Cascade-like Oculomotor Misinnervation

A 6-month-old boy with a history of microcephaly and a type II atrioseptal defect was investigated for oculomotor abnormalities. There was no family history of oculomotor abnormalities.

Examination showed an absence of adduction of the left eye on attempted right gaze and during attempted convergence. Instead, simultaneous abduction of both eyes was observed (Figure 1A). However, rhythmic adductive movements with an amplitude of approximately 10° were elicited upon sucking (Figure 1D). We observed both a slight narrowing of the left palpebral fissure and adductive movements simultaneously. Exotropia was observed in the primary position and abduction of the left eye was limited on left gaze (Figure 1B and C). Upgaze and downgaze were normal. No ptosis or enophthalmos were correlated with these findings. Pupillary function was normal, with no unusual changes in eye movements. Right-eye motility was unremarkable.

Congenital ocular misinnervation can occur in a variety of forms. It typically involves the sixth cranial nerve. Most common is the Duane, or congenital retraction, syndrome, which consists of hypoplasia of the sixth nerve (nucleus) and innervation of the lateral rectus muscle by a branch of the oculomotor nerve.¹

Depending on the relative contribution of third nerve fibers to the medial and lateral rectus, the patient is first with either predominant limitation of abduction or adduction. Some of our patient's clinical features can be explained by an extreme form of the Duane syndrome, where most, if not all, oculomotor nerve branch fibers originally directed to the medial rectus innervate the lateral rectus, thus leading to simultaneous abduction on attempted adduction (also referred to as synergistic divergence).²

Interestingly, although no left-eye adduction could be elicited on lateral gaze or convergence, we observed adductive movements during sucking. Missing innervation of the medial rectus by oculomotor nerve fibers was replaced by fibers, most likely originating from a motor branch of the trigeminal nerve. Thus, the lack of innervation of the lateral rectus (Figure 2A) appears to have triggered a sequence of aberrant nerve sprouting, resulting, initially, in a shift of fibers originally meant for the medial rectus toward the lateral rectus (Figure 2B). Second, and possibly as a consequence of lack of innervation of the medial rectus muscle, a shift of trigeminal nerve motor fibers to the medial rectus took place (Figure 2C), leading to a trigemino-oculomotor synkinesis between the lateral pterygoid or one of the suprahyoid muscles and the medial rectus. The slight narrowing of the palpebral fissure observed during sucking can be explained by synkinetic contraction of the medial muscle against a tight lateral rectus muscle in that specific situation, which led to discrete retraction of the globe.

The combination of the Duane and Marcus Gunn syndromes or other misinnervation syndromes involving the sixth, third, and fifth cranial nerves^3,4 recurs. Other cases with synergistic divergence and a trigemino-oculomotor synkinesis have been reported⁴ (also associated with the congenital fibrosis syndrome⁵). The recurring combination may lead to the assumption that these miswirings coincide and are precipitated by the same teratogenic disturbance during the second month of pregnancy, when extraocular muscle innervation develops.⁶ However, the apparent sequence of misinnervations seen in our patient and a similar patient who was previously reported⁷ prompt us to postulate that, in some cases, such miswirings not only happen simultaneously, but that one provokes the other to make a sufficient innervational stimulation possible at the motor end plate in the absence of original nerve sprouting.

Correspondence: Dr Safran, Ophthalmology Clinic, Department of Clinical Neurosciences, Geneva University Hospitals, 22 Rue Alcide Jentzer, 1211 Geneva 14, Switzerland (a.b.safran@hcuge.ch).

Financial Disclosure: None reported.