Exotropia, with or without internuclear ophthalmoplegia (INO), has been reported as a presenting feature of vertebrobasilar ischemic strokes. The combination of INO and contralateral exotropia was first described as “non-paralytic pontine exotropia” (NPPE) by Bogousslavsky and Regli in 1983. Although the terminology is not widely adopted, it distinguishes a milder form from the classic paralytic pontine exotropia. In patients with vascular risk factors, the most common etiology of INO is ischemic infarction.

A 58-year-old woman with hypertension, and diabetes mellitus, presented with acute left exotropia and gait imbalance upon awakening.  Examination revealed left exotropia with right hypertropia, consistent with a skew deviation. On attempted leftward gaze, the right eye failed to adduct, while the left eye fully abducted with associated left-beating nystagmus. An adduction limitation was initially misinterpreted as partial third nerve palsy. Preservation of convergence helped differentiate INO from third nerve palsy and a pseudo-INO from myasthenia or Guillain-Barre. Optokinetic testing demonstrated absent horizontal optokinetic nystagmus (OKN) in the right eye, while vertical OKN remained intact bilaterally. These findings supported a diagnosis of right INO with contralateral non-paralytic exotropia. Brain MRI revealed an acute infarct in the right dorsomedial pons, although up to 36% of INO cases may show no abnormalities on imaging. Mechanism of stroke was related to uncontrolled vascular risk factors and patient was placed on dual anti-platelet therapy. 

INO is a clinical diagnosis, classic presentations are straightforward, but milder or atypical variants can be overlooked or misdiagnosed. INO should be considered in patients with unexplained adduction deficits and suspected brainstem involvement, especially those with vascular risk factors. Early recognition is critical in the context of stroke, as timely reperfusion can prevent permanent neurologic damage. While imaging is valuable, absence of radiographic evidence does not rule out INO, reinforcing the importance of detailed neuro-ophthalmological examination.