Nature of language impairment in motor neurone disease
Rewaj, Phillipa Jane
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Background: Language impairment associated with Motor Neurone Disease (MND) has been documented since the late 19th century, yet little is understood about the pervasiveness or nature of these deficits. The common clinical view among healthcare professionals is that communication difficulties can be attributed solely to the motor speech disorder dysarthria. Recent literature raises the possibility of more central processing deficits. Impairments in naming ability and comprehension of complex grammatical constructs have been frequently reported in some patients with MND. However, there is now growing evidence of spelling impairment, which could suggest the contribution of a more phonologically based deficit. In addition, the close relationship between MND and frontotemporal dementia (FTD) raises questions about the connection between the language impairments seen in MND patients and those documented in patients with the primary progressive aphasia (PPA) syndromes associated with FTD. Aims: This thesis examines the nature of speech and language deficits in people with MND and the extent to which expressive communication impairment can occur above and beyond dysarthria. In particular, the study explores: i) to what extent these language impairments can be attributed to deficits in working memory, executive functioning and/or disease severity; ii) what spelling errors can reveal about the integrity of lexical, phonological and orthographic processing; iii) whether similar patterns of impairment can be seen in PPA syndromes; iv) the relationship between language impairment and bulbar onset; and v) the impact these findings have on clinical management of MND patients. Methods: MND patients from across Scotland with changes in speech and/or language were tested using a neuropsychological battery of experimental and standardised tests of naming, spelling, syntactic comprehension, prosody and phonological and orthographical awareness. Patients were also screened for levels of dysarthria, executive functioning and working memory deficits, and results compared to those of matched controls. Findings: As a group, MND participants performed significantly worse than matched controls on measures of naming, spelling, orthographical awareness, grammatical comprehension, affective prosody and verbal fluency, but not working memory. However, based on patterns of individual impairment, of which spelling impairment formed a distinctive marker, the patient group divided into dichotomous subgroups, with 44% of participants categorised as ‘linguistically impaired’, while the remainder displayed little to no impairment. Those participants identified as linguistically impaired did not differ significantly from other MND participants on measures of disease severity, disease duration or dysarthria severity, although significantly more bulbar onset than limb onset participants were linguistically impaired. Spelling error patterns were suggestive of deficits at both a lexical and sublexical level, and were comparable to those reported in PPA literature. These findings suggest that dysarthria may be masking linguistic deficits in almost half of dysarthric MND patients, and highlight the importance of multidimensional assessment of language for effective clinical management.