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In most European legislation HD is not mentioned specifically and HD usually falls under a category “general neurological conditions”. Legislations differ between countries in who (i.e. drivers themselves or physicians) should report changes in medical condition and who (i.e. drivers themselves, physicians or licensing agency) should initiate and monitor fitness to drive assessment in HD.
In the Netherlands people with group 1 licenses (car and motorcycle) who have HD should have their fitness to drive assessed by means of an on road driving test when there are physical or mental impairments (or a suspicion that these are there) that could interfere with the ability to drive a vehicle. People with group 2 licenses (bus, lorry and professional use of car and motorcycle) who have HD are unfit unless there are no physical or mental impairments that could interfere with the ability to drive a vehicle. In that case they can be declared fit for a maximum period of 3 years.
Huntington’s disease (HD) is an autosomal dominant neurodegenerative disorder, which is characterized by motor, cognitive and emotional-behavioural symptoms. Gene carriers are initially presymptomatic, typical disease onset is between 30 and 50 years. Depending on the number of so called CAG-repeats in the huntingtin gene on chromosome 4, disease symptoms may develop where more CAG-repeats are related to a younger onset. The first symptoms differ between patients. The initial motor symptoms are jerky, unintentional movements known as chorea. First cognitive impairments generally involve executive dysfunctions, slowing of psychomotor speed and social cognition (most specific emotion recognition in facial expressions). Emotional-behavioural symptoms usually start with mood or anxiety complaints. As the disease progresses chorea become more apparent, muscle control decreases and rigidity, bradykinesia (slowness of movements), dystonia and balance problems develop. Cognition becomes increasingly impaired and more cognitive functions, including memory, attention, and visuospatial functions, are affected, eventually leading to HD dementia. Emotional-behavioural problems worsen and may lead to apathy, anxiety, depression, aggressive and compulsive behaviour and commonly to a lack of disease insight.
HD is a progressive neurodegenerative disorder, however, a presymptomatic period (premanifest stage) precedes clinical disease onset (manifest stage). During the premanifest stage minor disease related changes may occur, but these do not interfere with activities of daily living and neither do they appear to impact fitness to drive1.
With a typical disease onset during midlife, the decision to stop driving may have massive impact on activities in daily living and participation, both socially and professionally. Furthermore, with increasing motor problems people with HD become even more dependent on transportation by car. Therefore, driving cessation may likely impact well-being and mental health. However, as the disease progresses cognitive and motor functions required for safe driving show a progressive decline and eventually all people with HD cease driving. Some drivers with HD give up driving in an early stage of the disease. Others continue driving while symptoms are still mild and they are able to compensate for their decreased driving skills by adjusting their driving behaviour, e.g. driving slower, avoiding rush hours, etc. However, a lack of insight in disease impairment is common in HD and consequently many drivers with HD overestimate their fitness to drive 2.
Complications or other symptoms of HD that mat occur and that may be relevant to safe driving are:
Potential ‘red flags’ to be aware of are changes in driving behaviour such as:
Described in other sections.
Motor symptoms associated with HD are jerky or writhing unintentional movements of the limbs, torso, head or facial muscles known as chorea. People with HD may also show a decrease in muscle control and fine motor coordination, leading to problems in gait, posture and balance but also in the operation of devices. Rigidity and dystonia (excessive muscle contracture) may develop and movements become increasingly slow (bradykinesia). Problems in mouth and tongue muscles lead to difficulties in speech and swallowing. Furthermore, fatigue is a common complaint in people with HD.
Higher order visual cognitive deficits may develop in manifest HD, however, lower order impairments in e.g. contrast sensitivity have only been described in sporadic cases. Oculomotor deficits, such as slow saccades or slow tracking eye movement, are preset in the majority of people with HD and may even be regarded as an early sign of onset.
Cognitive impairments may already be mildly present in the premanifest stage of HD3. The most common affected cognitive domains in this stage are executive functions, psychomotor speed and social cognition. Executive function impairments include task switching, diving attention, planning but also evaluation of task performance, error recognition and adjusting strategy. Psychomotor speed may be slowed. Impairments in social cognition present in the ability to recognise emotions in facial expression, specifically in recognizing negative emotions such as fear, disgust and anger. In the manifest stage of HD cognitive impairments become more evident and more cognitive domains are affected, including memory and language. Interference of cognitive impairments with daily life increases and eventually people with HD will develop HD dementia.
In the course of the disease emotional-behavioural problems worsen and a lack of disease insight is common among people with HD. Consequently, insight in driving ability is impaired as well. People with manifest HD may not always adequately adjust driving behaviour according to their impairments, resulting in dangerous driving. Furthermore, they may have problems in adequately recognizing potential danger or dangerous situations in traffic and they take more risks and unsafe decisions in challenging situations4. People with manifest HD are also more likely to be involved in accidents5.
Several screening tools (e.g. UHDRS, Beglinger 2012; DSDA, Farrell 2019) have been suggested as instruments to predict fitness to drive in HD, however, none of these were properly evaluated against driving ability. Deficits are found in a wide range of driving skills6 which is probably due to the extensiveness of symptoms in HD.
Devos and colleagues found that with a cognitive screening consisting of the Symbol Digit Modalities Test, the word reading subtest of the Stroop Color and Word Test, and the Trail Making Test B they could correctly classify 87% of the on-road fitness to drive verdict in 30 people with manifest HD2. However, these results have not yet been verified in a new and larger population.
The inability to recognize fear in emotional facial expressions has been shown to correlate with increased risk taking behaviour in several neurological conditions under which people with HD. It has therefor been suggested that measuring emotion recognition may add to the cognitive screening for predicting fitness to drive in HD4.
In general, the conclusion should be that no well-defined cognitive screening yet exists to predict fitness to drive in HD. Due to the extensiveness of motor, cognitive and behavioural symptoms in HD it is likely that a more comprehensive test battery is needed to validly predict fitness to drive.
Due to excessive motor and cognitive impairments recommendations of specific vehicle adaptations remain a challenge. No specific recommendations exist for people with HD, however, the use of an automated gearbox is advised in the early stages of HD.
People with manifest HD may have motor, cognitive and emotional-behavioural symptoms. Consequently they may develop problems in operational (e.g. signalling, steering, braking), tactical (e.g. adapting speed, keeping distance) and strategic (e.g. adequately planning a trip, not driving during rush hours) driving skills. All these aspects should be addressed in an on road assessment, although assessing strategic skills might be challenging in this setting.
Insight in disease symptoms may be impaired in people with HD and consequently they may not always adequately adjust their driving vehicle or driving behaviour according to their impairments. See also sections Emotional-behavioural symptoms and Vehicle Control implications and vehicle adaptations
Driving Licence Coding related to specific vehicle control or adaptations relevant to Huntington’s disease. There will be a separate section under the Vehicle Control implications which will refer to Driver Licence Coding, so please do not worry about detailing specific codes. This heading is here for conditions where coding is required mainly for more functional deficits. Also, in some countries the driving licence is restricted to specific conditions such as driving only in a defined geographical area or, for example within the hours of daylight. There will be text provided on the website under vehicle adaptations explaining this.
Dr. Rients Huitem, University Medical Center Groningen
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