Paralytic strabismus or squint is caused by weakness or paralysis of one or more of the extraocular muscles. Dysfunction of the fourth cranial nerve (trochlear nerve) can result from lesions anywhere along its path between the fourth nerve nucleus in the midbrain and the superior oblique muscle within the orbit. These lesions can be congenital or acquired.
The manifestations and diagnosis of fourth nerve palsy, also known as superior oblique paralysis and trochlear nerve palsy, are reviewed here. Palsies of the third and sixth cranial nerves are discussed separately. (See "Third cranial nerve (oculomotor nerve) palsy in children" and "Sixth cranial nerve (abducens nerve) palsy in children".)
The fourth cranial nerve (trochlear nerve) has the longest intracranial course and is the only cranial nerve that has a dorsal exit from the brainstem (figure 1). It begins in the midbrain at the level of the inferior colliculus as fascicles extending from the fourth nerve nuclei. The trochlear nerves cross as they exit dorsally in the anterior medullary velum. The fourth nerve passes between the superior cerebellar artery and the posterior cerebral artery and runs in the subarachnoid space. It travels within the lateral wall of the cavernous sinus and enters the orbit via the superior orbital fissure to innervate the superior oblique muscle [1-10]. The primary action of the superior oblique muscle is intorsion of the eye, but it has additional eye movements of abduction and depression (table 1).
The long course of the fourth cranial nerve renders it particularly prone to injury from blunt head trauma or compression from changes in intracranial pressure, brain tumors, or swelling anywhere along its course. (See "Elevated intracranial pressure (ICP) in children".)
Children with fourth nerve palsies may complain of binocular (ie, present with both eyes open) vertical diplopia and/or subjective tilting of objects (torsional diplopia). This subjective torsion can be measured objectively with a double Maddox rod. The affected eye usually is extorted because the superior oblique muscle is an intorter of the eye. Objects viewed in primary position or especially in down-gaze may appear double (eg, when going down a flight of stairs so that the child does not know which step to select).