Definitions Used In HD
"E" through "J"


"A" through "B"
"C" through "D"
"E" through "J"
"K" through "P"
"Q" through "X"
Color coding:
Other diseases similiar to HD
Electrical stimulation (ES): A test in which a small electrical charge is applied to a muscle via a needle to determine the level of muscle responsiveness.

Electroencephalography (EEG): A noninvasive, diagnostic technique that records the electrical impulses produced by brain cell activity. An EEG reveals characteristic brain wave patterns that may assist in the diagnosis of particular neurologic conditions, such as seizure disorders, impaired consciousness, and brain lesions or tumors.

Electrodiagnostic tests
(i.e., electromyography (EMG) and nerve conduction velocity, or NCV) - studies that evaluate and diagnose disorders of the muscles and motor neurons. Electrodes are inserted into the muscle, or placed on the skin overlying a muscle or muscle group, and electrical activity and muscle response are recorded.

EEG: (electroencephalography) The diagnostic test that is used to study the brain wave activity. It is most useful to evaluate the seizure disorders.

EMG/NCV: (electromyography/nerve conduction study) A test that is used to study the nerves and muscles to help diagnose disorders that can affect
them. A small needle is placed in the muscle in the EMG. Electrical conduction is studied in the NCV. The results are seen on an oscilloscope screen and compared to normal values. During this test, a small needle is inserted into a muscle to record the level of activity

Electrophysiologic studies: The study of the electrical activity associated with a specific body function (e.g., EMG, EEG, etc.).

ENCEPHALOPATHY: Any disease of the brain.

ENZYME: A protein that speeds up a chemical reaction or causes a chemical
change in another substance. Enzymes do their work without being changed or used up in the process. A chemical substance that is acted upon by an enzyme is called a "substrate." In many cases, enzymes are named by adding the suffix "-ase" to the name of the substrate upon which the enzyme exerts its action.
Epilepsy (Seizures) Said to be present in 30% of individuals with juvenile-onset HD. A first seizure in an HD patient should not be attributed to HD without further
evaluation as it may be indicative qf an additional neurologic problem, such
as a subdural hematoma sustained in a fall.
The treatment of a seizure disorder in a person with HD depends on the nature of the seizures. In the juvenile HD patient, myoclonic epilepsy or other generalized seizures may suggest divalproex sodium as a first treatment choice. Although seizure
management in HD is not usually difficult, for the occasional patient seizure control is quite difficult to achieve, requiring multiple
medications or specialized referral.

Ergot: A plant alkaloid produced by the fungus Claviceps purpurea.

Ergot-derived medication: A medication that has a chemical structure based on ergot, a plant alkaloid produced by a fungus called Claviceps purpurea. Permax® and Parlodel® are examples of ergot-derived medications that may be used to treat certain neurologic movement disorders.

Esophageal atony: Lack of normal muscle tone within the esophagus, the muscular tube that transports food from the throat to the stomach.

Essential tremor (ET): A common, slowly and variably progressive neurologic movement disorder characterized by involuntary, rhythmic, "back and forth" movements (i.e., tremor) of a body part or parts.   In ET patients, tremor is primarily a "postural" or "kinetic" tremor or may be a combination of both types: i.e., tremor occurs while voluntarily maintaining a fixed position against gravity (postural tremor) and/or when conducting self-directed, targeted actions (kinetic intention tremor).
ET may appear to occur randomly for unknown reasons (sporadically) or be transmitted as an autosomal dominant trait.  This form of tremor is maximal when the body part is being used, such
as when writing or when making fine coordinated hand movements. It is less common when at rest.

Extrapyramidal system: Refers to central nervous system structures (i.e., outside the cerebrospinal pyramidal tracts) that play a role in controlling motor functions. The extrapyramidal system includes substructures of the basal ganglia and the brainstem and interconnections with certain regions of the cerebellum, cerebrum, and other areas of the central nervous system.
Extrapyramidal disturbances may result in postural and muscle tone abnormalities as well as the development of certain involuntary movements.
Falls - Falls are common in persons with HD, and can be a source of significant
morbidity. Usually seen more in the moderate to advanced stages, they often
result from the combination of spasticity, rigidity, chorea, and loss of balance.
Pharmacotherapy (drugs) to prevent falls could include treatment of chorea,
rigidity, spasticity and dystonia, while minimizing the use of drugs such as
neuroleptics and benzodiazepines, whose side effects include sedation,ataxia, or parkinsonism. Most efforts at prevention, however, involve not drugs, but modification of the environment and behavior of the patient. Occupational and physical therapists can instruct patients in how to sit, stand, transfer, and walk more safely.

Fasciculation: Twitching of the muscles that is seen in diseases involving the peripheral nervous system.

Free radicals: Unstable molecular fragments that can damage cells.

Frequency: number of cycles or repetitions within a fixed unit of time such as the number of cycles per second (Hertz or Hz). For example, essential tremor is typically 4 to 12 Hz.

Froment's sign: Increased resistance to passive movements of a limb that may be detected upon voluntary activity of another body part.

Functional Magnetic Resonance Imaging (fMRI): A noninvasive, diagnostic scanning procedure that produces detailed, computerized images. The use of a "contrast agent" or dye enhances the detail of the images. Sequencing of these images may help physicians visualize the body's functioning, thus aiding differential diagnosis.

Gadolinium: A contrast agent that is given intravenously during MRI (magnetic resonance imaging) to increase visualization of specific abnormalities

Gait: The style or manner of walking. Gait disturbances may be associated with certain neurologic or neuromuscular disorders, orthopedic conditions, inflammatory conditions of the joints (i.e., arthritic changes), or other abnormalities.
Gait apraxia (walking)
Loss of the ability to consciously sequence and execute the movements required to coordinate walking. Gait apraxia may result in unsteady walking patterns; "toe-walking"; a widely based, jerky gait; and balance difficulties. It is
 an abnormality in which the patient is able to carry out all of the movements required for gait normally when lying down, but is unable to walk in the standing position, thought to be associated with frontal disorders or normal pressure hydrocephalus.

Gamma-aminobutyric acid (GABA): An amino acid neurotransmitter that inhibits or decreases the electrical activities of nerve cells. GABA is the primary  inhibitory neurotransmitter in the brain

Gastrostomy tube: (feeding tube)
A plastic tube inserted into the stomach through a surgical incision in the abdomen. A gastrostomy tube is used to deliver liquified food  to the digestive system when swallowing becomes dangerous or difficult.

Genetic anticipation: A phenomenon in which the onset of symptoms of a hereditary disease appears to occur at a progressively earlier age in successive
generations. Genetic anticipation has been
demonstrated in a number of hereditary disorders (such as Huntington's disease,
dentatorubropallidoluysian atrophy, etc.) in which the gene mutation consists of abnormally long sequences or "repeats" of particular coded instructions (e.g., unstable expansion of CAG repeats). With other disorders in which genetic anticipation has previously been suggested (e.g., essential tremor), studies have indicated that increased awareness of the condition in affected families may be responsible for earlier recognition of symptom onset.

GENE: The fundamental unit of heredity. Genes are located on strands of DNA found in the cells and mitochondria.

Genetic heterogeneity: Manifestation of similar or the same observable characteristics or traits (i.e., phenotype) resulting from different genetic
mechanisms, such as changes (mutations) of different genes or a combination of genes.

Gliosis: A proliferation of astrocytes in damaged areas of the central nervous system (CNS). Astrocytes are relatively large glial cells, which are the connective tissue cells of the CNS. Astrocytes have various functions, including accumulating in areas where nerve cells (neurons) have been
damaged. Gliosis and neuronal loss in certain brain regions are findings seen in various neurodegenerative disorders.

Global aphasia - This is the most severe form of aphasia, and is applied to patients who can produce few recognizable words and understand little or no spoken language. Global aphasics can neither read nor write.
Global aphasia may often be seen immediately after the patient has suffered a stroke and it may rapidly improve if the damage has not been too extensive.
However, with greater brain damage, severe and lasting disability may result.

Globus pallidus: A major substructure of the basal ganglia deep within the brain. Specialized groups of nerve cells in the globus pallidus function as an "intermediate relay system." This system processes and transmits information from the basal ganglia by way of the thalamus to areas of the brain that regulate complex motor functions (e.g., motor cortex, premotor area of frontal lobe

Glutamate: An amino acid that is a primary excitatory neurotransmitter in the central nervous system. This chemical agent plays an essential role in initiating and transmitting nerve impulses, crossing synapses to stimulate postsynaptic neurons.
Gray matter: Nerve tissue that primarily consists of nerve cell bodies, dendrites, and unmyelinated axons, thus having a gray appearance. In contrast, white matter predominantly contains myelinated nerve fibers.
Globus pallidus: A major substructure
of the basal ganglia deep within the brain. Specialized groups of nerve cells in the globus pallidus function as an "intermediate relay system." This system processes and transmits information from the basal ganglia by way of the thalamus to areas of the brain that regulate complex motor functions (e.g., motor cortex, premotor area of frontal lobe).
Hallervorden-Spatz disease:
A slowly progressive hereditary disorder characterized by abnormal accumulations of iron pigments in certain regions of the basal ganglia (e.g., globus pallidus and substantia nigra). This autosomal recessive disorder typically becomes apparent during late childhood or adolescence.
Associated symptoms may include progressive stiffness and resistance to movement (rigidity); increasing difficulties speaking and eating; sustained muscle contractions that result in repetitive twisting motions or unusual postures or positions (dystonia); the presence of multiple, irregular, jerky motions that may combine to appear flowing or writhing in nature (choreoathetosis); and/or other abnormalities.
Hallucinations: Perceptions that occur in the absence of external stimuli and while an individual is awake. Hallucinations may involve any of the senses, including hearing (auditory hallucinations), vision (visual hallucinations), smell (olfactory hallucin-ations), taste (gustatory hallucinations), and touch (tactile hallucinations).
Hallucinations may occur with high-dose
administration of certain medications or the abuse of particular drugs (hallucinogens) that cause increased stimulation or excitation of the central nervous system.
They may also be caused by chronic alcohol abuse; sensory deprivation; certain types of seizures due to impairment or damage of a limited area of the brain (e.g., temporal lobe epilepsy); psychotic illnesses characterized by disturbances in thought, behavior, and emotional reactions (schizophrenia); or certain organic disorders, such as Huntington's disease.
Hemichorea: Irregular, abrupt, relatively rapid involuntary (i.e., choreic) movements that are unilateral or involve muscle groups on one side of the body.

Hemiparesis: Weakness that affects one side of the body.

Hereditary: Inherited; inborn; referring to the genetic transmission of a trait, condition, or disorder from parent to offspring.

Heredodegenerative: Pertaining to genetically transmitted (hereditary) neurologic disorders characterized by progressive neuronal loss and other neurodegenerative changes (e.g., associated gliosis). For example, the term "heredo-degenerative dystonias" is used to refer to several, usually hereditary disorders in which neurodegenerative changes may lead to dystonia as a primary feature and other neurologic symptoms are typically present.
However, in some patients with such disorders, dystonia may not always develop and other neurologic features may be primary findings.
Disorders that have been categorized as heredodegenerative dystonias include ataxia-telangiectasia, Hallervorden-Spatz disease, Huntington's disease, Parkinson's disease, progressive supranuclear palsy,
neuroacanthocytosis, and Wilson's disease.

Homocystinuria: A metabolic abnormality characterized by excessive amounts of the amino acid homocystine in the urine. Homocystinuria, which may be transmitted as an autosomal recessive trait, may result from deficient activity of certain enzymes involved in the metabolic conversion of the amino acid methionine to cysteine.

hemorrhage: Bleeding; (such as in brain hemorrhage)

Huntington's disease (HD):
A hereditary, progressive, neuro-degenerative disorder primarily characterized by the development of emotional, behavioral, and psychiatric abnormalities; gradual deterioration of thought processing and acquired intellectual abilities; and movement abnormalities, including involuntary, rapid, irregular jerky movements (chorea) of the face, arms, legs, or trunk.
HD may be inherited as an autosomal dominant trait or, less commonly, appear to occur randomly for unknown reasons (sporadically). The disorder results from abnormally long sequences or "repeats" of certain coded instructions (i.e., unstable expanded CAG repeats) within a gene (located on chromosome p16.3). Progressive nervous system dysfunction associated with HD results from loss of neurons in certain areas of the brain, including the basal ganglia and cerebral cortex.

Hyperkalemic: Pertaining to hyperkalemia, a condition characterized by unusually high concentrations of potassium in the blood.  Potassium, a positively charged ion, helps to regulate water balance, nerve conduction, and muscle contraction.
Hyperkalemia may lead to various signs and symptoms, such as diarrhea, nausea, abdominal cramps, weakness, and, with increasing severity, abnormal heart rhythms and muscle paralysis.

Hyperkinetic: Characterized by excessive movement because of abnormally increased motor activity or function. Certain movement disorders are termed "hyper-kinetic" such as tics or essential tremor.

Hyperventilation: Abnormally rapid breathing usually accompanied by air swallowing.  Hyperventilation results in excessive intake of oxygen and increased elimination of carbon dioxide, which may eventually lead to a disturbance in the body's acid-base balance.

Hypokinetic: Diminished movement and decreased motor function. Some movement disorders are hypokinetic, such as Parkinson's disease.

An endocrine condition characterized by insufficient secretion of parathyroid hormone due to injury, disease, or congenital abnormalities of the parathyroid glands.  Parathyroid hormone plays an essential role in regulating calcium balance in the body.
Hypoparathyroidism results in low calcium blood levels (hypocalcemia), leading to various symptoms  and findings.  For example, hypoparathyroidism may cause a condition known as tetany in which increased excitability of nerves and muscles results in uncontrollable, painful muscle spasms and cramping; abnormal involuntary movements resembling chorea; spasmodic closure of the larynx (laryngospasm);  and other findings.
Hypoparathyroidism may also lead to seizures; abnormal calcium deposits (calcifications) in the basal ganglia; loss of transparency of the lenses of the eyes (cataracts); dry, scaly skin; weakened tooth enamel; and/or other abnormalities.

Idiopathic: A disorder or condition of spontaneous origin; self-originated or of unknown cause. The term is derived from the prefix "idio-" meaning one's own and "pathos" indicating disease.

Idiopathic epilepsy: Recurrent seizures of unknown origin or cause; repeated episodes of uncontrolled electrical activity in the brain that occur in the absence of detectable brain lesions. Also known as essential or primary epilepsy, these conditions are often thought to have a genetic component. This may suggest that affected individuals have an inherited predisposition for recurrent seizures.

Immunogenicity: The ability to or the degree to which a particular substance may provoke an immune response; having the properties of an antigen or any substance that may trigger a particular immune reaction, such as the production of anti-bodies.

Implantable Pulse Generator (IPG): A device that is placed under the skin near the collarbone as part of a surgical procedure known as deep brain stimulation. Wire leads from electrodes implanted in the brain are connected to the pulse generator, which then delivers continuous high frequency electrical stimulation to the thalamus via the implanted electrodes.
This form of stimulation probably "jams" the nucleus and therefore modifies the message in the movement control centers of the brain, serving to suppress tremor.

Inhibition: The restraint, suppression, or arrest of a process or the action of a particular cell or organ; the prevention or slowing of the rate of a chemical or
an organic reaction. The term "reciprocal inhibition" refers to the restraint or "checking" of one group of muscles upon stimulation (excitation) and contraction
of their opposing (antagonist) muscles.

Inhibitor: A substance that blocks, restricts, or interferes with a particular chemical reaction or other biologic activity.

Innervate: To supply a body part, tissue, or organ with nerves or nervous stimulation.

Of subtle, gradual, or imperceptible development; referring to the development of symptoms that may not be recognized by an affected individual until the disorder in question is established.

Ion: An electrically charged atom or group of atoms due to the loss or gain of one or more electrons. Positively charged ions, known as cations, include potassium, calcium, sodium, hydrogen, ammonium, and magnesium. Negatively charged ions, called anions, include chloride, phosphate, and bicarbonate.
The transfer of ions across cellular membranes plays an essential role in vital bodily processes, including enzyme activation, protein metabolism, acid-base balance, nerve impulse transmission, and muscle contraction.

Joint contractures: Permanent flexing or extension of joints in fixed postures due to shortening of muscle fibers. Contractures, abnormal fixation of the limbs, and associated deformity may result from prolonged immobility of developing joints.

Juvenile myoclonic epilepsy: A form of idiopathic epilepsy or recurrent seizures of unknown origin, with symptom onset typically occurring from approximately
12 to 16 years of age. The condition is characterized by sudden, involuntary, "shock-like" muscle jerks (myoclonus) that primarily occur during the morning
or with stress, fatigue, or alcohol consumption. Patients may later develop generalized tonic-clonic seizures associated with loss of consciousness and rhythmic contraction and relaxation of all muscle groups.