Introduction
Huntington's disease is a multi-faceted disorder, which involves changes in behaviour and intellect as well as involuntary movements. Although the movement disorder is the most visible aspect of the condition and hence is often regarded as the hallmark of the disease, it is frequently the behavioural changes which are the most incapacitating for HD sufferers and distressing for their families. Behaviour is in part dependent upon how one feels, and there are certain mood changes which occur in HD such as irritability, depression and blunting of emotions. However, how one behaves is also a product of the way in which one thinks. To understand HD behaviour fully it is necessary to understand the way in which the HD patients thinking is altered by the disease process.Overview of Cognitive Changes
Medical textbooks typically refer to the cognitive changes of HD as a dementia. This is appropriate in so far as the word 'dementia' is simply a descriptive term meaning 'loss of mental ability'. Unfortunately, however, the term dementia has tended historically to carry the connotation of a "global' impairment, as if the patient is no longer capable of any useful mental or intellectual activity. That notion is far from accurate. HD does not damage the brain in a global or uniform way. Rather, it selectively damages and impairs the function of certain parts of the brain while sparing others. The brain is highly specialised, each part having a very specific role. That means that the mental changes in HD are specific and characteristic, and are determined by the particular parts of the brain which are damaged. In HD the part of the brain particularly affected is the striatum, deep structures within the brain which have strong connections with the front part of the brain.What abilities are preserved in HD?
HD sufferers can make sense of what they perceive through their senses. They can, for example, see and recognise the things that they see around them and discriminate between similar and dissimilar objects. They can understand the meaning of words and retain much of the general knowledge about the world that is acquired in childhood, such as what objects are for. These abilities are often regarded as the 'tools' or instruments of thought. They provide the foundation for efficient thinking.What abilities am impaired in HD?
It is not sufficient merely to have available the 'tools' of thought i.e. words and visual percepts. To illustrate by analogy: on an ocean liner there are a variety of navigational instruments and communication systems. These are the instruments or tools which are necessary for the ship to function. However, they are not enough. The ship also needs a captain and crew, whose job it is to make plans, to organise, to attend to the instrument panels and communication systems, and abstract out relevant information and ignore what is not relevant. They need to check incoming information with the intended goal and to have the flexibility to alter a course of action if circumstances change. That is, the captains role is to regulate or control what happens. We too need to be able to plan, to organise behaviour, to attend and abstract information, to self-monitor and to have the mental flexibility to adapt and alter behaviour if circumstances change. It is these regulatory control aspects of behaviour which are particularly dependent upon the subcortical-frontal pathways in the brain which are affected in HD.How is it possible to detect patients' difficulties?
A variety of clinical tests map the difficulties in planning, organisation, attention, abstraction and mental flexibility in people with HD. In one such test patients are asked to sequence a set of pictures to tell a story. HD patients have considerable difficulty doing so even though they have no difficulty identifying the individual elements of each picture. That is, they are able to see the content of pictures, but have difficulty in the organisational aspect of the ordering task. In another test patients are asked to name the colour of the ink of incongruous colour words (i.e. the word 'red' written in blue ink). In order to produce the correct response 'blue' the individual needs to focus attention on the ink colour and suppress attention to what the word says. People with HD can have difficulty in this kind of attentional task.In a further test, patients are asked to join up a random series of numbered or lettered circles in sequential order, alternating between numbers and letters (i.e. 1 A 2 B 3 C). The task requires that the person keeps two things in mind (numbers and letters) and is able to switch attention.
In another task patients are asked to sort a pile of cards, showing coloured shapes, underneath four key cards according to a sorting 'rule'. The cards could potentially be sorted according to the shapes on a card, the colour of those shapes, or the number of shapes on the card. HD patients generally have no difficulty finding one way of sorting cards (i.e. by shape, colour or number). However, having found one sorting rule they can show difficulty thinking of alternative possible ways of sorting - having thought in one way they have difficulty seeing things in a novel way.
HD patients show a similar difficulty in flexibility of thought in a drawing task which requires the person to construct abstract shapes or patterns made up only of four lines. People who do not have HD tend to show a consistent pattern: they produce variations of one idea (e.g. permutations of a rectangle) and having exhausted that idea take a different tack (e.g. shapes made up of intersecting lines or circles). They can switch from one way of thinking to another. HD patients typically have no difficulty understanding the instructions and are able to produce shapes from four lines. Nevertheless, they commonly become 'stuck on one idea', producing variations of a single theme, and are not able to generate alternative ideas.
Practical Implications of Cognitive Change
The ability to plan and to think ahead is a great motivator of behaviour. In our daily lives much of what we do is stimulated by this capacity for forward thinking. We may, for example, think that we are running short of milk or bread, that there will not be enough for tomorrow's breakfast, so plan to stop by at the shop on the way home from work; we may think that a relatives birthday is approaching, so we will need to buy a card or present now if it is to arrive on time. If the capacity for planning and forward thinking is lost as in HD then people become essentially passive; that is, they react to events as they happen but they are no longer proactive. They lose the stimulus to initiate activities themselves. The loss of initiative that occurs in HD can thus be seen in part as a reflection of the impairment in forward thinking. People with HD may seem content to lie in bed all day or sit doing nothing. They often need an external stimulus to action, such as encouragement or prompting from carers, since they lack the self-motivation. Under normal circumstances we organise what we do. For example, if we need to go to several shops we will arrange them in the most efficient order, so that we are not retracing our steps. People with HD often have the capability of doing tasks or components of tasks individually, yet have difficulty in the organisation aspects of behaviour, so that their performance may seem disorganised and inefficient. This can represent a problem in the early stages of HD when people are still at work.In our daily lives we constantly divide our attention between different activities or have to switch our attention away from what we are doing. In the home we may, for example, do the ironing while watching television, or go to answer the telephone when in the middle of cooking. Difficulties with attention in HD make it difficult for sufferers to do two things at once and to switch from one task to another.
People with HD have difficulty self-monitoring and checking their performance. They may therefore be unaware of errors in their performance. This difficulty too can represent an early occupational problem: the person may know how to do their job but errors creep in of which they are unaware. The impression given to an employer may be that the person has become lazy or lackadaisical. That is not the case. It is simply that the person is no longer able to carry out efficiently the monitoring procedures that would keep errors in check.
A loss of flexibility and mental adaptability has quite widespread implications. It means that people with HD may seem rather rigid in their behaviour, they may like their own routine, and seem unwilling or unable to adapt to new situations. Loss of flexibility also has implications for interpersonal relationships. To have sympathy or empathy with other people one needs to be able to see things from the other person's point of view. HD patients have difficulty seeing things from alternative perspectives. They cannot easily put themselves in another person's shoes, and appreciate how another person may be feeling. They may therefore appear cold and uncaring: they are not being deliberately unkind, it is simply that the disease robs them of the ability to appreciate how someone may be feeling. Difficulty in thinking forward and in mental flexibility also means that people with HD can have difficulty seeing the repercussions of their own behaviour. They are not able to see the practical or emotional consequences that their behaviour may have on other people.
Memory in HD
Poor memory is a common complaint of HD sufferers and their families. Memory problems arising from brain disease can occur for different reasons. They may potentially result from a fundamental problem in information storage, occurring because of damage to the parts of the brain which are critical for laying down memories. However, the efficiency with which we remember is also determined by our mental activity as we take information in and retrieve it. If, in listening to a conversation, we engage our attention fully, actively think over what is being said, and consider how it matches up with our own experiences or opinions, then we will remember the conversation more effectively than if we listen only passively, or have our minds on other things. Similarly, in recalling a past event, if we actively recreate in our minds the situational context of the event, then we will recall details more effectively than if we wait purely passively for the details to come to mind.There are reasons to think that HD memory problems relate to those secondary factors - what the person does with information as it is taken in and retrieved, rather than a primary problem in the ability to lay down or store memories. The first clue to this comes from the fact that memory performance in people with HD can be variable. They may seem forgetful and absent-minded, yet recall in detail an event or what someone said a month or a year ago. That is, despite their forgetfulness they demonstrate the capability for new learning and remembering. There is also more direct, formal evidence. One way of assessing memory formally is to ask people to learn pairs of words (e.g. friend-train, gold-sugar). After being presented verbally with six or eight words the person is then told one of the words (e.g. friend) and asked to recall the other (i.e. train). Healthy individuals show a learning curve; that is, they may remember three or four of the pairs after one hearing, and increasingly remember more with additional hearings until they have learnt all of the word pairs. People with HD find the task extremely difficult and show little improvement in their memory performance with successive hearings of the word pairs. The same is true for people with Alzheimer's disease, for whom memory problems are often the most prominent feature of their condition. However, if a strategy is provided for linking up words in pair (i.e. the person is encouraged to imagine a friend getting of a train, or a gold bar standing up in a sugar bowl) then a different pattern emerges. For people with Alzheimer's disease there is no significant improvement; provision of the strategy for linking words does not help. In contrast, in HD there is a dramatic improvement in memory performance.
How do we interpret these differences? It seems likely that in Alzheimer's disease there is a fundamental problem in retention; in laying down and storing new information, and this arises because of damage to the parts of the brain particularly important for information storage. In HD the capability for new learning is potentially available. However, people with HD do not spontaneously adopt strategies that promote efficient memorising: they do not actively think about and link up information in their minds. If a method (external aid) is provided which does this linking process for them, then their memory performance can improve. These findings are important, since they have implications for management of memory problems in HD. They suggest that people with HD should be able to benefit from external memory aids, such as a noticeboard or memo book, since these can help to structure and organise information and act as a trigger to assist information retrieval.
Emotional Changes in HD
There are a number of emotional changes that may occur in HD. People may become irritable, and experience feelings of anxiety and internal agitation. They may be emotionally volatile, losing their temper for no apparent reason. They may become depressed. They may also show emotional blunting, a loss of the emotional warmth and range of emotional expression that they demonstrated before they became ill. Some of these mood changes may occur independently of the changes in cognition/thinking. However, there may be potential interactions between emotional and cognitive change. An HD sufferer who, for example, loses his temper when spoken to while he is watching the television, is demonstrating the volatility and loss of emotional control which occur in HD. However, the feeling of irritability might stem from or be exacerbated by the fact that the person actually finds it difficult to switch attention efficiently away from the television to the conversation at hand and then back to the television. The patient has difficulty coping with more than one thing at a time. If people with HD appear to lose their temper for no apparent reason it is worth considering whether there may be precipitating factors. Is it the case that cognitive demands are being placed on the person with which he/she is unable to cope?Individual Differences
Alterations in behaviour and intellect are an inevitable part of HD. However, just as the involuntary movements vary in their severity from one individual to another so too do the mental changes. For some people, the changes cause few practical problems, whereas for others they have profound consequences for the affected persons inter-personal relationships and place very considerable burden on carers. In addition to individual differences in severity, there are also differences in the types of behavioural changes demonstrated. Not all patients will show all possible behavioural abnormalities all of the time. We have developed in Manchester a questionnaire of behavioural symptoms to try to ascertain the frequency with which different aspects of behavioural disorder are present and how they change over the course of the disease. Our preliminary studies based on interviews with 108 HD patients and their relatives attending an HD clinic (most of the patients were in the mild and moderate stages of the disease) suggested that some behavioural symptoms are certainly quite common and present in the majority of people affected with HD. These include many of the features described above such as reduced drive and initiative and poorer quality of performance on tasks. Some of these features correlate with severity of cognitive disorder, emphasising the relationship between such behavioural features and HD patients' thinking processes. Mood changes such as depressive symptoms are also relatively common, occurring in as many as 50% of individuals with HD, but these mood changes have a much less strong relationship to the degree of cognitive (thinking) problems. These diverse findings highlight the complexity and multi-faceted nature of behavioural changes in HD.Effects of disease versus reaction to disease
It is sometimes questioned whether the behavioural changes in HD are in inherent part of the disease process (i.e. a result of the physical changes that take place in the brain) or a reaction to having a distressing and debilitating condition. Much of the behavioural change, as described above, is an intrinsic part of the disease. That is not to say that there are not also reactive components. People with HD have life changes imposed on them: they may lose their job, their social life, their mobility and their independence. They may be misunderstood and ostracised by others. It is not surprising that they may show signs of irritability and frustration. Sometimes behaviour may represent an interaction between the direct effects of the disease and the consequences of it. To illustrate, a man with HD who was walking along the road was stopped by the police and accused of drunkenness. Having drunk nothing the man felt insulted and outraged, hit the policeman who then arrested him and charged him with assault. The HD patient's feeling of anger can be seen as an understandable reaction to the consequences of HD i.e. being misunderstood and falsely accused. However, in a comparable situation most people would see in a split second the potential repercussions of hitting a policeman and realise that it would not be in their interest to do so, and moreover would have the capability of suppressing their feelings of anger. The HD patient, by virtue of his illness, could neither foresee the consequences of his actions, nor could he keep his feelings of anger in cheek.Conclusion
HD can be a destructive condition, which can damage social and family relationships. It is, however, the disease which is at fault. People with HD are not being deliberately inconsiderate and awkward. It is the disease which may rob them of ability to be flexible, considerate and empathic. If new treatments for HD are to be successful then they will need to make an impact not only on the movement disorder but also the behavioural aspects of the condition.