Definition of Alternating hemiplegia syndrome

Alternating hemiplegia syndrome: See: Alternating hemiplegia of childhood.

Definition of Alport syndrome

Alport syndrome: An hereditary condition characterized by kidney disease, sensorineural (nerve) deafness and sometimes eye defects.
The classic disorder as described by Alport in 1927 is nephritis (inflammation of the kidney), often progressing to renal failure, and sensorineural (nerve) hearing loss affecting both sexes in successive generations. The kidney disease becomes evident as recurrent microscopic or gross hematuria (blood in the urine) as early as childhood, and usually earlier in males than in females. Progression to renal failure is gradual and usually occurs in males by the fifth decade.
The sensorineural (nerve) hearing loss in Alport syndrome primarily affects high tones and occurs in 30 to 50% of relatives with renal disease. The severity of the auditory and renal features do not correlate in a given individual with Alport syndrome.
Alport syndrome is not one disease. It has diverse genetic causes; there are X-linked and autosomal dominant and recessive forms of Alport syndrome. There are also diverse clinical types of Alport syndrome including, for example, juvenile-onset Alport syndrome with deafness, adult-onset Alport syndrome with deafness, and adult Alport syndrome without deafness.
The X-linked disorder that has come to be known as Alport syndrome is characterized by hematuria (blood in the urine), progressive renal failure, and sensorineural (nerve) hearing loss and is frequently associated with both eye abnormalities (such as lens and retinal anomalies) as well as the identification of mutations in the gene encoding what is called the basement membrane specific type IV collagen alpha-5 chain (COL4A5), an X-linked gene. This syndrome was proven to be an X-linked dominant disorder.

Definition of Aicardi syndrome

Aicardi syndrome: A genetic disorder characterized by the partial or complete agenesis of the corpus callosum (the structure that links the 2 hemispheres of the brain), infantile spasms (a characteristic form of childhood seizures), mental retardation, and an ocular (eye) abnormality called chorioretinal lacunae in which there are lacunae (holes) in the retina of the eye.
Aicardi syndrome may be associated with other brain defects such as microcephaly (small brain) or porencephalic cysts (cerebrospinal fluid-filled cavities or gaps in the brain). Features associated with Aicardi syndrome include cleft lip and/or palate, fatty tumors (lipomas) of the scalp, blood vessel malformations (cavernous hemangiomas), rib and vertebral defects and scoliosis (curved spine).
The genetics of Aicardi syndrome are extraordinary. The disorder affects only females. It is an X-linked dominant trait lethal in males. In a hemizygous male (whose only X chromosome contains the Aicardi gene), the Aicardi gene is fatal before birth. In a heterozygous female (with the Aicardi gene on one of her two X chromosomes), the Aicardi gene causes Aicardi syndrome. The Aicardi gene has been charted to the short (p) arm of the X chromosome and is in band Xp22.
There is no cure for Aicardi syndrome. Nor is there a standard course of treatment. Treatment is purely symptomatic. It generally involves management of seizures and programs for the mental retardation.
The prognosis for individuals with Aicardi syndrome varies according to the presence and severity of symptoms.

Definition of Agenesis, sacral

Agenesis, sacral: Failure of formation of all or part of the sacrum (the lowest section of the
spine).
Currarino syndrome is a condition characterized by the combination of:

Partial absence of the sacrum (the lowest portion of spine),
Anorectal (anal and rectal) abnormalities, and
An abnormal mass in front of the sacrum (due to a meningocoele or
teratoma).

The malformations in Currarino syndrome are all in tissues that have their
embryological origin in the tail bud and may reflect disturbances in its
formation during early embryonic life. A mutation (change) in a gene called the
HLXB9 homeobox gene has been identified as responsible for autosomal dominant
Currarino syndrome, also known as hereditary sacral agenesis.

Definition of Agenesis of the corpus callosum

Agenesis of the corpus callosum: A congenital abnormality (a birth defect) in which there is partial or complete absence (agenesis) of the corpus callosum, the area of the brain which connects the two cerebral hemispheres (the two halves of the brain).
Agenesis of the corpus callosum can occur as a severe syndrome in infancy or childhood, as a milder condition in young adults, or as an asymptomatic incidental finding.
If there are symptoms, the first ones are usually seizures followed by feeding problems and delays in holding the head erect, sitting, standing, and walking. The seizures may constitute a very common disorder called infantile spasms. There may also be retardation in mental and physical development and impairment of hand-eye coordination and visual and auditory memory. Hydrocephalus is also a complication. In mild cases, symptoms (such as seizures, repetitive speech, and/or headaches) may not appear for years.
Girls with agenesis of the corpus callosum may have a specific condition called Aicardi's syndrome in which there is severe mental retardation, infantile spasms and chorioretinal lacunae. Agenesis of the corpus callosum can occur as an isolated condition or in association with other cerebral anomalies (such as the Arnold-Chiari malformation and Dandy-Walker syndrome, Andermann syndrome with progressive neuropathy, schizencephaly, holoprosencephaly, and migrational anomalies). Agenesis of the corpus callosum is also associated with several chromosome anomalies, including trisomy 13 and trisomy 18.
There is no standard course of treatment for agenesis of the corpus callosum. Treatment usually involves management of signs and symptoms such as hydrocephalus and seizures if they occur.
The prognosis (outlook) with agenesis of the corpus callosum is variable. The condition does not cause death in the majority of patients. Although many children with the disorder lead normal lives and have average intelligence, careful neuropsychological testing reveals subtle differences in higher cortical function compared to individuals of the same age and education without ACC. Children with agenesis of the corpus callosum accompanied by developmental delay and/or seizure disorders should be screened for metabolic disorders. The mental retardation associated with agenesis of the corpus callosum is not progressive.

Definition of Adrenal cortex

Adrenal cortex: The outer portion of the adrenal gland located on top of each kidney. The adrenal cortex produces steroid hormones which regulate carbohydrate and fat metabolism and mineralocorticoid hormones which regulate salt and water balance in the body.
Underfunction of the adrenal cortex results in Addison disease while overfunction occurs in the adrenogenital syndrome and in Cushing syndrome.

Definition of Acute radiation syndrome

Acute radiation syndrome: An acute illness caused by a dose greater than 50 rads of penetrating radiation to most or all of the body in a short time, usually a matter of minutes. Examples of persons who suffered from acute radiation syndrome (ARS) are the survivors of the Hiroshima and Nagasaki atomic bombs and the firefighters that first responded after the Chernobyl Nuclear Power Plant event in 1986.
A person with ARS usually goes through four stages. In the prodromal stage, the classic symptoms are nausea, vomiting, and possibly diarrhea (depending on dose) that occur from minutes to days following exposure. These symptoms may last (episodically) for minutes up to several days. Then comes the latent stage. In this stage the patient looks and feels generally healthy for a few hours or even up to a few weeks. Then comes the overt or manifest illness stage, In this stage the symptoms depend on the specific ARS syndrome and last from hours up to several months. The last stage is recovery or death. Most patients who do not recover die within several months of exposure. For those who recover, the process lasts from several weeks up to two years.
There are three ARS syndromes: the bone marrow syndrome (or hematologic syndrome), the gastrointestinal (GI) syndrome, and the cardiovascular (CV)/ central nervous system (CNS) syndrome.
The bone marrow syndrome is characterized by anorexia (lack of appetite), fever, and malaise. There is a drop in all blood cell counts for several weeks. The primary cause of death is infection and hemorrhage. The chance of survival decreases with increasing dose of radiation.
The GI syndrome is more severe. It includes severe diarrhea, fever, dehydration, and imbalance in the electrolytes (sodium, potassium, etc). Death is due to infection, dehydration and electrolyte imbalance and usually occurs within 2 weeks of exposure.
The CV/CNS syndrome is the most severe. There is initially extreme nervousness; confusion; severe nausea, vomiting, and watery diarrhea; burning sensations of the skin; and loss of consciousness. After the latent period, 5 to 6 hours after exposure there is return of watery diarrhea, convulsions, and coma and death comes within 3 days of exposure.
ARS is also known as radiation sickness or radiation toxicity.

Definition of Acute lymphoblastic leukemia

Acute lymphoblastic leukemia: An acute (sudden onset), rapidly progressing form of leukemia that is characterized by the presence in the blood and bone marrow of large numbers of unusually immature white blood cells destined to become lymphocytes. Acute lymphoblastic leukemia is also called acute lymphocytic leukemia and is abbreviated ALL (spoken not as the word "all", but as the three letters A-L-L). ALL is the most common cancer occurring in children, representing almost 25% of cancer among children. There is a sharp peak in the incidence of ALL incidence among children ages 2 to 3. This peak is approximately fourfold greater than that for infants and is nearly 10-fold greater than that for youths who are 19 years old.

For unexplained reasons, the incidence of ALL is substantially higher for white children than for black children, with a nearly threefold higher incidence at 2 to 3 years of age for white children compared to black children. The incidence of ALL appears to be highest in Hispanic children.

Factors associated with an increased risk of ALL have been identified. The main environmental factor is radiation, namely prenatal exposure to x-rays or postnatal exposure to high doses of radiation. Children with Down syndrome (trisomy 21) also have an increased risk for both ALL and acute myeloid leukemia (AML). About two-thirds of acute leukemia in children with Down syndrome is ALL. Increased occurrence of ALL is also associated with certain genetic conditions, including neurofibromatosis, Shwachman syndrome, Bloom syndrome, and ataxia telangiectasia.

The malignant lymphoblasts from a particular ALL patient carry antigen receptors unique to that patient. There is evidence to suggest that the specific antigen receptor may be present at birth in some patients with ALL, suggesting a prenatal origin for the leukemic clone. Similarly, some patients with ALL characterized by specific chromosome translocations have been shown to have cells containing the translocation at the time of birth.

Seventy-five to 80% of children with ALL now survive at least 5 years from diagnosis with current treatments that incorporate systemic therapy (e.g., combination chemotherapy) and specific central nervous system (CNS) preventive therapy (i.e., intrathecal chemotherapy with or without cranial irradiation). Ten-year event-free survival of multiple large prospective trials conducted in different countries for children treated primarily in the 1980s is approximately 70%.

Since nearly all children with ALL achieve an initial remission, the major obstacle to cure is bone marrow and/or extramedullary (e.g., CNS, testicular) relapse. Relapse from remission can occur during therapy or after completion of treatment. While the majority of children with recurrent ALL attain a second remission, the likelihood of cure is generally poor, particularly for those with bone marrow relapse occurring while on treatment.

Definition of Acute idiopathic polyneuritis

Acute idiopathic polyneuritis: Also known as the Guillain-Barre syndrome, a disorder characterized by progressive symmetrical paralysis and loss of reflexes, usually beginning in the leg, with in most cases nearly complete or complete recovery.
The Guillain-Barre syndrome is not associated with fever. There is paralysis involving more than one limb, most commonly the legs, and that paralysis is progressive. There is areflexia (loss of reflexes) or hyporeflexia (diminution of reflexes) in the legs and arms. Other conditions that may mimic the Guillain-Barre syndrome need to be ruled out.
The Guillain-Barre syndrome is due to an immune response that results in the direct destruction of the myelin sheath surrounding the peripheral nerves or the axon of the nerve itself.
The syndrome sometimes follow triggering events, including vaccinations. Among the vaccines reportedly associated with Guillain-Barre syndrome are the swine influenza vaccine (in 1976-1977), the oral poliovirus vaccine, and tetanus toxoid. Aside from vaccinations, infection with the bacteria Campylobacter jejuni and viral infections can trigger the Guillain-Barre syndrome.

Definition of Acute brain syndrome

Acute brain syndrome: See: Delirium.

Definition of Acrocephalosyndactyly

Acrocephalosyndactyly: An
inherited disorder causing abnormalities of the skull and
face and the hands and feet.
In acrocephalosyndactyly there is closure too-early of
some of the sutures of the skull (craniosynostosis). This
results in an abnormally shaped head, which is unusually
tall and peaked, and an abnormally shaped face with shallow eye
sockets and underdevelopment of the midface. There is fusion of
fingers and toes (syndactyly) and broad ends of the thumbs and big
toes.
Surgery is often useful to correct the abnormalities of the
skull, face, hands and feet.
Acrocephalosyndactyly is an autosomal dominant trait with boys and
girls affected equally. A affected parent can have transmit the gene
for the disorder or both parents can be normal with the disorder
appearing in the child due to a new mutation.
The best-known type of acrocephalosyndactyly is Apert syndrome
which is due to a mutation in the fibroblast growth factor receptor 2
(FGFR2) gene on chromosome 10. Different mutations in FGFR2 are
responsible for two other genetic diseases, namely,
Pfeiffer syndrome (another type of acrocephalosyndactyly)
and Crouzon syndrome (purely a craniofacial disorder with
no hand or foot problems). All are dominant traits.

Definition of Acquired epileptiform aphasia

Acquired epileptiform aphasia: See Landau-Kleffner syndrome.

Definition of Achoo syndrome

Achoo syndrome: A disorder characterized by nearly uncontrollable paroxysms of sneezing provoked in a reflex fashion by the sudden exposure of a dark-adapted subject to intensely bright light, usually to brilliant sunlight. The number of successive sneezes is usually 2 or 3, but can be up to about 40. The achoo syndrome is also called the photic sneeze reflex or the helio-ophthalmic outburst syndrome.
The syndrome is much more common than has been generally recognized. In one study it was found in 23% of medical students. The syndrome is one of the most frequent of all known genetic traits. It is inherited in an autosomal dominant manner, affecting males and females alike, who have a 50:50 chance of passing the achoo gene to each of their children who will have the disorder. The syndrome was first described in 1978.
One physician reported that he had suffered from photic sneezing for over 20 years and, having just learned of the existence of the syndrome, found that the "symptoms are more easily tolerated if you can put a name to them, even if that produces only an illusory understanding of their significance." He commented on the potential hazards of photic sneezing if it occurs while one is driving a car on a sunny day. Tunnels and shadows created by tall buildings and forested terrain can induce sneezing upon re-entering the bright sunlight.

References alluded to in the text:

WR Collie, RA Pagon, JG Hall, MHK Shokeir: ACHOO syndrome (helio-ophthalmic outburst syndrome). Birth Defects Orig. Art. Ser. XIV(6B): 361-363, 1978. (The original description of the syndrome.)
EW Benbow: Practical hazards of photic sneezing. Brit. J. Ophthal. 75: 447, 1991. (The physician who discovered he had the syndrome.)

Definition of Aarskog-Scott syndrome

Aarskog-Scott syndrome: A syndrome of wide spaced eyes (ocular hypertelorism), front-facing (anteverted) nostrils, a broad upper lip, a malformed ("saddle-bag") scrotum, and laxity of the ligaments resulting in bending back of the knees (genu recurvatum), flat feet, and overly extensible fingers. There are X-linked and autosomal forms of the disease. The gene for the X-linked form has been mapped to chromosome band Xp11.21 and identified as the FGD1 gene.
The disease is named for DJ Aarskog (1928-) and CI Scott, Jr. (1934-), Norwegian and American pediatricians, who described it in 1970 and 1971. It is also known as Aarskog syndrome, faciodigitogenital dysplasia, and faciogenital dysplasia.

Definition of Aarskog syndrome

Aarskog syndrome: See: Aarskog-Scott syndrome.