Correct! The pattern of ocular misalignment fulfills the features of the Parks-Bielschowsky “three-step test,” a strong indication of a fourth nerve palsy. Acquired causes of fourth nerve palsy, including trauma, tumor, ischemia, and inflammation, typically cause a hypertropia that is greater in downgaze than in upgaze. Because the hypertropia in left gaze is greater here in upgaze than in downgaze, the palsy is likely to be very long-standing, possibly even congenital, with “decompensation” in adulthood. Why does this decompensation occur? Consider the anatomy of the superior oblique muscle and its tendon. It is awkward. The muscle passes forward along the medial orbital wall, passes through a sleeve of dura called the trochlea, and must turn more than 90 degrees to insert onto the top of the eye.

Perhaps it does not take much for the muscle to give out if its innervation was not perfect to begin with. Sometimes the diplopia in this condition can be palliated with a spectacle prism. But because of the incomitance, the prism is usually an imperfect or temporary solution, and eye muscle surgery is the answer. A minor procedure (weakening of the inferior oblique muscle) is likely to provide relief of diplopia. With regard to the other answer choices here: although myasthenia gravis can produce any pattern of ocular misalignment, it is unlikely to cause such “three-step test” positivity. The same is true of inflammatory orbitopathy. Cerebral aneurysm can cause an isolated third nerve palsy, and you are right to worry about that diagnosis. However, the pattern of misalignment does not fit. By the way, the intermittency of the diplopia is likely to vex you. Here are two good reasons for that phenomenon in any case of ocular misalignment: 1) the misalignment is present only in certain fields of gaze, and 2) fusion is breaking down because of fatigue or CNS depressants, converting a phoria to a tropia.