Moebius Syndrome

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Moebius syndrome

Definition

Moebius syndrome is a condition in which the facial nerve is underdeveloped, causing paralysis or weakness of the muscles of the face. Other nerves to the facial structures may also be underdeveloped.

Description

Moebius syndrome has been called "life without a smile" because the paralysis of the facial muscles, the most constant feature, leads to the physical inability to form a smile even when happy feelings are experienced. The facial nerve is one of a group of 12 nerves known as the cranial nerves because they originate in the brain. The facial nerve is also known as the seventh cranial nerve. The sixth cranial nerve, also called the abducens, controls blinking and back-and-forth eye movement and is the second most commonly affected cranial nerve in Moebius syndrome. Additional cranial nerves affected in some patients control other eye movements and other functions such as hearing, balance, speech, and feeding.

Individuals with Moebius syndrome may also have abnormalities of their limbs, chest muscles, and tongue. The chance of mental retardation appears to be increased in people with Moebius syndrome, but most people with the disorder have normal intelligence.

Genetic profile

Most cases of Moebius syndrome are isolated and do not appear to be genetic, but occurrence in multiple individuals within some families indicates that there are multiple genetic forms. One study in 1991 suggested that forms of Moebius syndrome which included abnormalities of the limbs and skeleton were less likely than other types to be genetic. During pregnancy, certain exposures, such as to the drug misoprostol, appear to increase the risk of Moebius syndrome.

Chromosomes 13, 3, and 10 appear to contain genes causing forms of Moebius syndrome, now named, respectively, types 1, 2, and 3. The presence of a gene on chromosome 13 was first suggested based on a family in which several members had facial weakness and finger abnormalities along with a chromosome rearrangement called a balanced translocation involving chromosomes 1 and 13. In a balanced translocation, two chromosomes have broken and exchanged pieces. Balanced translocations are usually not associated with physical abnormalities unless (1) material has been lost or gained during the breaks, or (2) a gene is disrupted by one of the breaks. When a child with Moebius syndrome in an unrelated family was found to have a deletion (missing piece) of chromosome 13 in the same area as the break in the first family, this suggested that there might be a gene causing Moebius syndrome on chromosome 13 rather than on 1.

The genes on chromosomes 3 and 10 were localized using a technique called linkage mapping, which involves using molecular genetics and statistical methods to look throughout all of the chromosomes in families with several affected members for areas associated with the disease. As of 2001, the actual genes on chromosomes 3, 10, and 13 have not been identified. These three forms of the disease are inherited in an autosomal dominant manner, which means that only one altered copy of the gene is required to have the disease, and people with the disease have a 50% chance of having an affected child with each pregnancy. However, in the chromosome 3 and 10 families, some individuals who appear to carry a gene do not show signs of Moebius syndrome, suggesting that factors other than genetics, such as uterine environment, are involved even in these highly familial cases.

One family was reported in which two brothers and their male cousin who were the sons of sisters all had Moebius syndrome along with other physical abnormalities and mental retardation. Boys only have one X chromosome and can inherit an X-linked disease from their unaffected mothers, who have two X chromosomes. The pattern of affected children in this family is therefore typical of X-linked inheritance , so it is suggested that there may be a gene involved in Moebius syndrome on the X chromosome as well. If this is the case, the son of a woman with an altered Moebius gene on one X-chromosome would have a 50% chance of inheriting the gene and having the condition. A man with this type of Moebius syndrome would be unlikely to have affected children since his daughters would likely have one normal X chromosome from their mother and his sons would not receive his X chromosome but his Y chromosome. In another family, a brother and sister with unaffected parents had Moebius syndrome, suggesting autosomal recessive inheritance, in which two altered copies of a gene are required to have the disorder. In an autosomal recessive disorder, a couple in which each parents carry one altered copy of the disease gene have a 25% chance of having a child with the condition with each pregnancy.

Demographics

Moebius syndrome is extremely rare and does not seem to affect any particular ethnic group more than others. The families in which genes on chromosomes 3 and 10 were mapped were Dutch.

Signs and symptoms

The first sign of Moebius syndrome in newborns is an inability to suck, sometimes accompanied by excessive drooling and crossed eyes. Also seen at birth in some patients are abnormalities of the limbs, tongue, and jaw. Children also often have low muscle tone, particularly in the upper body. The lack of facial expression and inability to smile become apparent as children get older.

When cranial nerve palsy is associated with limb reduction abnormalities and the absence of the pectoralis muscles, the condition is known as Poland-Moebius or Möebius-Poland syndrome. Common limb abnormalities are missing or webbed fingers and clubfoot .

The prevalence of mental retardation in Moebius syndrome is uncertain. It has been estimated in the past to be between 10% and 50%, but these numbers are thought to be overestimates resulting from the lack of facial expression and drooling seen in people with Moebius syndrome. In one study of familial cases of Moebius syndrome, 3% were reported to be mentally retarded.

Diagnosis

Diagnosis of Moebius syndrome is made on the basis of clinical symptoms, especially the lack of facial expression. Since exact genes involved in Moebius syndrome have not yet been identified, molecular genetic testing is not available at this time.

Treatment and management

The ability to smile has been restored in some cases of Moebius syndrome by surgery which transfers nerve and muscle from the thigh to the face. Other surgeries can be used to treat eye, limb, and jaw problems. In children with feeding problems, special bottles or feeding tubes are used. Physical and speech therapy are used when necessary to improve control over coordination, speech, and eating.