Huntington’s disease (HD) is an inherited neurodegenerative disorder that progressively affects motor control, thought processes, and emotional well-being. The complexity of its symptoms often raises the question of whether it constitutes a form of dementia, which is primarily associated with cognitive decline. This article clarifies how the cognitive impairment in HD aligns with the clinical definition of dementia while maintaining its unique characteristics.
Understanding Clinical Dementia
Dementia is a syndrome characterized by a significant decline in cognitive ability that is severe enough to interfere with a person’s independence in daily life. Major diagnostic manuals, such as the DSM-5, often use the term Major Neurocognitive Disorder. This classification requires evidence of a decline from a previous level of functioning in one or more cognitive domains, including attention, executive function, memory, language, and social cognition.
The decline must be persistent and represent a change from the person’s prior abilities, and it cannot be attributed solely to acute delirium or another psychiatric disorder. Clinicians use these criteria to determine if cognitive impairment rises to the level of dementia, regardless of the underlying cause. Many diseases can lead to this syndrome, including Alzheimer’s disease, vascular disease, and Huntington’s disease.
The Underlying Cause of Huntington’s Disease
Huntington’s disease is a genetic, autosomal dominant disorder, meaning only one copy of the mutated gene is needed to inherit the condition. It is caused by an abnormal expansion of a CAG trinucleotide repeat sequence within the HTT gene on chromosome 4. An expansion to 40 or more repeats results in full disease penetrance, whereas a healthy gene typically has 26 or fewer repeats.
This expanded repeat sequence leads to the production of a toxic protein known as mutant huntingtin (mHTT). The mHTT protein causes progressive neurodegeneration, initially targeting the striatum, which is part of the basal ganglia. This selective loss of neurons is the core pathology that drives the motor, cognitive, and psychiatric symptoms seen in HD.
The Triad of Huntington’s Disease Symptoms
HD is characterized by a progressive combination of three distinct categories of symptoms: motor, cognitive, and psychiatric disturbances. Motor symptoms often begin with involuntary movements known as chorea, which are random, jerky, and uncontrollable motions. As the disease advances, these hyperkinetic movements transition into hypokinesia, which includes slow movement (bradykinesia) and rigidity.
The cognitive changes in HD often present as a specific pattern of decline, sometimes referred to as a “subcortical” dementia. Patients experience slowed thinking (bradyphrenia) and significant impairment in executive functions like planning and problem-solving. Unlike Alzheimer’s disease, which features prominent early memory loss, HD cognitive impairment is characterized by difficulty retrieving information rather than an inability to store new memories.
The third component involves various psychiatric and behavioral symptoms that can appear many years before the motor or severe cognitive symptoms. Common features include depression, anxiety, irritability, and apathy. Obsessive-compulsive behaviors and, less frequently, psychosis can also occur, contributing significantly to the overall burden of the disease.
Classifying HD’s Cognitive Decline
The cognitive decline in HD meets the clinical criteria for dementia, or Major Neurocognitive Disorder, because the impairment significantly interferes with a person’s independence. Although HD is a specific neurological disorder, its cognitive manifestations are classified under the broader umbrella of dementia. The DSM-5 specifically lists “Major Neurocognitive Disorder due to Huntington’s Disease” as an etiological subtype.
This classification recognizes that the distinctive pattern of cognitive deficits in HD differs from common cortical dementias, such as Alzheimer’s disease. Since the underlying pathology primarily affects subcortical structures like the basal ganglia, the resulting cognitive profile is termed a subcortical dementia. This type is characterized by slowness of thought and executive dysfunction, contrasting with the pronounced early memory and language deficits seen in cortical dementias.