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It is well established that selective cognitive impairments may be present in amyotrophic lateral sclerosis (ALS) patients who do not display a full blown frontotemporal dementia (FTD) syndrome.1 ,2 Over the last 20 years there has been a surge of publications demonstrating predominantly executive dysfunction and most recently social cognition deficits in these patients.3–5 This may be taken as evidence of potentially subclinical levels of FTD in this group, enhancing the link between the two disorders with the demonstration of a spectrum of cognitive dysfunction from ALS patients who only display motor involvement (ALS-motor) to those with dementia (ALS-FTD).
The Taylor et al 6 study makes a significant contribution to this field by highlighting the presence of language deficits in ALS patients without dementia, which although occasionally demonstrated for example,1 ,7 ,8 and certainly long suspected, is an area which has been relatively neglected. Previous investigations have often been confined to the use of simple confrontation naming tasks to determine the presence of language dysfunction and have lacked systematic and detailed study. In the Taylor et al 6 report a comprehensive language assessment was undertaken and although the profile of impairment was not analysed in detail the study provides the simple yet striking demonstration that these language deficits may indeed be at least as common if not more common than the typically reported executive dysfunction. Moreover the data demonstrates that such language deficits may be found in patients who do not have executive dysfunction; regression analysis demonstrated that executive dysfunction accounted for only 44% of the variance in language scores indicating some independence of these cognitive deficits and more specifically that executive dysfunction in ALS is not the whole story.
If language dysfunction is so common in ALS patients the question arises as to why this area has been neglected. The presence of bulbar dysfunction in ALS patients is likely to be a contributing factor with the inherent difficulties of distinguishing between dysarthria from dysphasia. Moreover the reported link between ALS and FTD has highlighted the more frontal nature of the impairment with corresponding executive and behaviour change although clearly language variants are present in the FTD syndromes. It is also possible the proposed ALS to FTD spectrum might be obscuring some of the potential avenues for research and future investigation of a more ALS specific cognitive profile may be fruitful.
The finding of such common language dysfunction in ALS may also address reasons as to why verbal fluency tests (in particular letter fluency) appear particularly sensitive to impairment in ALS across studies3 ,4 ,7 ,9 ,10 and highlights that this is an essential measure to include in any cognitive screen. We have demonstrated that letter fluency deficits are most likely caused by executive dysfunction, are prevalent in ALS patients without word naming deficits4) and are reflective of dysfunction of the more dorsolateral regions of the prefrontal cortex.7 However, it is clear that this test not only measures executive dysfunction with processes of initiation, strategy formation and attention switching, but also word retrieval and lexical search.
The demonstration of these language deficits also suggests that the current consensus criteria for diagnosing cognitive impairment in ALS (based on executive dysfunction alone) will need to be updated to include language assessment.11 In fact a patient may currently be diagnosed as ALS without cognitive impairment, but have prominent language changes which may affect daily life, use of aids such as light-writers and other potential intervention. The finding of common language deficits in ALS also highlights that cognitive screening measures for ALS which only employ tests of executive function will inevitably not be sensitive to proportion of patients with isolated language dysfunction.
The Taylor et al 6 report is likely to be one of the first of many investigations in this field demonstrating heterogeneity of the cognitive profile in ALS. Future investigations will inevitably define more clearly the exact nature of the language impairments in ALS and determine whether we are indeed seeing subclinical expressions of language variants of FTD or a pattern of impairment which is more specific to ALS.
Competing interests SA receives research support from the Motor Neurone Disease Association, UK, MND Scotland, Silvia Aitken Charitable Trust and the University of Edinburgh. She has also received support from the Wellcome Trust, UK. She has received salary support from the University of Edinburgh and NHS Lothian and travel expenses and honoraria for speaking and education activities not funded by industry. The views expressed here are those of the author.
Provenance and peer review Commissioned; internally peer reviewed.
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