Duane syndrome

Duane syndrome (DS) is a rare, congenital eye movement disorder most commonly characterized by the inability of the eye to abduct or move outwards. The syndrome was first described by Jakob Stilling (1887) and Siegmund Türk (1896), and subsequently named for Alexander Duane who discussed the disorder in more detail in 1905

Other names for this condition include: Duane's Retraction Syndrome (or DR syndrome), Eye Retraction Syndrome, Retraction Syndrome, Congenital retraction syndrome and Stilling-Turk-Duane Syndrome.

Characteristics
As described by Duane, the characteristic features of the syndrome are:


 * 1) Limitation of abduction (outward movement) of the affected eye.
 * 2) Less marked limitation of adduction (inward movement) of the same eye.
 * 3) Retraction of the eyeball into the socket on adduction, with associated narrowing of the palpebral fissure (eye closing)
 * 4) Widening of the palpebral fissure on attempted abduction. (N.B. Mein and Trimble point out that this is "probably of no significance" as the phenomenon also occurs in other conditions in which abduction is limited).
 * 5) Poor convergence
 * 6) A face turn to the side of the affected eye to compensate for the movement limitations of the eye(s) and to maintain binocular vision.
 * 7) Eye is 45ﾟ to left or right, resulting in "correct movement", but wrong placing of eye. (i.e. when an unaffected eye looks to the right, the affected eye looks straight forward, and when the unaffected eye looks straight forward, the affected eye looks to the left)

In addition to the above, other characteristics may or may not be present, and these include:


 * 1) Elevation or 'upshoot' of the affected eye on adduction. More rarely, 'down shoots' can also occur.
 * 2) Head movements to compensate for loss of eye movement when attempting to view an object outside of binocular viewing range (which may be very narrow).
 * 3) Absence of diplopia: Few patients with the syndrome complain of diplopia when their eyes are misaligned.

In about 80% of cases, only one eye is affected (with the left being predominant in a 3:1 ratio). However, in some cases, both eyes are affected, with one eye usually more affected than the other.

Causes
DS is a miswiring of the eye muscles, causing some eye muscles to contract when they shouldn't and other eye muscles not to contract when they should. Alexandrakis G & Saunders RA state that: ''In most cases, the abducens nucleus and nerve are absent or hypoplastic, and the lateral rectus muscle is innervated by a branch of the oculomotor nerve. However, there may be contributing mechanical abnormalities'' This view is supported by the earlier work of Hotchkiss et al. who reported on the autopsy findings of two patients with Duanes syndrome. In both cases the sixth cranial nerve nucleus was absent, as was the sixth nerve, and the lateral rectus muscle was innervated by the inferior division of the third or oculomotor cranial nerve. This misdirection of nerve fibres results in opposing muscles being innervated by the same nerve. Thus, on attempted abduction, stimulation of the lateral rectus via the oculomotor nerve will be accompanied by stimulation of the opposing medial rectus via the same nerve; a muscle which works to adduct the eye. Thus, co-contraction of the muscles takes place, limiting the amount of movement achievable and also resulting in retraction of the eye into the socket. This process is called synkinesis.

The mechanical factors noted by Miller and Clark above are generally regarded as arising secondary to loss of innervation. During corrective surgery fibrous attachments have been found connecting the horizontal recti and the orbital walls and fibrosis of the lateral rectus has been confirmed by biopsy. This fibrosis can result in the lateral rectus being 'tight' and acting as a tether or leash. Co-contraction of the medial and lateral recti allows the globe to slip up or down under the tight lateral rectus producing the up and down shoots characteristic of the condition.

Epidemiology
Most patients are diagnosed by the age of 10 years and DS is more common in girls (60 percent of the cases) than boys (40 percent of the cases). A recent French study reports the following findings: ''The incidence of this syndrome in the population of strabismic patients was 1.9%. The number of women affected was 83 (53.5%). The syndrome was unilateral in 121 cases (78.1%). The left eye (71.9%) was affected more frequently than the right.'' Around 10-20% of cases are familial; these are more likely to be bilateral than non-familial Duane syndrome. Duane syndrome has no particular race predilection. While usually isolated to the eye abnormalities, Duane syndrome can be associated with extraocular problems (so-called "Duane's Plus"), including cervical spine abnormalities (Klippel-Feil syndrome), Goldenhar syndrome, autism, heterochromia, and thalidomide-induced embryopathy.

Classification
Two different classification systems have been proposed, by Brown and by Huber, of which Brown's remains in more common clinical use.

Brown's classification
Brown classified the condition using the following three sub-types, based upon clinical observations: Type A: Limited abduction and less limited adduction (as described originally by Duane). Type B: Limited abduction but normal adduction, and Type C: In which limitation of adduction is greater than limitation of abduction, giving rise to a divergent deviation and a head posture in which the face is turned away from the side of the affected eye.

Huber's classification
Huber's classification system was based upon electromyographical examinations: Type I: Marked limitation of abduction (corresponds to Brown's Type B) explicable by maximum innervation reaching the lateral rectus only when the affected eye is adducted. Type II: Limitation of adduction (corresponds to Brown's Type C)which Hubel explains as being caused by co-innervation of both medial and lateral recti on attempted adduction, and Type III: Limitation of both adduction and abduction (corresponds to Brown's Type A) which Hubel explains as due to co-contraction, accompanied by a loss of innervation to the lateral rectus on attempted abduction.

Differential diagnosis
Disorders similar in presentation to Duane syndrome can be acquired as a result of trauma, or following localised infection of the orbit leading to inflammation and consequent mechanical restrictions of eye movement. In such cases a full case history will usually help in distinguishing between these conditions. In the clinical setting, the principal difficulties in differential diagnosis arise as a consequence of the very early age at which patients with this condition first present. The clinician must be persistent in examining abduction and adduction, and in looking for any associated palpebral fissure changes or head postures, when attempting to determine whether what often presents as a common childhood squint is in fact Duane syndrome. Fissure changes, and the other associated characteristics of Duane's such as up or down shoots and globe retraction, are also vital when deciding whether any abduction limitation is the result of Duane's and not a consequence of VI or abducens cranial nerve palsy.

Treatment
The majority of patients remain symptom free and able to maintain binocularity with only a slight face turn. Amblyopia is uncommon and, where present, rarely dense. This can be treated with occlusion, and any refractive error can also be corrected.

Duane syndrome cannot be cured, as the "missing" cranial nerve cannot be replaced and, traditionally there has been no expectation that surgery will result in any increase in the range of eye movement. Surgical intervention, therefore, has only been recommended where the patient is unable to maintain binocularity, where they are experiencing symptoms, or where they are forced to adopt a cosmetically unsightly or uncomfortable head posture in order to maintain binocularity. The aims of surgery are to place the eye in a more central position and, thus, place the field of binocularity more centrally also, and to overcome or reduce the need for the adoption of an abnormal head posture. Occasionally surgery is not needed during childhood, but becomes appropriate later in life, as head position changes (presumably due to progressive muscle contracture).

Surgical approaches
In types A and B the most commonly adopted approach remains the recession of the medial recti of the affected eye, or of both eyes where the condition is bilateral. In type C, recession of the lateral recti is the method of choice.

Dr. Arthur Rosenbaum at UCLA has championed the use of Vertical Rectus Transposition surgery. He reports significant increases in lateral eye movements in many cases. The VRT realigned muscles work with the good medial muscle to provide a "tripod" of musculature for the eye; the newly moved muscles provide torque and tension against the medial muscle, which is what allows for the central alignment. And, because they are "working" muscles, they also may allow for some range of lateral movement in their new positions. Morad et al. report the use of a similar approach in Duane's Type C.

Prominent down- or upshoots may be treated with a special form of muscle surgery, a "Y-splitting" procedure, often combined with a recession.