's disease, some spinocerebellar ataxias, and a variety of inherited metabolic disorders, such as metachromatic leucodystrophy, Kuf's disease, lysosomal storage disorders, and mitochondrial cytopathies.
Normal-pressure hydrocephalus can present as the classic triad of dementia, incontinence, and gait disturbance, but has tended to be overdiagnosed. The cognitive impairment is very much that of a subcortical impairment with cognitive slowing. Patients with prominent cognitive impairment are more likely to have a coexisting degenerative disease, and rarely, if ever, respond to shunting.
Multiple sclerosis patients will often develop cognitive impairment, and in some this can be a prominent, and even presenting, feature.
Syphilis is the classic infection which can be associated with dementia. However, late cases of Borrelia burgdorfei (Lyme disease) can also be associated with dementia (Krupp et al. 1991) and Whipple's disease can rarely involve the cerebral cortex, with cognitive impairment that responds to antibiotic treatment. The other major infection associated with dementia is HIV encephalopathy, and dementia can be a presenting feature (Janssen et al. 1992).
There is evidence that this is becoming less common with treatment with zidovudine (Melton et al. 1997). Progressive multifocal leucoencephalopathy is frequently associated with cognitive impairment and occurs in a variety of immunosuppressed patients as well as those with HIV.
One of the main reasons for neuroimaging of patient with dementia is to identify tumours, the treatment of which can result in cognitive improvement. However, with malignant tumours, irradiation itself may give rise to late cognitive impairment (Keime-Guibert et al. 1998).
The paraneoplastic phenomenon of limbic encephalitis, usually associated with carcinoma of the bronchus, can result in a memory impairment that precedes diagnosis of the tumour by a number of years and can mimic Alzheimer's disease. However, examination of the CSF usually reveals oligoclonal bands and occasionally pleocytosis (Bakheit et al. 1990).
Drugs, particularly barbiturates, can lead to cognitive slowing, as can heavy metals, such as lead, arsenic, manganese, and mercury. Workers in the felt hat industry who were exposed to mercury frequently developed a confusional state with cognitive impairment, hence the term ‘mad as a hatter’. Dementia pugilistica, arising from recurrent head injury, particularly in boxers, is associated with tangles and presents as a cognitive impairment with dysarthria and an extrapyramidal syndrome.
A variety of rare degenerative dementias are gradually being delineated, although some, such as argyrophilic grain dementia (Braak et al. 1989) are really only diagnosed at autopsy. Kosaka (1994) described a series of patients with basal ganglia calcification on neuroimaging, with neurofibrillary tangles but no senile plaques.
The rare Worster–Drought syndrome, a familial disorder with dementia and spastic paraparesis, is now recognized as a novel amyloidosis due to a stop codon mutation in the BRI gene (Vidal et al. 1999).