History
Mr. Lam was a 70-year-old man. He is an ex-social drinker and a non-smoker.
He has a history of hyperlipidemia, mild coronary artery disease, and a transient ischemic attack in 2022 (transient left monocular visual loss).
He presented with a 6-month history of gradual-onset binocular multidirectional diplopia with bilateral asymmetrical ptosis. These symptoms were variable over time, with diurnal variation within a day, segregated by asymptomatic periods in between. The ocular symptoms were exacerbated by prolonged reading and in the evenings.
He denied the presence of limb or neck weakness, bulbar symptoms, or respiratory symptoms.
Physical examination
Physical examination was notable for an initial unremarkable extra-ocular movement testing and the absence of ptosis.
Mr. Lam began to report multidirectional diplopia upon prolonged testing, with a right eye elevation deficit becoming apparent towards the end of the examination. The elevation velocity of the right eye from a depressed position to a neutral position was normal. The elevation of the right eye from a neutral position to the elevated position was halted abruptly midway. The actions of depression, adduction, and intorsion were intact.
Investigations
Baseline blood tests and a plain CT of the brain were unrevealing.
Though process
With the variable bilateral ocular symptoms, the pathology was likely localized to the neuromuscular junction.
A strategic midbrain lesion could result in such bilateral ocular symptoms without contralateral limb involvement. This is because the superior rectus subnucleus crosses the midline, and the levator palpebrae superioris muscles are controlled by a single central subnucleus. However, one would also anticipate a fixed extraocular movement impairment in the oculomotor nerve-innervated muscles.
The etiology was typically ocular-onset myasthenia gravis. The differential diagnosis would include a structural orbital lesion leading to the isolated right eye elevation deficit.
Next steps
Blood was sent for anti-AChR. Nerve conduction test with repetitive nerve stimulation would be the next step if the antibody testing were to be negative. A CT scan of the thorax with contrast should be arranged to look for a thymoma if the antibody serology came back to be positive.
A trial of Mestinon was initiated.