Motor neurone disease, dementia and aphasia: coincidence, co-occurrence or continuum?
Cognitive, and particularly aphasic, symptoms associated with motor neurone disease (MND) are still frequently described as rare and "recently discovered". This review demonstrates that the association between MND and dementia was described and recognised as an entity as early as 1929, and its close relationship to Pick's disease was postulated in 1932. Changes in language production and comprehension were also observed by early authors, although they were rarely described as aphasia. The striking similarity to the contemporary descriptions is, however, sometimes obscured by diverging terminology. The syndromes of MND/dementia and MND/aphasia are well established but represent a comparatively small subgroup of MND. In addition, subtle cognitive alterations have also been reported in non-demented MND patients; most studies have found evidence of frontal-executive dysfunction, similar in pattern, but much milder than in patients with frank MND/dementia. These findings are strengthened further by post-mortem studies demonstrating pathological changes in the frontal lobes, and functional neuroimaging studies, showing reduced frontal activation. The issue of whether memory, visuospatial skills and language are affected in non-demented subjects remains, however, controversial. Further studies are required to establish whether MND/dementia and MND/aphasia form separate disease entities or can be viewed as extreme forms of a cognitive deficit characteristic of MND in general.