Correct! The likely cause of the diplopia is a right sixth nerve palsy. His ear abnormalities are likely caused by an infected middle ear (“otitis media”) with spread of the infection to the mastoid region. How can you connect that infection with the sixth nerve palsy? By assuming that the infection has spread through the petrous bone to its apex, where the bone forms the lateral wall of Dorello’s canal, the narrow space through which the sixth nerve travels to get from the subarachnoid space into the cavernous sinus.

The association between middle ear infection and sixth nerve palsy was described long ago by Gradenigo and is now known as “Gradenigo’s syndrome.” The sixth nerve in the tight space of Dorello’s canal is vulnerable to a “compartment syndrome” as infections and cancers come from a lateral source in the petrous bone, from a medial source in the clivus (a favorite site for cancer metastasis), and from an inferior source in the sphenoid sinus (infections and cancer). You cannot be faulted here if you chose “transverse-sigmoid (dural) sinus junction” as your answer. Mastoid inflammation from middle ear infections can cause venous sinus thrombosis at the transverse-sigmoid junction, raising intracranial pressure and causing a secondary sixth nerve palsy. Why does high intracranial pressure cause a sixth nerve palsy? Because the high pressure drives the brain downward, tugging on the sixth nerve where it is tethered in Dorello’s canal. But in such cases, papilledema is usually present (and it was not in this boy). By the way, low intracranial pressure also drives the brain downward, which is why sixth nerve palsy also occurs in intracranial hypotension. A final tip: when intracranial pressure goes up, you might instead encounter a comitant esotropia with full ocular ductions, representing a breakdown in “ocular fusion.”

Incorrect