It
is usually apparent in the forth or fifth decades, but may occur at almost any
age. No clear sex preponderance is evident. Low prevalence rates have been
noted in Japan and among African and American blacks, and most patients are of
northern European ancestry.
Early signs of the disease vary greatly from person to person. A common observation is that the earlier the symptoms appear, the faster the disease progresses.
Family
members may first notice that the individual experiences mood swings or becomes
uncharacteristically irritable, apathetic, passive, depressed, or angry. These
symptoms may lessen as the disease progresses or, in some individuals, may
continue and include hostile outbursts or deep bouts of depression.
In
some individuals, the disease may begin with uncontrolled movements in the
fingers, feet, face, or trunk. These movements - which are signs of chorea -
often intensify when the person is anxious. HD can also begin with mild
clumsiness or problems with balance. Some people develop choreic movements
later, after the disease has progressed. They may stumble or appear
uncoordinated. Chorea often creates serious problems with walking, increasing
the likelihood of falls.
The
disease can reach the point where speech is slurred and vital functions, such
as swallowing, eating, speaking, and especially walking, continue to decline.
Some individuals cannot recognize other family members. Many, however, remain
aware of their environment and are able to express emotions.
In
general, the duration of the illness ranges from 10 to 30 years. The most common
causes of death are infection (most often pneumonia), injuries related to a
fall, or other complications.
The
rate of disease progression and the age at onset vary from person to person.
Adult-onset HD, with its disabling, uncontrolled movements, most often begins
in middle age. There are, however, other variations of HD distinguished not
just by age at onset but by a distinct array of symptoms. For example, some
persons develop the disease as adults, but without chorea. They may appear
rigid and move very little, or not at all, a condition called akinesia.
Some
individuals develop symptoms of HD when they are very young - before age 20.
The terms 'early-onset' or 'juvenile' HD are often used to describe HD that
appears in a young person. A common sign of HD in a younger individual is a
rapid decline in school performance. Symptoms can also include subtle changes
in handwriting and slight problems with movement, such as slowness, rigidity,
tremor, and rapid muscular twitching, called myoclonus. Several of these
symptoms are similar to those seen in 'Parkinson's disease', and they differ
from the chorea seen in individuals who develop the disease as adults. These
young individuals are said to have 'akinetic-rigid' HD or the Westphal variant
of HD. People with juvenile HD may also have seizures and mental disabilities.
The earlier the onset, the faster the disease seems to progress. The disease
progresses most rapidly in individuals with juvenile or early-onset HD, and
death often follows within 10 years.
Individuals
with juvenile HD usually inherit the disease from their fathers. These
individuals also tend to have the largest number of CAG repeats. The reason for
this may be found in the process of sperm production. Unlike eggs, sperm are
produced in the millions. Because DNA is copied millions of times during this
process, there is an increased possibility for genetic mistakes to occur. To
verify the link between the number of CAG repeats in the HD gene and the age at
onset of symptoms, scientists studied a boy who developed HD symptoms at the
age of two, one of the youngest and most severe cases ever recorded. They found
that he had the largest number of CAG repeats of anyone studied so far - nearly
100. The boy's case was central to the identification of the HD gene and at the
same time helped confirm that juveniles with HD have the longest segments of
CAG repeats, the only proven correlation between repeat length and age at
onset.
A
few individuals develop HD after age 55. Diagnosis in these people can be very
difficult. The symptoms of HD may be masked by other health problems, or the
person may not display the severity of symptoms seen in individuals with HD of
earlier onset. These individuals may also show symptoms of depression rather
than anger or irritability, or they may retain sharp control over their
intellectual functions, such as memory, reasoning, and problem-solving.
There
is also a related disorder called senile chorea. Some elderly individuals
display the symptoms of HD, especially choreic movements, but do not become
demented, have a normal gene, and lack a family history of the disorder. Some
scientists believe that a different gene mutation may account for this small
number of cases, bu this has not been proven.
The
most important issue in the management of HD is the education of the patient
and family about the disease and the implication of the diagnosis for other
family mambers. The organisations are invaluable sources of information and
support for HD families, as well as help with chronic care patients.
