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Adrenoleukodystrophy

Adrenoleukodystrophy (ALD) is a degenerative disorder of the sheath covering nerve fibers, known as myelin. A type of leukodystrophy, the victims of ALD are typically male, as the disease is usually inherited in a sex-linked manner on the X chromosome. Leukodystrophies are disorders that affect the growth and/or development of myelin, a complex fatty neural tissue that insulates many nerves of the central and peripheral nervous systems. Without myelin, nerves are unable to conduct an impulse, leading to increasing disability as myelin destruction increases and intensifies. more...

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Leukodystrophies are different from demyelinating disorders such as multiple sclerosis, in which myelin is formed normally, but is lost by immunologic dysfunction or other reasons.

Symptoms

The clinical presentations is largely dependent on the age of onset of the disease. The most frequent type is the childhood-onset one, which normally occurs in males between the ages of 5 and 10 and is characterized by failure to develop, seizures, ataxia, adrenal insufficiency and degeneration of visual and auditory function.

In the adolescent-onset form, the spinal cord dysfunction is more prominent and therefore is called adrenomyeloneuropathy or "AMD". The patients usually present with weakness and numbness of the limbs and urination or defecation problems. Most victims of this form are also males, although female carriers rarely exhibit symptoms similar to AMD.

Adult and neonatal (which tend to affect both males and females and be inherited in an autosomal recessive manner) forms of the disease also exist but they are extremely rare. Some patients may present with sole findings of adrenal insufficiency (Addison's disease).

Diagnosis

The diagnosis is established by clinical findings and the detection of serum long chain fatty acid levels. MRI examination reveals white matter abnormalities, and neuroimaging findings of this disease are quite reminiscent of the findings of multiple sclerosis. Genetic testing for the analysis of the defective gene is available in some centers.

Pathophysiology

The most common form of ALD is X-linked (the defective gene is on the X chromosome, location Xq28), and is characterized by excessive accumulation of very long chain fatty acids (VLCFA) - fatty acids chains with 24-30 carbon atoms (particularly hexacosanoate, C26) in length (normally less than 20). This was originally described by Moser et al in 1981.

The gene (ABCD1 or "ATP-binding cassette, subfamily D, member 1") codes for a protein that transfers fatty acids into peroxisomes, the cellular organelles where the fatty acids undergo β-oxidation (Mosser et al 1993). A dysfunctional gene leads to the accumulation of long-chain fatty acids.

The precise mechanisms through which high VLCFA concentrations cause the disease are still (2005) unknown, but accumulation is severe in the organs affected.

The prevalence of X-linked adrenoleukodystrophy is approximately 1 in 20,000 individuals. This condition occurs with a similar frequency in all populations.

Treatment

While there is no cure for the disease, some dietary treatments, for example, Lorenzo's oil in combination with a diet low in VLCFA, have been used with limited success, especially before disease symptoms appear. A recent study by Moser et al (2005) shows positive long-term results with this approach.

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Adrenoleukodystrophy
From Gale Encyclopedia of Medicine, 4/6/01 by John T. Lohr

Definition

Adrenoleukodystrophy is a rare genetic disease characterized by a loss of myelin surrounding nerve cells in the brain and progressive adrenal gland dysfunction.

Description

Adrenoleukodystrophy (ALD) is a member of a group of diseases, leukodystrophies, that cause damage to the myelin sheath of nerve cells. Approximately one in 100,000 people is affected by ALD. There are three basic forms of ALD: childhood, adult-onset, and neonatal. The childhood form of the disease is the classical form and is the most severe. Childhood ALD is progressive and usually leads to total disability or death. It affects only boys because the genetic defect is sex-linked (carried on the X chromosome). Onset usually occurs between ages four and ten and can include many different symptoms, not all of which appear together. The most common symptoms are behavioral problems and poor memory. Other symptoms frequently seen are loss of vision, seizures, poorly articulated speech, difficulty swallowing, deafness, problems with gait and coordination, fatigue, increased skin pigmentation, and progressive dementia.

The adult-onset form of the disease, also called adrenomyeloneuropathy, is milder, progresses slowly, is usually associated with a normal life span, and usually appears between ages 21-35. Symptoms may include progressive stiffness, weakness, or paralysis of the lower limbs and loss of coordination. Brain function deterioration may also been seen. Women who are carriers of the disease occasionally experience the same symptoms, as well as others, including ataxia, hypertonia (excessive muscle tone), mild peripheral neuropathy, and urinary problems. The neonatal form affects both male and female infants and may produce mental retardation, facial abnormalities, seizures, retinal degeneration, poor muscle tone, enlarged liver, and adrenal dysfunction. Neonatal ALD usually progresses rapidly.

Causes & symptoms

The genetic defect in ALD causes a decrease in the ability to degrade very long chain fatty acids. These build up in the adrenal glands, brain, plasma, and fibroblasts. The build-up of very long chain fatty acids interferes with the ability of the adrenal gland to convert cholesterol into steroids and causes demyelination of nerves in the white matter of the brain. Demyelinated nerve cells are unable to function properly.

Diagnosis

Diagnosis is made based on observed symptoms, a biochemical test, and a family history. The biochemical test detects elevated levels of very long chain fatty acids in samples from amniocentesis, chorionic villi, plasma, red blood cells, or fibroblasts. A family history may indicate the likelihood of ALD because the disease is carried on the X-chromosome by the female lineage of families.

Treatment

Treatment for all forms of ALD consists of treating the symptoms and supporting the patient with physical therapy, psychological counseling, and special education in some cases. There is no cure for this disease, and there are no drugs that can reverse demyelination of nerve and brain cells. Dietary measures consist of reducing the intake of foods high in fat, which are a source of very long chain fatty acids. A mixture called Lorenzo's Oil has been shown to reduce the level of long chain fatty acids if used long term; however, the rate of myelin loss is unaffected. Experimental bone marrow transplantation has not been very effective.

Prognosis

Prognosis for childhood and neonatal ALD patients is poor because of the progressive myelin degeneration. Death usually occurs between one and ten years after onset of symptoms.

Prevention

Since ALD is a genetic disease, prevention is largely limited to genetic counseling and fetal monitoring through amniocentesis or chorionic villus sampling.

Key Terms

Amniocentesis
The collection of amniotic fluid through a needle inserted through the abdomen. Used to collect fetal cells for genetic analysis.
Ataxia
Loss of coordination of muscular movement.
Hypertonia
Having excessive muscular tone.
Myelin
A layer that encloses nerve cells and some axons and is made largely of lipids and lipoproteins.
Neuropathy
A disease or abnormality of the peripheral nerves.

Further Reading

For Your Information

    Books

  • Berkow, Robert. Merck Manual of Medical Information. Whitehouse Station, NJ: Merck Research Laboratories, 1997.

    Other

  • Adrenoleukodystrophy. http://www.ninds.nih.gov/healinfo/DISORDER/Adrenoleukodystrophy/adrenoleuko.htm.
  • Adrenoleukodystrophy (ALD): A Peroxisomal Disorder. http://www.biology.washington.edu/bsa/bio...Papers/AB%Adrenoleukodystrophy%202.html.

Gale Encyclopedia of Medicine. Gale Research, 1999.

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