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Purpose To summarize the clinical, neuroradiologic, and genetic observations in a group of patients with unilateral synergistic divergence (SD). Methods Five unrelated patients with unilateral SD underwent ophthalmologic and orthoptic examinations; three of them also had magnetic resonance imaging of the brain and orbits. Three patients underwent genetic evaluation of genes known to affect ocular motility (KIF21A, PHOX2A, HOXA1, and ROBO3). Results Patients did not meet clinical criteria for CFEOM types 1, 2, or 3. Each patient had severe adduction weakness on the affected side and a large angle exotropia in primary gaze that increased on attempted contralateral gaze because of anomalous abduction. Magnetic resonance imaging revealed a much smaller medial rectus muscle in the involved SD orbit. Oculomotor cranial nerves were present in the one patient imaged appropriately. Genetic sequencing in three patients revealed no mutations in KIF21A, PHOX2A, HOXA1, or ROBO3. Conclusions SD should be classified as a distinct congenital ocular motility pattern within congenital cranial dysinnervation disorders. It is possibly caused by denervation of the medial rectus with dysinnervation of the ipsilateral lateral rectus by the oculomotor nerve precipitated by genetic abnormalities (some currently identified) or by local environmental, teratogenic, or epigenetic disturbances.

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