Nuclear lesions: Isolated lesions are very rare. Trauma, ischemia, tumor, and inflammatory diseases may cause nuclear lesions. Patients will present with vertical strabismus and vertical diplopia. Patients usually present with their head tilting in a direction opposite to the muscle paresis. Lesions will affect the contralateral superior oblique muscle if the lesion is in the nucleus and the fascicles before decussation. Congenital absence of the nerve is possible [3].

Cisternal lesions: Neoplasm, aneurysmatic compression, ischemia, increased intracranial hypertension, or traumatic avulsion/contrecoup forces are main causes of trochlear nerve damage (Fig. 15.2). Inflammation/neuritis is also possible. Lesions will cause ipsilateral palsy of the superior oblique muscle. There may be associated hypotrophic superior oblique muscle due to chronic denervation (Fig. 15.3).

Cavernous sinus lesions: Neoplasm, carotid-cavernous fistula, ICA aneurysms, inflammatory granulomatous disease, pituitary lesions, or apoplexy may cause damage to the trochlear nerve in its intradural segment [4]. Direct injury from trauma or surgery may also damage the nerve [5]. Isolated damage to CN IV is unlikely, and usually associated neurologic findings may be found.