The pathophysiology is congenital agenesis of the sixth nerve nucleus and innervational misdirection of part of the medial rectus nerve to the lateral rectus muscle. The result is limited abduction and co-contraction of the medial and lateral recti on adduction. The co-contraction causes the globe to retract, producing lid fissure narrowing on adduction. If the medial and lateral recti receive the same reciprocal innervation, the eye will rest in primary position (equal). The result is a stand-off with equal medial and lateral forces, so there is limited abduction and adduction or Duane’s type 3 syndrome. In cases where the lateral rectus muscle receives more innervation, an exotropia Duane’s type 3 syndrome occurs. In most cases, the medial rectus muscle gets most of the medial rectus nerve, so the resting eye position is in adduction. This is esotropia Duane’s syndrome type 1 (limited abduction, intact adduction). Duane’s syndrome type 2, characterized as good abduction with poor adduction, is very rare (if it exists at all), and most cases are actually Duane’s type 3. Synergistic divergence is a rare type of Duane’s syndrome with abduction on attempted adduction. The lateral rectus muscle receives almost all the innervation from the medial rectus nerve, and there is agenesis of the sixth nerve nucleus [1]. Duane’s syndrome is usually sporadic without systemic manifestations, but it can be familial or associated with systemic disease (Goldenhar syndrome, Klippel-Feil syndrome, and intrauterine thalidomide exposure).

Congenital fibrosis syndromes (CFS) are associated with tight, fibrotic muscles, with the medial and inferior rectus most often involved. CFS is usually inherited as an autosomal dominant trait. There has been evidence that maldevelopment of the oculomotor (III), trochlear (IV), and abducens (VI) nuclei is associated with congenital fibrosis [6]. In rare cases, there may be hypoplasia or aplasia of the inferior rectus muscles associated with muscle fibrosis without craniofacial dysostosis syndromes or neurofibromatosis [7].

These conditions are discussed in other chapters:

Dissociated strabismus complex occurs in approximately 40 % of patients with infantile esotropia, usually presenting after 2 years of age. Dissociated strabismus can, however, occur primarily or secondarily with any disorder that disrupts normal binocular visual development. Two types have been described: dissociated vertical deviation (DVD) and dissociated horizontal deviation (DHD), with DVD being the most commonly recognized form. Dissociated strabismus has three components: elevation, abduction, and extorsion. The vertical component is predominant with DVD, and the horizontal component is predominant with DHD. Dissociated strabismus is typically latent and is manifest when one eye is occluded for several seconds. It can also become manifest spontaneously, often occurring when the patient is fatigued or is daydreaming.

DVD is characterized by a slow drift of one eye up and out with slight extorsion. It is almost always bilateral but is often asymmetric. DVD can be distinguished from a true hypertropia by the lack of a corresponding hypotropia in the contralateral eye when the ipsilateral hypertropic eye returns to primary position (Fig. 7.3).

The appearance of DHD resembles consecutive exotropia, with an eye that will intermittently drift outward, especially when the child is tired or daydreaming. In unilateral cases, covering the DHD eye induces an exodrift, but there is no drift when the fellow eye is covered. DHD can be unilateral, bilateral, or asymmetric. In cases where there is a small residual esotropia plus a unilateral DHD, alternate cover testing will disclose an exo-shift (inward movement) from the DHD eye, while the fellow eye will show an eso-shift (outward movement). If the DHD is larger than 10–15 PD, it can be symptomatic and may need surgery.

Strabismus associated with thyroid eye disease occurs as inflamed muscles change to become fibrotic and stiff. All the extraocular muscles are affected, but the inferior rectus (IR) and medial rectus muscles are most severely involved. Asymmetric IR fibrosis will cause a hypotropia with limited elevation of both eyes, worse on the side of the hypotropic eye. Fibrosis of the medial rectus muscles results in an esotropia. A combination of hypotropia and esotropia is common. Involvement of the lateral rectus muscle is uncommon, however; if there is an exotropia, one must rule out a history of prior orbital decompression or coexistent myasthenia gravis. Surgery should be performed only after the acute inflammatory phase subsides, when the angle of strabismus is stable.