Palsies of the trochlear nerve: diagnosis and localization--recent concepts

In this review, the anatomy of the trochlear nerve, the diagnosis of palsies of the trochlear nerve, and the localization of lesions of the trochlear nerve are discussed. Paresis of the superior oblique muscle is often not evident on duction testing; therefore, subjective diplopia testing with use of a Maddox rod is often necessary. The torsional component of the deviation may be evaluated by double Maddox rod testing. Palsies of the trochlear nerve must be distinguished from other causes of vertical diplopia, such as oculomotor palsy, skew deviation, myasthenia gravis, and Graves' ophthalmopathy. Trauma is the most common cause of isolated, unilateral or bilateral, acquired palsies of the trochlear nerve when a cause can be determined. The localization of lesions of the trochlear nerve to the nucleus or fascicles (or both), subarachnoid space, cavernous sinus and superior orbital fissure, or orbit depends on the associated damage to neighboring neurologic structures. Myokymia of the superior oblique muscle is usually idiopathic and benign but may rarely be an isolated manifestation of tectal disease.