Definitions Used In HD
"Q" through "X"


"A" through "B"
"C" through "D"
"E" through "J"
"K" through "P"
"Q" through "X"
 Color coding:
Other diseases similiar to HD
Q- none
Range of motion (ROM): The extent of a joint's free movement. The normal ROM of the elbow, for instance, carries the forearm through a half-circle. Passive ROM is tested while the limb is relaxed. Active ROM is movement controlled by the patient.

Receptor: A molecule on a neuron that receives a neurotransmitter. Reception of the neurotransmitter causes changes in the neuron which increase or decrease its likelihood of "firing," or sending its own signal to other neurons. Dopamine receptors are located on corpus striatum neurons, and on nigral cells.

Reduced penetrance: Reduced expression of a genetic disorder. The term penetrance refers to the frequency with which a specific genetic mutation produces its typical effect in those with the genetic abnormality. For example, if fewer than 100 percent of individuals who inherit a gene mutation for an autosomal dominant disorder develop the disease, the specific trait is said to have "reduced penetrance."

Reflex: Involuntary, predictable response to a particular stimulus.

Refractory: Resistant to or not readily yielding to treatment.

REM sleep: The period of sleep that is associated with dreaming, rapid eye movements (REM), and certain involuntary muscle movements.

Restless legs syndrome (RLS):
A neurologic movement disorder characterized by unusual, uncomfortable sensations(paresthesias/dysesthesias) deep within the calves and/or thighs, resulting in an irresistible urge to move the legs, and motor restlessness in response to or in an effort to alleviate discomfort. In some patients, the arms may also be affected. Symptoms become obvious or worse during periods of relaxation or inactivity; occur most frequently during the evening or the early part of the night; and may be temporarily relieved by voluntary movements of the affected area. Most patients experience associated sleep disturbances, including difficulties drifting off to and remaining asleep. RLS is also often associated with periodic limb movements of sleep (PLMS) or repeated, stereotypic, upward extension of the great toe and foot, potentially followed by flexion of the knee, hip, or ankle. Episodes of PLMS typically occur during periods of lighter (i.e., non-REM) sleep.

Reye syndrome: A potentially life-threatening disease characterized by sudden inflammation and swelling of the brain (acute encephalopathy) and rapid fat accumulation within certain internal organs (viscera), particularly the liver, occurring subsequent to certain viral infections, such as chickenpox or upper respiratory tract infections (e.g., influenza B).
Reye syndrome primarily occurs in children and adolescents, although it has sometimes been reported during infancy or young adulthood. About a week after the onset of a viral infection, patients may develop uncontrollable vomiting, followed by a rapid onset of listlessness, confusion, and memory loss, a state of unconsciousness (coma), seizures, and/or other findings, potentially leading to life-threatening complications. Evidence suggests that the use of aspirin-containing preparations (salicylates) as a treatment for particular viral infections plays a role in the development of Reye syndrome. Therefore, experts advise that such medications be avoided for the treatment of viral infections during infancy, childhood, adolescence, and young adulthood.

Rhythmical myoclonus: Involuntary, shock-like contractions or spasms of a muscle or muscle group that occur in a rhythmical pattern. This usually occurs as a result of a lesion in the central nervous system.

Rigidity:  Stiffness in the limbs or body due to dysfunction of the basal ganglia and related structures.
Medications derived from salicylic acid, including aspirin (acetylsalicylic acid). Such compounds have anti-inflammatory, pain-relieving (analgesic), and fever-reducing (antipyretic) activities. Salicylates act to reduce the production of certain hormone-like chemicals known as prostaglandins that may have varying actions, potentially leading to inflammatory effects, increased pain sensitivity, fever, etc.
Prolonged aspirin use may damage the stomach or intestinal lining, causing peptic ulcers and bleeding from the digestive tract. In addition, excessive intake of salicylates may lead to salicylate toxicity,
characterized by rapid breathing, irritability, vomitingand other findings.

Sandhoff's disease:
A neurodegenerative metabolic disorder that is characterized by symptoms and findings similar to those associated with Tay-Sachs disease as well as possible, moderate enlargement of the liver and spleen (hepatosplenomegaly). Sandhoff's disease is a lysosomal storage disease in which deficiency of the enzymes hexo-saminidase A and B results in an abnormal accumulation of certain fats (i.e., gangliosides) in particular tissues of the body. The disorder is transmitted as an autosomal recessive trait and affects only non-Jewish individuals (as opposed to Tay-Sachs disease, which primarily occurs in individuals of Ashkenazi Jewish ancestry).

Sandifer Syndromrigidity: are dramatic dystonic postures in children that have been reported in association with gastroesophageal reflux. Sandifer syndrome has been associated with gastroesophageal reflux. GERD is very common in otherwise asymptomatic infants but may be associated with apnea, choking, cyanosis, as well as posturing. The diagnosis is established by history, ph probe testing, and disappearance of abnormal postures with medical management of reflux with posturing during the testing.

Lateral or "sideways" curvature of the normally vertical line of the spine. Progressive spinal deformity may be associated with numerous neuromuscular and neurodevelopmental disorders (neurogenic scoliosis), such as cerebral palsy, spinal muscular atrophy, and Rett syndrome. The severity of the deformity varies, depending upon the degree of weakness, the nature and progression of the underlying disorder, or other factors.

SEP: Somatosensory-evoked potentials use EEG electrodes to record the response of the brain to a sensory stimulus. Usually a small electrical pulse is given at the wrist or behind the knee. The response in the EEG is then measured. The pulse needs to be repeated at least several hundred times
in order to have enough of a signal to analyze. From this information, the evaluator may determine whether there is a delay in conduction to the brain, a blockage at any point, or abnormally low or high activity in the brain.

SEIZURES: The abnormal electrical discharge of brain cells (neurons) that results in a transient disturbance in brain function. Disturbances of brain function, manifested as episodic impairment or loss of consciousness, abnormal movement, or sensory disturbances. Caused by paroxysmal disturbances
in the electrical activity of the brain.
  • Myoclonic: Seizures characterized by jerking a body extremity or generalized tonic-clonic seizures within an hour or two of waking from sleep.
  • Partial: (Formerly known as focal seizures.) The seizure is limited to one area in the brain. During this type of seizure, the child may experience a range of strange or unusual sensations including sudden, jerky movements of one body part, distortions in hearing or seeing, stomach discomfort, or a sudden sense of fear. Partial seizures are classified as either simple or complex. In simple partial seizures, there is no loss of consciousness. In partial complex seizures, consciousness is impaired.
  • Petit-mal:
    Now called generalized absence seizures. These are characterized by 5 to 15 second lapses in consciousness. During an absence seizure, the child appears to be staring into space and the eyes may roll upwards. Absence seizures typically occur in childhood and resolve in adolescence. Absence seizures are rare in adults.
  • status epilepticus:
    Seizures that continue for more than 20 minutes without an intervening period of responsiveness.
Sensorimotor:  Pertaining to both the sensory and motor aspects of a bodily function serotonin: An important neurotransmitter (communicates information chemically between brain cells) that is involved in the pain disorders and emotional perceptions.

Sequelae: Plural of sequela, which is any abnormal condition that occurs subsequent to and/or is caused by disease, injury, or treatment.

Serotonin (3-[2-aminoethyl]-5-indolol): A vasoconstrictor found in many tissues of the body that is present in relatively high concentrations in portions of the central nervous system (e.g., hypothalamus, basal ganglia, etc.). Serotonin functions as a neurotransmitter, regulating the delivery of messages between nerve cells (neurons). This neurotransmitter is thought to play some role in regulating consciousness and mood states.  Serotonin is also present in other tissues of the body such as the intestines and blood platelets.

Sialorrhea: Excess production of saliva, or increased retention of saliva in the mouth, due to difficulty swallowing.

Side effect: An effect of a drug that is not the main or intended effect. Side effects may be of no concern, or they may be bother-some or even dangerous, in which case they may limit the upper dose a patient can tolerate. Side effects are also called adverse effects.

Single photon emission computed tomography (SPECT): A noninvasive scanning procedure during which a radioactive substance known as a
radionuclide is introduced into the body to help evaluate the function and structure of certain organs or tissues. The amount of the substance taken up by particular tissues may depend upon the amount of blood flow within such regions. For example, absence of radionuclide uptake in a targeted region may indicate a lack of blood flow in certain areas.
Following intravenous administration of the radioactive compound, a specialized rotating camera detects the radiation emanating from the radionuclides in the form of particles known as protons. The recorded images may produce colorized, horizontal and vertical cross sections and be reconstructed by computer to create three-dimensional images. By evaluating the blood supply to particular tissues, SPECT may be particularly helpful in detecting certain changes within the central nervous system or the heart.

Spasmodic dysphonia (SD):
A manifestation of dystonia. SD involves the muscles of the larynx and surrounding muscles and therefore involves speech. In individuals with SD, speech in blocked by intermittent spasms of the voice box (larynx).

Spasmodic torticollis (ST): A form of dystonia involving the muscles of the neck, and therefore called "cervical dystonia." As a result of the abnormal involuntary contractions of the neck muscles, the head may be rotated, tilted, flexed, extended, or any combination of these postures. The movements may be quick, sustained, or patterned and, therefore, may be associated with tremor.

Spasmolytic: Antispasmodic; referring to agents that may eliminate or relieve spasms, typically of involuntary (i.e., smooth) muscle, such as within the
arteries, the intestine, the ring-shaped muscles around certain natural openings or passages (sphincters), the bladder, the muscular tubes that carry urine from the kidneys to the bladder (ureters), etc.

Spasticity: stiffness of the body involving the limbs that results from dysfunction of the corticospinal tracts. An abnormal increase in muscle tone that may be caused by certain types of damage to the nerve
pathways regulating muscles. Spasticity is a common complication of cerebral palsy, brain injuries, spinal cord injuries, multiple sclerosis, and stroke. Spasticity can lead to incoordination, loss of function, pain, and
permanent muscle shortening, or contracture.

Spinal cord: The cylindrical structure of nerve tissue that, together with the brain, comprises the central nervous system. The spinal cord is an extension of the medulla oblongata--which is part of the lowest region of the brain (brainstem)--and is contained within a central canal in the spinal column.

Sporadically: Occurring intermittently, randomly, or in isolation.

SSRIs: Selective serotonin reuptake inhibitors. Drugs belonging to this class are antidepressant agents that selectively inhibit the absorption of serotonin
at certain nerve membranes (e.g., presynaptic neuronal membranes). These drugs increase the concentration of serotonin within the central nervous system and enhance serotonin's neurotransmission activities.

Stereotactic: refers to use of precise coordinates to identify deep structures of the brain. The coordinates may be obtained by fitting a patient's head with a special frame and taking a CT or MRI scan. The position of the brain structures relative to the frame permits fine localization of the deep brain structures. Stereotactic methods are used during brain surgery for tremor, Parkinson's disease, and dystonia. These brain structures are located with precise, three-dimensional coordinates.

Stereotypic: Inappropriate, persistent repetition of particular bodily postures, actions, or speech patterns. These are typically involuntary, rhythmic,
coordinated, and purposeless movements, postures, or vocalizations that may appear ritualistic or purposeful in nature. Stereotypies may be associated with a variety of neurologic and behavioral disorders, such as Tourette syndrome, obsessive-compulsive disorders, Rett syndrome, restless legs syndrome, schizophrenia, and autism. Stereotypical: Conforming to a repetitive pattern as in repetition of particular movements or gestures.

Stretch reflex: Contraction of a muscle stimulated by rapid stretching.

Stretch-loop circuits: Pathways of electrical impulses along specific nerve fibers (alpha motoneurons) that result in a "stretch" reflex in a muscle.

Striatum: An area of the brain that controls movement and balance. It is connected
to and receives signals from the substantia nigra.

Subarachnoid hemorrhage: Bleeding in the area surrounding the brain, that is usually a result of the rupturing of a cerebral aneurysm in the brain.

Substantia nigra: A dark band of gray matter deep within the brain where cells manufacture the neurotransmitter dopamine for movement control. Degeneration of cells in this region may lead to a neurologic movement disorder such as Parkinson's disease.
Sydenham's chorea:
A usually self-limited condition in which chorea develops in association with an inflammatory disease caused by certain strains of streptococci bacteria. This disease, known as rheumatic fever, is characterized by the sudden onset of fever and joint pain, with subsequent inflammation of the heart (carditis), chest pain, skin rash, and other symptoms. If rheumatic fever involves the nervous system, Sydenhams chorea may develop.
This condition commonly affects children aged 5 to 15 or women during pregnancy. Sydenham's chorea involves involuntary, uncontrollable, jerky movements that gradually worsen in severity, potentially affecting arm movements, the manner of walking (gait), and speech. In most patients, the condition spontaneously resolves in weeks or months.
Sympathetic nervous system:
Part of the nervous system that along with the parasympathetic nervous system forms the autonomic nervous system (ANS).
The ANS regulates the functioning of involuntary structures, such as the glands, smooth muscle, and heart.
The sympathetic nervous system regulates certain involuntary responses during times of strong emotion, such as fear or anger; exercise; or other forms of stress. These responses, sometimes referred to as the "fright-or-flight response," include widening of the pupils; increased heart and breathing rates; constriction of most blood vessels, raising blood pressure; widening of those blood vessels that supply skeletal muscles; and reduction in the rate of peristalsis.

Synapse: The junction between two neurons or between a neuron and an effector organ. As a nerve impulse reaches a synapse, the terminal or end of the "presynaptic" neuron's axon releases neurotransmitters, which diffuse across the gap and bind to receptors of the "post-synaptic" neuron or the effector organ (i.e., muscle or gland). As the electrical impulse is conducted across the gap, electrical changes are triggered that serve to continue or
hinder transmission of the impulse.

Synthesis: The formation of a complex chemical compound through the union of simpler substances.

Tardive dyskinesia: A movement disorder that may result from extended therapy with certain anti-psychotic medications such as haloperidol.   The condition is characterized by involuntary, rhythmic movements of the face, jaw, mouth,
and tongue, such as lip pursing, chewing movements, or protrusion of the tongue.
Facial movements are sometimes accompanied by involuntary, jerky or writhing motions (choreoathetoid movements) of the trunk, arms, and legs.
In some patients, symptoms discontinue months or years after withdrawal of antipsychotic therapy. However, in others, the condition may not be reversible.
Tardive dystonia: A form of tardive dyskinesia characterized by chronic dystonia due to administration of medications that block dopamine D2 receptors (dopamine receptor antagonists), such as certain antipsychotic agents. (Dopamine receptors are molecules on the surfaces of receiving nerve cells that are sensitive to stimulation by dopamine, a neurotransmitter that controls movement and balance. Several types of dopamine receptors have been identified, including D1, D2, and D3.)
Dystonia is a neurologic movement disorder characterized by sustained muscle contractions that often result in repetitive twisting motions or unusual postures or positions.
Tardive dystonia is the most common form of secondary dystonia--i.e., dystonia that results from certain environmental factors or "insults" that affect the brain. In adults, tardive dystonia often initially affects facial or neck muscles. Dystonia may remain limited to such regions or extend to affect adjacent muscles of the trunk and arms. Children are more likely to be affected by generalized dystonia that involves muscles of the trunk and legs.

Tay-Sachs disease:
A progressive neurodegenerative metabolic disorder that belongs to a group of diseases called lysosomal storage diseases. Also known as GM2 gangliosidosis type I or infantile type, Tay-Sachs disease results from deficiency of the enzyme hexo-saminidase A, which leads to an abnormal accumulation of certain fats (i.e., gangliosides) in particular tissues, particularly nerve cells of the brain. An autosomal recessive disorder, Tay-Sachs disease primarily affects individuals of northeastern European Jewish (Ashkenazi Jewish) ancestry. Symptom onset typically begins from about 3 to 6 months of age. Associated symptoms may include an exaggerated startle response, increasing listlessness, loss of previously acquired skills (psychomotor regression), severely
diminished muscle tone (hypotonia), and the development of characteristic, cherry-red circular areas within the middle layers of the eyes (Tay's sign).
With disease progression, affected infants and children may develop increasing muscle stiffness or rigidity; seizures; sudden, involuntary, "shock-like"
contractions of multiple muscle groups in response to certain stimuli (generalized, stimuli-sensitive myoclonus); enlargement of the brain (metabolic megalencephaly); deafness; blindness; and dementia. Life-threatening complications may
develop between 2 to 5 years of age.

Tendon: A tough fibrous cord of tissue that attaches muscle to bone (or other structures of the body).

Teratogenic: Possessing the ability to disrupt normal fetal development and causing fetal abnormalities.

Thalamus: An area of the brain consisting of 2 relatively large masses of gray matter.
The thalamus relays information from most sensory organs to the outer region of the cerebrum or cerebral cortex; receives and processes messages from the body concerning heat, cold, pain, pressure, and touch; and influences motor activity of the cerebral cortex.

There are motor, vocal, and sensory tics.
These are  brief, intermittent stereotyped movements such as blinking, nose
twitching, head jerking, or transient abnormal postures.
Ties which involve
the respiratory and vocal apparatus may result in sounds including sniffs,
snorts, grunts, coughs, and sucking sounds
HD patients may be unaware of vocal tics, but family members may find the incessant noises grating. They should be helped to understand that the tics are not under voluntary control. Tics generally do not by themselves require treatment, but may
respond to neuroleptics, benzodiazepines, or SSRIs.
Tics are involuntary movements, but patients will usually be able to suppress them temporarily. They are increased by stress, excitement, fatigue, and anxiety and decreased by absorbing activities and sleep; a key feature is that they wax and wane.
The range of tic manifestations is broad. Motor tics include abdominal twiddling, fist clinching, foot dorsiflexion, grimacing, head shaking, head jerking, hip flexion/abduction, hitting, jumping, kicking, knee extension, lip protusion, mouth opening, neck stretching, nose flaring, pelvic thrusting, platysma contractions, rubbing, shoulder shrugging, skipping, smelling, sphincter tightening, spitting, squatting, stammering, etc
Vocal tics include barking, belching, clicking, coprolalia, coughing, echolalia, snorting, squeaking etc. There are two general types of motor tics: myoclonic and dystonic. In dystonic tics, a posture is briefly sustained. About 5% of patients with Tourette syndrome have dystonia. In myoclonic tics, the movements are very brief, typically clustered about the face and shoulders.

Transcranial magnetic stimulation is a research technique that uses a magnet to generate a pulse that goes through the skull and directly stimulates the brain beneath it. This activates the fibers controlling muscles, and a surface EMG electrode may be used to measure the response in the muscles. Just as stimulating a nerve may give information about conduction through the nerve, stimulating the motor areas of the brain gives information about
conduction from the brain to the spinal cord and muscles. TMS is currently used only for research; however, in the future, it may become a more widely
used technique.

Tone: Resistance or tension in a muscle when a limb or other body part is moved passively and in a relaxed state about a joint. A state of muscle tension balanced by partial contraction or alternate contraction and relaxation.

Torticollis: The involuntary turning of the neck to one side that can be seen in disorders of the basal ganglia.

Tourette Syndrome
May be the most dramatic and publicized manifestation of tics. By definition it is a chronic syndrome of both motor and vocal tics that last more than a year according to the DSM III. It occurs in 1 in 10,000 and mosten in males with an onset at 2 to 15 years.
The syndrome may be very mild and go untreated throughout life or may be so severe as to confine the patient to bed. The patient is able to voluntarily suppress the movements for minutes or hours but then experiences an irrestible pressure to discharge the movements or the
vocalizations. The symptoms may vary over time and remit spontaneously in one third by late adolescence.
Tourette syndrome is a neuropsychiatric disorder. Comorbid distrubances include obsessive-compulsive disorder, attention-deficit hyperactivity disorder, learning difficulties, and other behavioral, emotional, or psychiatric problems, visual and auditory perceptual abnormalities, language disorders, and sleep disturbances.
Obsessive-compulsive disorder and  ADHD may precede the appearance of tics and occur in about 50% to 60% of children with Tourette syndrome. Behavioral or mental problems may overshadow the tic disorder.
Toxic metabolites: Potentially harmful substances formed as the result of normal body functions.

A parasitic disease that affects the brain that occurs in patients who are immunosuppressed (such as those individuals with AIDS)

Tremor: Rhythmic, involuntary, oscillatory (or to-and-fro) movements of a body part.

Tropical spastic paraparesis (HTLV-1-associated myelopathy): A rare disorder characterized by slowly progressive weakness (paraparesis), stiffness (rigidity), and spasticity of the leg muscles due to infection with the human T-cell lymphotropic virus-1 (HTLV-1). Modes of transmission include sexual contact, mother-to-child transmission (e.g., via breastfeeding), and blood transfusion.
Truncal ataxia (gait/walk)
Patients with truncal ataxia caused by damage to the cerebellar vermis or
associated pathways tend to have a wide-based, unsteady gait, and become more unsteady when attempting to keep their feet close together.

Unilateral:  Affecting, pertaining to, or confined to one side only.

Unverricht-Lundborg's disease
(Baltic myoclonic epilepsy): A form of progressive myoclonic encephalopathy (PME) characterized by the development of repeated seizures or episodes of uncontrolled electrical activity of the brain (epilepsy); sudden, "shock-like" muscle contractions that may be induced by voluntary movements or in response to certain external stimuli (action or reflex myoclonus); and eventual impairment of coordination, postural instability, and other associated findings (i.e., cerebellar ataxia).
Although mental deterioration may also be associated with the disorder, it is typically milder than that seen with Lafora's disease (another form of PME). Unverricht-Lundborg's disease is thought to be
inherited as an autosomal recessive trait. Symptom onset typically begins from about age 6 to 13.
The disorder is slowly progressive;
however, the degree of disease progression and disability may be extremely variable among affected family members.

Upper motor neurons: Nerve cells extending from the brain to the spinal cord that control movement.

Variable expressivity:
Varying manifestation of a genetic trait. The term "expressivity" refers to the degree to which a hereditary trait appears in
an individual. Thus, in individuals with a gene mutation for an autosomal dominant disorder that has variable expressivity, the specific characteristics that are manifested may vary in range and degree from mild to severe.

Vascular: Pertaining to or containing blood vessels.

Vasomotor: Pertaining to the muscles and nerves that control blood vessel diameter, thereby regulating or modulating blood pressure.

Ventral intermediate (VIM) nucleus:
A specific region of the thalamus. This area of the brain is involved in the control of movement and is the "target" area for thalamotomy and deep brain stimulation when treating patients with tremor.

Visual-evoked potentials - are similar to somatosensory potentials; however, the stimulation is applied as patterns or flashes of light seen by the eye, and the brain's response to the visual stimulus is then assessed with EEG electrodes.

Referring to or characterized by virulence or the degree to which an invading microorganism, such as a bacterium or virus, is able to produce disease. Measures of virulence may be based upon the micro-organism's ability to invade bodily tissues and the severity of the disease produced.

White matter: The lipid rich myelinated portion of the brain and spinal cord.

Wilson disease:
(Often ruled out first when testing a patient for a neurological disease of "unknown origin" such as HD.) A rare genetic disorder of copper metabolism, leading to an excessive accumulation of copper in certain tissues and organs, including the liver, brain, kidneys, or corneas of the eyes. Without prompt, appropriate treatment, the disorder may result in progressive liver disease, degenerative changes of the brain, psychiatric abnormalities, and other symptoms.
Neurologic findings may include tremor; involuntary, rapid, jerky movements combined with relatively slow, writhing movements (choreoathetosis); impaired muscle tone and sustained muscle contractions, producing repetitive movements and abnormal posturing; increasingly slurred speech; and difficulties swallowing.
Some patients may also experience increasing irritability, anxiety, severe depression, unusual behaviors, or other psychiatric problems.

X-linked dominant trait: Human traits, such as an individual's specific blood group, eye color, or expression of certain diseases, result from the interaction of one gene inherited from the mother and one from the father. In X-linked dominant disorders, the gene mutation for the disease trait is transmitted as a dominant gene on the X chromosome and therefore may "override" the instructions of the normal gene on the other chromosome, resulting in expression of the disease.
Because females have two X chromosomes yet only one functioning copy is necessary, a percentage of X chromosomes with the mutated gene may not be expressed (random X chromosome inactivation). As a result, females often have less severe symptoms than affected males. In contrast, because males have one X chromosome from the mother and one Y chromosome from the father, those who inherit an
X-linked dominant disease trait typically fully express the mutated gene on the X chromosome, causing a more severe form of the disorder that may result in lethality before or shortly after birth.