Behavioural abnormalities contribute to functional decline in Huntington’s disease
- J M Hamilton1,
- D P Salmon1,
- J Corey-Bloom1,
- A Gamst2,
- J S Paulsen3,
- S Jerkins1,
- M W Jacobson1,
- G Peavy1
- 1Department of Neurosciences, University of California, San Diego, California, USA
- 2Department of Family and Preventative Medicine, University of California, San Diego, California
- 3Departments of Psychiatry and Neurology, The University of Iowa, Iowa City, Iowa, USA
- Correspondence to: Dr Joanne M Hamilton, Alzheimer’s Disease Research Center, 9500 Gilman Drive, MC 0948, La Jolla, California, USA;
- Received 19 October 2001
- Accepted 5 September 2002
- Revised 21 August 2002
The independent and relative contributions of motor, cognitive, and behavioural deficits to functional decline in patients with Huntington’s disease are examined. Twenty two patients with Huntington’s disease were assessed with rating scales for motor dysfunction, cognitive measures of executive functions, and behavioural measures of apathy, executive dysfunction, and disinhibition. Their functional status was assessed with informant based and clinician based ratings of activities of daily living (ADL). A composite apathy/executive dysfunction behavioural index was strongly related to decline in ADL independently and after controlling for motor and cognitive deficits. These results suggest that behavioural dysfunction contributes to functional decline in patients with Huntington’s disease and may impede their ability to utilise motor or cognitive skills that remain available in the early stages of the disease.
- ADL, activities of daily living
- BDI, Beck depression inventory
- CANTAB, Cambridge neuropsychological test automated battery
- FLOPS, frontal lobe personality scale
- HD-ADL, Huntington’s disease activities of daily living scale
- TFC, total functional capacity scale
- UHDRS, unified Huntington’s disease rating scale
Huntington’s disease is a genetically transmitted neurodegenerative disease that results in a severe movement disorder (chorea, dystonia, bradykinesia, and oculomotor deficits) because of atrophy of the basal ganglia1 and related brain structures.2 The movement disorder is accompanied by notable cognitive impairment (for example, executive dysfunction) and behavioural changes such as depression, irritability, apathy, and inflexibility.3 In some combination, the triad of motor, cognitive, and behavioural deficits associated with Huntington’s disease contributes to profound functional decline with a gradual loss of independence in performing the usual activities of daily living (ADL).
Previous studies have shown that the severity of functional impairment in patients with Huntington’s disease is at least moderately related to the severity of their motor and cognitive dysfunction.4–6 However, these factors account for only a portion of the variance in ADL decline, and additional variance might be explained by the behavioural abnormalities that occur in the disease. The profound apathy, lack of initiative, and irritability shown by some patients with Huntington’s disease may interfere with their ability to perform certain ADL, even though they retain the necessary motor and cognitive capacity.
To address this issue, we examined the relation between functional disability and motor, cognitive, and behavioural deficits in individuals with Huntington’s disease. We hypothesised that disease related changes in behaviour would significantly influence ratings of functional capacity even after the effects of motor and cognitive deficits were taken into account.
The participants were 22 patients with Huntington’s disease (13 women, 9 men) who had an informant available to complete behavioural and functional ratings. All patients were recruited from the Huntington’s Disease Centers of Excellence at the University of California, San Diego (UCSD) (n = 16) or the University of Iowa (n = 6). The institutional review boards at each institution approved all the procedures. Written consent was obtained from the participants after the study was fully explained to them.
The subjects had a positive family history of Huntington’s disease or a positive genetic test for the mutation, and presented with at least one major neurological sign of the disease (chorea or dystonia) on the unified Huntington’s disease rating scale (UHDRS).7 Mean (SD) age of the participants was 46.3 (8.4) years, and they had 13.9 (2.0) years of education, an estimated premorbid IQ of 112.5 (9.9), and a Mattis dementia rating scale (DRS) score of 120.0 (13.2), which is indicative of mild to moderate dementia. English was the first language of all the patients. Most were classified as having no (n = 12) or minimal (n = 4) depressive symptoms, according to the Beck depression inventory (BDI); however, a subset had mild to moderate (n = 2), moderate to severe (n = 1), or severe (n = 3) depressive symptoms. Many of the patients were taking antidepressants (n = 12), neuroleptics, (n = 7), anticonvulsants (n = 4), or anxiolytics (n = 3), either alone or in combination. No patient had a reported history of stroke, brain tumour, brain surgery, head injury with loss of consciousness for more than five minutes, or substance abuse/dependence within the past year.
The severity of motor dysfunction was assessed using the motor examination from the UHDRS.7 Total scores range between 0 and 124, with higher scores signifying greater motor dysfunction.
The pattern recognition, spatial recognition, spatial span, and spatial working memory subtests of the Cambridge neuropsychological test automated battery (CANTAB) were administered according to the standard test protocol.8 A CANTAB cognitive composite score was derived by computing z scores for each of the four subtests and averaging across the four scores. The controlled oral word association test and the symbol digit modalities test from the UHDRS were also administered and a composite UHDRS cognitive z score derived.
The frontal lobe personality scale (FLOPS) was completed by an informant who had at least daily contact with the patient. The FLOPS is a 46 item questionnaire for identifying the types of behavioural change commonly associated with frontal lobe damage.9 Three subscales are derived: apathy, executive dysfunction, and disinhibition. To correct for different numbers of items in the three subscales, a behavioural “change ratio” was computed for each by dividing the difference between ratings of predisease and current behaviour by the number of subscale items. Change ratios increased with increasing occurrence of behavioural problems.
The same informant who completed the FLOPS also completed the Huntington’s disease activities of daily living scale (HD-ADL).10 The HD-ADL scale was divided into separate instrumental (for example, communication, finances) and physical (eating, dressing) ADL subscales. The Shoulson–Fahn total functional capacity scale (TFC) from the UHDRS was also completed by a clinician.
Values are given as mean (SD) throughout. The average UHDRS motor score (mean = 29.9 (20.8)), the CANTAB composite score (mean z score = −1.5 (1.3)), the UHDRS composite score (mean z score = −2.0 (1.1)), the TFC (mean = 8.7 (3.9)), and instrumental (mean = 60.6 (28.5)%) and physical (mean = 77.3 (22.6)%) HD-ADL scores were indicative of mild to moderate impairment. Change scores for the FLOPS subscales differed significantly from zero (p values < 0.001; indicating worsening behaviour), with the apathy score (mean = 1.2 (0.7)) greater than the executive dysfunction score (mean = 0.9 (0.7); p < 0.05) and the executive dysfunction score greater than the disinhibition score (mean = 0.5 (0.5); p < 0.01). For remaining analyses apathy and executive dysfunction subscales were combined because they were highly correlated (r = 0.83, p < 0.001).
The disinhibition subscale was not significantly correlated with any of the three functional measures (instrumental HD-ADL, r = −0.17; physical HD-ADL, r = 0.07; TFC, r = 0.09). In contrast, the apathy/executive score was highly related to the scores obtained on the instrumental HD-ADL (r = −0.92), the physical HD-ADL (r = −0.83), and the TFC (r = −0.77) functional measures (all p values < 0.001; fig 1).
Multiple regression analyses showed that a combination of the motor score, the CANTAB cognitive score, and the apathy/executive score accounted for 91% of the variance in the instrumental HD-ADL score (adjusted R2 = 0.90; p < 0.001), 83% of the variance in the physical HD-ADL score (adjusted R2 = 0.80; p < 0.001), and 82% of the variance in TFC score (adjusted R2 = 0.79; p < 0.001).
The unique variance in function explained by each variable was then determined after controlling for the variance accounted for by both of the other two variables (table 1). The apathy/executive score accounted for significantly more unique variance in the instrumental HD-ADL scores than did the motor score (36% v 3% (95% confidence interval 6% to 63%)) or the CANTAB cognitive score (36% v < 1% (9% to 67%)). There was no significant difference in the percentage of variance in physical HD-ADL or TFC scores explained by the behavioural, motor, and cognitive scores.
Similar multiple regression analyses using the UHDRS cognitive score instead of the CANTAB cognitive score showed that the motor score, the cognitive score, and the apathy/executive score accounted for 92% of the variance in the instrumental HD-ADL score (adjusted R2 = 0.90; p < 0.001), 84% of the variance in the physical HD-ADL score (adjusted R2 = 0.81; p < 0.001), and 84% of the variance in TFC score (adjusted R2 = 0.81; p < 0.001). Additional analyses showed that the apathy/executive score accounted for significantly more unique variance (table 1) in the instrumental HD-ADL scores than did the UHDRS cognitive score (18% v 1% (3% to 37%)). No other comparisons were significantly different.
Our results show that a composite behavioural index of apathy/executive dysfunction in patients with Huntington’s disease is strongly related to informant based or clinician based ratings of decline in their everyday activities. This behavioural measure accounted for a significant amount of unique variance in the ADL measures even after controlling for the effects of motor and cognitive deficits, and this remained the case even if instrumental or physical ADL were considered separately. Thus behavioural dysfunction may be quite disabling in patients with Huntington’s disease and could impede their ability to utilise motor or cognitive skills that may still be available in the early stages of the disease.
Apathy was the most significant behavioural abnormality to develop in patients with Huntington’s disease, consistent with a previous finding of apathy in roughly 50% of affected patients who were assessed with the neuropsychiatric inventory.11 This does not appear to be a reflection of depression as there was no significant relation between scores on the BDI and the FLOPS apathy subscale (or any other subscale) in the present study (r = 0.18, p > 0.4) or in a previous study with a larger sample of Huntington’s disease patients.12 Rather, the frontostriatal neuropathology that occurs in Huntington’s disease13 appears to lead to a primary reduction in drive and motivation that is manifested as behavioural apathy.
Behavioural dysfunction makes an important contribution to the decline in everyday functioning in patients with Huntington’s disease, and a full accounting of the antecedents of their functional decline must consider the interaction between motor, cognitive, and behavioural impairment. Although this conclusion must be generalised cautiously because of the relatively small sample size and the retrospective nature of the behavioural ratings, our results underscore the value of assessing behavioural change in Huntington’s disease.
Supported by NIMH grant R03 MH59430 and NIH grant P50 AG05131 to the University of California at San Diego (UCSD); NIMH grants K02 MH01579, R29 MH55331, and NINDS grant R01 NS40068 to the University of Iowa; and Center of Excellence Awards from the Huntington’s Disease Society of America to UCSD and the University of Iowa.