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Semantic Dementia

Information for clinicians

Semantic dementia (SD) is a clinical manifestation of frontotemporal lobar degeneration, a non-Alzheimer circumscribed degeneration of the frontal and temporal lobes. In Semantic dementia, cerebral atrophy is restricted to the anterior temporal lobes and gives rise to a distinctive clinical syndrome characterised by a selective loss of meaning.

Demographics and epidemiology
SD is a rare disorder and accounts for approximately 15% of cases of frontotemporal lobar degeneration. Age at onset is typically 50-65 years. The disease occurs with equal frequency in males and females. The mean duration of illness is 8 years, ranging from 3 to more than 15 years.

Cognitive changes
SD is characterised by a loss of word meaning and impaired recognition of faces and objects. Patients typically have profound difficulty naming and make semantic errors (e.g. “dog” for cow). They also show poor understanding of words. The disorder is not restricted to the verbal domain and patients may also show poor recognition of previously familiar faces and objects, as well as tastes, smells and environmental sounds (e.g. the ringing of a doorbell). By contrast, cognitive functions that do not depend on meaning are well preserved. Speech output is fluent and grammatically correct. Patients retain intact visuo-spatial skills and can find their way around their locality without becoming lost. Day-to-day memory is well preserved and patients have no difficulty remembering appointments and recalling recent personal events. Activities of daily living are well preserved and patients remain functionally independent at least in the early stages of the disease.

Behavioural changes
Although loss of meaning is the most prominent aspect of the disorder, behavioural changes are also common. Patients often become preoccupied with a narrow range of activities (e.g. sewing, walking), which they pursue obsessively. They develop rigid behavioural routines, carrying out the same activities at fixed times. Patients typically become more egocentric although they do not show the profound change in social conduct characteristic of FTD.

Physical changes
There is generally an absence of neurological signs in the early stages of the condition. However, primitive ‘grasp’ reflexes emerge in later stages. Parkinsonian signs of rigidity and akinesia appear only in the advanced stages of the disease. A small proportion of SD patients develop motor neuron disease.

Although a positive family history is present in a proportion of patients with frontotemporal lobar degeneration this is extremely rare in cases of SD.

Treatment and management
Clinical management of SD poses some unique challenges. Unlike most patients with dementia, SD patients are relatively youthful and physically fit. Nevertheless, services are most appropriately provided by psychiatric services for elderly people.

Currently, there exists no curative treatment for SD. Pharmacological treatments may alleviate some of the behavioural disturbances associated with the condition. For example, administration of selective serotonin reuptake inhibitors may reduce rigid, stereotypical behaviours in some patients.

Because of their profound problems in comprehension, SD patients become increasingly dependent on their families and the burden on carers and families is immense. Management is, therefore, generally focused on supporting care-givers through psychiatric and voluntary services and provision of day and respite care.

Further reading

Neary D, Snowden J, Mann D. Frontotemporal dementia. Lancet Neurology, 2005; 4: 771-780

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