Marcus Gunn phenomenon is a rare genetic disorder that is usually present at birth. It is characterized by the movement of one upper eyelid in a rapid rising motion each time the jaw moves. Other eye abnormalities and vision difficulties may also occur. The exact cause of this phenomenon is not known.
In most patients with Marcus Gunn phenomenon, the upper eyelid of one eye (unilateral) droops (blepharoptosis). The major symptom of this disorder is the rapid and involuntary upward movement of the affected eyelid after almost any movement of the lower jaw. As a result, that eye appears to open even wider. This first becomes apparent soon after birth, especially during feeding and sucking. These activities cause the eyelid to move up and down. A few cases have been seen in adults.
Individuals with Marcus Gunn phenomenon may experience visual impairment, such as crossing of the eyes (strabismus), slight impairment of vision in one eye as compared to the other (a difference in the refractive powers of the eyes [anisometropia], and other problems (e.g., superior rectus muscle palsy or double elevator palsy).
In some cases, symptoms may be produced by movement of the lips, whistling, smiling, clenching the teeth, chewing, puffing out the cheeks, or swallowing.
The exact cause of Marcus Gunn phenomenon is not known. For reasons that are unclear, there is a bad connection between the nerves that stimulate, and the muscles that move, the eyelids and the jaw. Some clinical researchers believe that about half of the cases of Marcus Gunn phenomenon are genetically determined. The site and nature of the gene(s) at fault in this disorder are not known, but familial studies indicate that it is transmitted as an autosomal dominant trait.
Chromosomes, which are present in the nucleus of human cells, carry the genetic information for each individual. Human body cells normally have 46 chromosomes. Pairs of human chromosomes are numbered from 1 through 22 and the sex chromosomes are designated X and Y. Males have one X and one Y chromosome and females have two X chromosomes. Each chromosome has a short arm designated p and a long arm designated q. Chromosomes are further sub-divided into many bands that are numbered. For example, chromosome 11p13 refers to band 13 on the short arm of chromosome 11. The numbered bands specify the location of the thousands of genes that are present on each chromosome.
Genetic diseases are determined by the combination of genes for a particular trait which are on the chromosomes received from the father and the mother.
Dominant genetic disorders occur when only a single copy of an abnormal gene is necessary for the appearance of the disease. The abnormal gene can be inherited from either parent, or can be the result of a new mutation (gene change) in the affected individual. The risk of passing the abnormal gene from affected parent to offspring is 50% for each pregnancy regardless of the sex of the resulting child.
Marcus Gunn phenomenon is a rare genetic disorder present at birth. It affects males and females in equal numbers. Approximately 300 cases have been reported in the medical literature.
The diagnosis is obvious and is often made by one or both of the parents who become aware of the major symptom during feeding.
Treatment of Marcus Gunn phenomenon is usually not needed because the condition tends to grow less noticeable with age. Genetic counseling may be of benefit for patients and their families. Other related eye problems such as strabismus, amblyopia, etc., may be corrected with eyeglasses, surgery and/or drugs.
Surgical correction of the droopiness of the eyelid or of the jaw winking is recommended only if the patient (or parents) agree on which of the symptoms is the more cosmetically objectionable. Surgical procedures are available for several different intensities of the droop of the eyelids.
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