INTRODUCTION of Dementia in Motor Neuron Disease

Background

Most patients with motor neuron disease (MND) are free of cognitive impairment, but there is growing evidence of an association between MND and frontal lobe or frontotemporal dementia (FTD). Some propose that frontotemporal lobe dementia with motor neuron disease (FTD/MND) is nosologically distinct; others suggest that it is part of a spectrum of diseases encompassing classic MND at one end and FTD at the other.

Pathophysiology

Pyramidal cell loss in frontal and temporal lobes and degeneration of motor neurons in the hypoglossal nucleus and spinal motor neurons characterize FTD/MND. Pyramidal neurons in the premotor cortex usually are preserved. Signs and symptoms reflect frontal and temporal lobe dysfunction with lower motor neuron-type weakness, muscle atrophy, and fasciculations.

Frequency

United States

Frontal lobe dementia is the second or third most common type of degenerative dementia in autopsy series. The precise frequency of the subgroup of FTD patients with FTD/MND in autopsy or population studies is unknown (but rare).

International

In a Scandinavian autopsy series, dementia was reported in 2-6% of patients with MND. The relative frequency of FTD/MND in all patients with dementia appears similar in the United States and Japan. Certain populations (eg, Chamorro Indians of Guam, indigenous residents of the Kii peninsula) have a disproportionately higher incidence and prevalence of overlap degenerative syndromes (MND, dementia, parkinsonism).

Mortality/Morbidity

Progressive dementia with symptoms of executive dysfunction, personality change, and motor weakness leads to severe morbidity.
Death usually occurs within 3 years of onset from inanition, pulmonary failure, and aspiration.
Patients with FTD/MND generally follow a more rapid course than patients with either FTD or MND alone. They are more likely to have a bulbar form of MND, which may help explain its more aggressive course.

Race

FTD/MND has been described in patients of Asian, European, and African descent. No data are available about incidence and prevalence among racial groups.

Sex

Men appear to be affected slightly more frequently than women, but this difference may not be significant.

Age

The mean age of onset in sporadic cases varies among series but overall is 55-65 years. Familial cases tend to be younger.