Case 8 – A 30-year-old man with double vision

A 30-year-old man presents to the ophthalmology clinic with a complaint of double vision. He states that when he gazes laterally to the right, he sees two images side by side. He does not have this problem when he looks to the left. Past medical history is significant for depression, for which he takes amitriptyline. He denies any ocular pain or recent viral illness. He also denies any history of tick bite and does not smoke cigarettes, drink alcohol, or use intravenous drugs. On physical examination the patient is a well-appearing, well-nourished man. Visual field testing reveals an adduction deficit in the left eye and horizontal nystagmus in the right eye during extreme right lateral gaze. Extreme right lateral gaze also recreates the painless horizontal diplopia that the patient has been experiencing. Conjugate eye movements are observed in all other directions. Accommodation and convergence are normal.

What is the most likely diagnosis?

Internuclear opthalmoplegia (INO). INO results from lesions of the medial longitudinal fasciculus (MLF), a fiber pathway that normally connects the abducens nerve nucleus (cranial nerve VI) located at the pontomedullary junction to the contralateral oculomotor nucleus (cranial nerve III) in the midbrain. Without coordination of these two nuclei, the medial rectus of the adducting eye cannot coordinate with the lateral rectus of the abducting eye. This leads to the characteristic disconjugate lateral gaze that is a hallmark of INO. In addition, the abducting eye also has end-gaze nystagmus. INO can either be unilateral (as in this patient) or bilateral, in which lateral gaze in either direction will produce diplopia.

What other condition should be considered in the differential diagnosis?

INO can easily be confused with a medial rectus palsy since the affected eye appears to have lost its ability to adduct. However, most patients with INO still have the ability to converge, as this ocular movement pattern (bilateral ocular adduction in response to focusing on an object moving closer to the eyes) does not require an intact MLF. Additionally, INO is often the presenting sign of multiple sclerosis (MS). Most (92%) patients who develop INO because of demyelination will progress to full-blown MS. Such patients require close follow-up so that the diagnosis of MS can be made at an early and more easily managed stage. Another important consideration is that myasthenia gravis (MG), which can be life threatening if left untreated, can also initially mimic the findings of INO. Half of patients with MG present initially with extraocular muscle weakness. As such, patients in whom there is any question of the diagnosis should undergo testing for anti-acetylcholine receptor antibody.

What test(s) should be used to determine the etiology of this condition?

The presence of INO suggests that the patient has a brain stem lesion involving the MLF. There are many disease processes that can create such a lesion, including:

1. Brain stem and fourth ventricular tumor
2. Brain stem infarction
3. Drug intoxication (eg, phenothiazines, tricyclic antidepressants, toluene, tacrolimus)
4. Lyme disease
5. Multiple sclerosis (the most common cause of bilateral INO in young adults)
6. Trauma
7. Subdural hematoma
8. Syphilis
9. Viral infection

In this patient, a history of tricyclic antidepressant use may explain his symptoms, as this medication is known to cause INO. Other tests to determine an etiology would include a toxin screen, MRI of the brain, FTA-ABS/VDRL (test for syphilis), Lyme titer, fasting blood glucose, complete blood count with differential, and blood pressure measurement.

What is the most appropriate management for this patient?

Treatment is focused on the etiology of the brain stem lesion associated with INO.

By Daniel Osei, MD, Resident in Orthopedic Surgery, Hospital for Special Surgery; in association with Le TT, Schabelman E, Shivaram A, and Klein J, eds: First Aid Cases for the USMLE Step 2 CK. New York: McGraw-Hill, 2007.