A 67-year-old man presents to the emergency department 1 hour after sudden-onset weakness and double vision. His medical history is significant for hyperlipidemia and poorly controlled hypertension. His current medications include atorvastatin and amlodipine. His temperature is 98.6°F (37.0°C), blood pressure is 175/95 mmHg, pulse is 90/min, respirations are 16/min. On cranial nerve exam, his left pupil is 6 mm in diameter compared to 3 mm on the right. His left eye is displaced laterally and downward and he also has left-sided ptosis. Strength is 2/5 in the right upper and right lower extremities and 5/5 in the left upper and left lower extremities. He has marked right-sided dysdiadochokinesia. Sensation is intact throughout. Which of the following regions of his brain is most likely affected?
Lateral medulla
Lateral pons
Posterior limb of internal capsule
Midbrain
Midbrain
This patient with a history of poorly controlled hypertension, acute onset of left-sided oculomotor nerve palsy (mydriasis, ptosis, and “down and out” pupil), right-sided paresis, and right-sided dysdiadochokinesia most likely has a lacunar stroke of the left midbrain.

Important structures in the midbrain often affected by stroke include the oculomotor nucleus, substantia nigra, corticospinal tract, red nucleus, and superior cerebellar peduncle. Occlusions to branches of the basilar artery lead to infarctions of various parts of the midbrain. Midbrain lesions almost always manifest with an ipsilateral oculomotor palsy (ptosis from the levator palpebrae muscle; mydriasis from loss of parasympathetic inflow; abduction and excyclotorsion of the globe from the superior rectus, medial rectus, inferior rectus, and inferior obliques). There may also be contralateral ataxia (cerebellar peduncle after decussation and red nucleus), contralateral hemiparesis (corticospinal tract before decussation at the medulla), and parkinsonian tremor (substantia nigra).

Sciacca et al. review the anatomy and clinical signs of strokes of the midbrain, pons, and medulla. They discuss how a stroke of the midbrain will present with oculomotor palsy and paresis. They recommend being vigilant for symptoms of strokes to allow for early diagnosis.

Incorrect Answers:
Answer A: Lateral medullary syndrome (Wallenberg syndrome) is usually due to a stroke of the posterior inferior cerebellar artery. This presents with ipsilateral ataxia, loss of pain and temperature sensation in the face, dysarthria, and Horner syndrome, as well as contralateral loss of pain and temperature sensation in the body. In this patient with intact sensorium and without dysarthria, lateral medullary syndrome is less likely.

Answer B: Lateral pontine syndrome can be caused by interruption of blood supply by the anterior inferior cerebellar artery. It presents with ipsilateral limb and gait ataxia, ipsilateral Horner syndrome, ipsilateral facial hemianesthesia, ipsilateral facial paralysis, nystagmus and vertigo, and contralateral loss of pain and temperature in the body. In this patient with intact sensorium and without facial paralysis, vertigo, and oculomotor palsy, midbrain stroke is more likely than a pontine stroke.

Answer C: Medial pontine syndrome can be due to occlusion of the paramedian branches of the basilar artery and disrupts the abducens nerve, the dorsal column-medial lemniscus, and the corticospinal tract. This results in contralateral spastic hemiparesis, ipsilateral lateral rectus paralysis (eye turned down and in), and contralateral loss of vibration and stereognosis. In this patient with ptosis, and an eye turned down and out, oculomotor palsy and a midbrain stroke are more likely than a pontine stroke.

Answer E: Strokes of the posterior limb of the internal capsule lead to contralateral hemiparesis. Strokes of the internal capsule would not be expected to lead to oculomotor nerve palsy or dysdiadochokinesia. The constellation of ataxia, hemiparesis, and cranial nerve palsy is more suggestive of a brainstem pathology than of a lacunar stroke in the internal capsule.

Bullet Summary:
Midbrain strokes commonly present with ipsilateral oculomotor nerve palsy and contralateral hemiparesis/hemiplegia, contralateral hemiataxia, and/or contralateral tremor.