Saturday, 11 January 2014

Duane Syndrome

Duane syndrome, also called Duane retraction syndrome (DRS), is a group of eye muscle disorders that cause abnormal eye movements. People with Duane syndrome have difficulty rotating one or both eyes outward (abduction) or inward (adduction).



Six muscles, which control the movement of the eye, are attached to the outside of the wall of the eye. In each eye, there are two muscles that move the eye horizontally. The lateral rectus muscle pulls the eye out towards the ear and the medial rectus muscle pulls the eye in towards the nose. There are four other muscles, which move the eye up or down and at an angle. Each eye muscle receives the command for movement from cranial nerves that exit the brain.

Cause

Duane syndrome is due to miswiring of the eye muscles. The “mistake” probably happens around the 6th week of pregnancy and is due to poor development of tiny parts of the brain stem that control the eye muscles.
In Duane syndrome, the sixth cranial nerve that controls the lateral rectus muscle (the muscle that rotates the eye out towards the ear) does not develop properly. Why the nerve does not develop is not yet understood.

                      


Thus, the problem is not primarily with the eye muscle itself, but with the nerve that transmits the electrical impulses to the muscle. There is also irregular innervations of a branch from the third cranial nerve, which controls the medial rectus muscle (the muscle that rotates the eye toward the nose). This is why abnormalities may be found in both left gaze and right gaze.


Affect

Duane syndrome affects girls more often than boys. In addition, the left eye is more often involved than the right eye. The reason for this is not known. Around 20% of Duane syndrome patients have both eyes affected. No particular race or ethnic group is more likely to be affected.

Characteristics of Duane syndrome

Strabismus-the eyes may be misaligned and point in different directions at all times
Head position-patients often maintain a head posture or head turn to keep the eyes straight.
Amblyopia-reduced vision in the affected eye.
Eyelid narrowing-the affected eye may appear smaller than the other eye Upshoot or downshoot-with certain eye movements, the eye may occasionally deviate upward or downward.



Onset

Duane retraction syndrome is present from birth, even if it is not recognized during infancy. An abnormal head posture and strabismus are often visible in old photographs taken in early childhood.
In 90% of cases, the patient has no family history of Duane syndrome. Ten percent of patients will have an affected family member and these tend to be cases where both eyes are involved. There is currently no test that can determine whether a patient has a hereditary form.


Types of Duane Syndrome

Duane syndrome is often characterized by whether the primary abnormality is a reduced ability to turn the affected eye(s)
 Outward (type I)
 Inward (type II)
 Both (type III)
Type I is the most common form of Duane syndrome. And affected patients will characteristically have a head turn towards the involved side, and will appear esotropic (crossing inward) in straight ahead gaze.

 

Association with other Eye Problems


The problem with the 6th cranial nerve is usually an isolated condition and the child is usually otherwise completely normal. With careful follow-up, the long-term prognosis for good vision is usually excellent.
Occasionally, Duane syndrome may be found in association with other eye problems, including disorders of other cranial nerves, nystagmus (an involuntary back-and-forth movement of the eyeball), cataract, optic nerve abnormalities, microphthalmos (abnormally small eye), and crocodile tears.
 Non-ocular medical problems
Not usually, however, some patients with Duane syndrome have other problems, such as hearing impairment, Goldenhar syndrome, spinal and vertebral abnormalities. There is also an increased frequency of Duane syndrome in patients with thalidomide exposure.

Treatment

For the majority of patients, Duane syndrome does not require surgical treatment. Surgery for Duane syndrome is indicated for one of four reasons:
 To reduce strabismus
 To eliminate a socially unacceptable head position
 To eliminate a significant upshoot or downshoot.
 To eliminate disfiguring enophthalmos.
The goal of treatment is to restore satisfactory eye alignment in the straight-ahead position, eliminate an abnormal head posture and to prevent amblyopia. In most cases, eye muscle surgery is required. Because the function of the affected nerve and muscle cannot be restored, the other eye muscles are adjusted to compensate and allow for better eye alignment.

Surgical Treatment


Surgery cannot fix the problem of nerves that are miswired.  By moving the eye muscles surgery can compensate for the miswiring. Because surgery doesn’t “really” fix the problem, surgery cannot restore normal eye movement, but surgery can (and usually does) substantially improve the situation. The full effect of the surgery may take some weeks to become apparent.  There is a low incidence of unexpected or inadequate results.