A 46-year-old man presents to the emergency department with complaint of diplopia. He reports that his symptoms began last night and reports no prior visual disturbance and has no associated symptoms. His past medical history is significant for hypertension and type 2 diabetes. On exam his pupils are symmetrical and reactive to light bilaterally, but extraocular movements reveal his left eye can only move laterally and the same eye is pointed ‘down and out’. What is the most appropriate next step in management of this patient’s condition?
1.Edrophonium (tensilon)test
2.MRI of the head
3.CT of the head without contreast
4.Reassure and follow up in 2 months
Explanation:
The patient in this scenario has an isolated palsy of the oculomotor nerve (CN III), as evidenced by his extraocular movements revealing that his left eye is “down and out”. The extraocular muscles are innervated by 3 different cranial nerves. Patterns of innervations are as follows:
Cranial 4 (Trochlear): innervates the superior oblique
Cranial Nerve 6 (Abducens): innervates the lateral rectus
Cranial Nerve 3 (Oculomotor): innervates all the remaining muscles (ie medial rectus, inferior oblique, superior rectus and inferior rectus).
Know this mnemonic: “SO-4, LR-6, All the rest 3” (ie Superior Obliqueby CN 4, Lateral rectus by CN 6, and all the other EOMs by CN 3).
When trying to isolate a problem, it can help to check movement in the direction in which that muscle is the primary mover. This can be assessed as follows:
Superior oblique: Depresses the eye when looking medially
Inferior oblique: Elevates the eye when looking medially
Superior rectus: Elevates the eye when looking laterally
Inferior rectus: Depresses the eye when looking laterally
Medial rectus: Adduction when pupil moving along horizontal plane
Lateral rectus: Abduction when pupil moving along horizontal plane
In a patient over 40 with a history of diabetes or hypertension, an isolated CN 3, 4 or 6 palsy without any other neurologic deficits or accompanying pain will typically resolve in 2 months on it’s own without requiring any further intervention or imaging. If it does not resolve in 2 months, you can perform an MRI of the head to rule out an intracranial pathology.