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A 10 year old boy complains of severe right ear pain of one week’s duration. Diplopia started two days ago. Examination shows cloudiness and redness of the
right eardrum (tympanic membrane). He has reduced abduction of the right eye and esotropia. The optic fundus examination is normal. Imaging shows opacification of the right petrous apex.
The lesion causing the neuro-ophthalmic abnormality likely to lie in
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Correct!
The probable cause of the diplopia is a right sixth nerve palsy. The eardrum abnormalities are likely due to 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 from the mastoid region of the temporal bone to its petrous 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.” Infections and cancers come from a lateral source in the petrous bone. They come from a medial source in the clivus (a favorite site for cancer metastasis). They come from an inferior source in the sphenoid sinus (infections and cancer).
You cannot be faulted if you chose “transverse-sigmoid (dural) sinus junction” as your answer. You might have been aware that 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 (“otitic hydrocephalus”). Why does high intracranial pressure cause a sixth nerve palsy? 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 encounter a comitant esotropia with full ocular ductions, representing a breakdown in the brain’s ability to maintain bifoveal fixation (“ocular fusion”).
How can you connect that infection with the sixth nerve palsy? By assuming that the infection has spread from the mastoid region of the temporal bone to its petrous 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.” Infections and cancers come from a lateral source in the petrous bone. They come from a medial source in the clivus (a favorite site for cancer metastasis). They come from an inferior source in the sphenoid sinus (infections and cancer).
You cannot be faulted if you chose “transverse-sigmoid (dural) sinus junction” as your answer. You might have been aware that 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 (“otitic hydrocephalus”). Why does high intracranial pressure cause a sixth nerve palsy? 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 encounter a comitant esotropia with full ocular ductions, representing a breakdown in the brain’s ability to maintain bifoveal fixation (“ocular fusion”).
Incorrect