Brainstem stroke presenting as isolated bilateral ptosis

Background

Among cerebral infarctions, pure midbrain infarctions are a rarity with prevalence rates ranging between 0.6% and 2.3%.1 The midbrain is primarily supplied by the posterior cerebral circulation, including the basilar, superior cerebellar and posterior cerebral arteries. The blood supply is complex, with variations in vascularity at the inferior colliculus, superior colliculus and pretectal levels. These three levels are further divided axially into medial and lateral zones. The medial zone at each level is supplied by the basilar artery and its paramedian branches.

At the level of the inferior colliculus, the lateral zone receives branches from the superior cerebellar artery. At the level of the superior colliculus and the pretectal level, the lateral zone is supplied by the posterior cerebral artery. However, the superior colliculus and adjacent tectum are supplied by the superior cerebellar artery. As such, posterior circulatory embolic strokes appear to be the most common aetiology for midbrain infarction, though often they do involve surrounding structures (pons, thalamus and cerebellum) with the resultant clinical manifestations.2 3

Pure midbrain lesions are infrequent and often are associated with oculomotor nerve palsy. Clinical manifestations are in keeping with the area and extent of the stroke; whereby, paramedian lesions result in ocular motor weakness and ataxia; anterolateral lesions result in hemiparesis and hemiataxia; lateral lesions cause sensory disturbances and combined anteromedial/anterolateral lesions result in hemiparesis and oculomotor disturbances.1 3–6 An isolated bilateral ptosis is a rare manifestation of a midbrain infarction.

We present the case of an elderly man who presented to the eye emergency with isolated bilateral ptosis and was subsequently diagnosed with a small acute perforator infarct in the midbrain.

Case presentation

Our patient, an 89-year-old man (background of diabetes and hypertension), presented to the Birmingham Midland Eye Centre emergency department with acute onset bilateral ptosis (right side worse than the left), which had suddenly occurred that morning, while he was watching television. He also reported feeling generally unwell and unsteady while walking. On examination, his ocular movements were normal and full range, with no nystagmus, no diplopia, no anisocoria and no relative afferent pupillary defect. He had normal colour vision and visual acuity. There was no obvious facial palsy. There was bilateral ptosis, more marked on the right side. He was transferred to the medical admissions unit for further investigation.

He also reported upper respiratory symptoms suggestive of a viral infection that self-resolved 3 weeks ago. There was no significant family history of neuromuscular disorders and no headache. His cranial nerve examination was normal except for bilateral ptosis, which was more marked on the right side. The rest of the cranial nerve examination was normal. He had no incoordination and had a normal gait. His limb tone and power were normal. There was no pronator drift. He had normal flexor plantar reflexes. A CT scan of his head reported changes in keeping with mild to moderate periventricular chronic ischaemic small vessel disease but no acute intracranial haemorrhage, extra-axial collection, space occupying lesion or acute major territorial infarction.

Neurology saw the patient and agreed with the majority of findings but noted a subtle element of fatigability. In keeping with an isolated bilateral ptosis without extraocular eye movement abnormality or pupil involvement, the differentials were ocular myasthenia, postviral sequelae or a stroke. Given his presentation, the patient was started on pyridostigmine 30 mg three times a day and also received 300 mg of aspirin while awaiting an MRI diffusion-weighted scan.

Investigations

The patient subsequently underwent a multiplanar MRI of the head, which reported a tiny focus of restricted diffusion seen in the right periaqueductal white matter of the midbrain with corresponding T2 high signal suggesting small acute perforator infarct (figure 1). At this point, the pyridostigmine was stopped and aspirin was continued. He had a carotid Doppler, which revealed diffuse atherosclerotic changes with no flow-limiting lesions in the internal carotid arteries and normal direction of flow in the vertebral arteries. A transthoracic echocardiogram revealed no obvious intracardiac source of emboli, good left ventricular function and no significant valve abnormalities.

Differential diagnosis

Given that our patient did demonstrate some forms of mild fatigability, a differential to consider would include a much more common cause of bilateral ptosis—myasthenia gravis.7 However, the acute onset and lack of extraocular muscle involvement made this less likely.8 Another differential to consider would be structural lesions in the midbrain such as an inflammatory granuloma, tumour or demyelination. Infectious causes such as tuberculoma9 or neurocysticercosis10 have been previously shown to present with isolated bilateral ptosis. However, these would have been identified on the MRI.10

Treatment

Our patient was started on aspirin for 2 weeks, followed by clopidogrel and discharged with stroke team follow-up.

Outcome and follow-up

Unfortunately, the patient then developed right-sided weakness both in the upper and lower limbs, 5 months after his presentation to us with ptosis. A repeat MRI showed an acute lacunar infarct in the left corona radiata. On examination, he had mildly reduced power in his right lower limb hip flexion 4/5 and knee flexion 4+. He had no arm weakness, facial weakness or speech disturbance and had no sensory deficit. His gait was slow but had no spastic features. He was seen by the stroke specialist and initiated on dual antiplatelet therapy with aspirin and clopidogrel for 1 month and thereafter to continue on clopidogrel alongside his usual statin, antihypertensive and antidiabetic medication.