Isolated Trochlear Nerve Palsy Associated with Carotid–Cavernous Sinus Fistula

SummaryCranial nerve ophthalmoplegia linked with a cavernous sinus lesion usually involves the third, fourth and sixth cranial nerve. Isolated fourth nerve palsy caused by carotid–cavernous sinus fistula (CCSF) is rare, and related case reports are sporadic in the literature. We report a 67-year-old woman with thunderclap-type headache and isolated right-side trochlear nerve palsy. The unique history and possible pathophysiologic mechanism are discussed. We propose that: (1) thunderclap-type headache could serve as a first symptom of CCSF; and (2) isolated trochlear nerve palsy within the cavernous sinus is not unusual. First-line clinicians should be alert and more aware of this entity.