Isolated Trochlear Nerve Palsy Caused by a Cavernous Sinus Dural Arteriovenous Fistula: A Case Report.

Cavernous sinus dural arteriovenous fistula (CS-dAVF) usually presents with ocular signs such as conjunctival hyperemia, proptosis, and diplopia. Isolated cranial nerve palsy without ocular signs is uncommon, and isolated trochlear nerve palsy as the sole manifestation is particularly rare. An octogenarian female presented to our institution with acute diplopia. The Bielschowsky head tilt test revealed her right trochlear nerve palsy. No abnormalities found in other cranial nerves. Magnetic resonance angiography and digital subtraction angiography identified a dural shunt from the right meningohypophyseal trunk to the posterolateral margin of the right cavernous sinus without reflux into cortical veins or the ophthalmic vein, establishing the diagnosis of CS-dAVF. The long course of the trochlear nerve makes it vulnerable to hemodynamic or mechanical stress. Localized shunt inflow to the lateral compartment can produce selective trochlear nerve palsy without ocular signs. This case indicates that CS-dAVF should be among the differential diagnoses of isolated trochlear nerve palsy. Given the absence of cortical venous reflux, the patient was managed conservatively with observation.