by vascular or neoplastic changes in the midbrain, increased intracranial pressure (ICP), large anterior midline craniofacial tumors (e.g., nasopharyngeal carcinoma), otitis media (OM) with involvement of the petrous portion of the sphenoid (Gradenigo syndrome), and any abnormality that involves the cavernous sinus An abnormality of the sixth cranial nerve will cause a reduction in ipsilateral abduction ( Fig 28.2 ) resulting in a possible ipsilateral esotropia The fourth cranial nerve innervates the superior oblique muscle It is the only cranial nerve that completely decussates and has a dorsal projection over the midbrain This position renders the fourth cranial nerve particularly vulnerable to blunt head trauma, one of the most common causes of fourth nerve palsy The fourth cranial nerve also has a relatively long intracranial course, which makes it susceptible to increased ICP and parenchymal shifts caused by cerebral edema It also runs through the cavernous sinus Fourth cranial nerve palsy may be congenital but asymptomatic for several years during childhood until the brain is no longer able to compensate Acquired or congenital palsy of this cranial nerve causes the eyes to become misaligned vertically (ipsilateral hypertropia) Patients with congenital fourth cranial nerve paresis compensate by tilting their head to the ipsilateral side, which allows for a rebalancing of the eye muscles such that alignment may be achieved Old photographs may demonstrate this tilt Facial asymmetry can also be seen after years of this compensatory tilting Ophthalmic consultation is usually needed to differentiate between congenital and acquired palsy The third cranial nerve supplies the remaining four extraocular muscles It is involved with downgaze, upgaze, and adduction Parasympathetic innervation to the pupil (see Chapter 29 Eye: Unequal Pupils ) and innervation to the eyelid muscle (levator palpebrae) are also carried in the third cranial nerve A complete third cranial nerve palsy results in an eye that is positioned down (from the remaining action of the unaffected superior oblique muscle) and out (from the remaining action of the unaffected lateral rectus muscle) with ipsilateral ptosis and ipsilateral pupillary dilation ( Fig 28.3 ) Because the third cranial nerve divides into a superior and an inferior division just as it enters the orbit from the cavernous sinus and because the fibers to individual muscles are segregated within the nerve throughout its course, partial third cranial nerve palsies may occur with or without ptosis and/or pupillary dilation This may leave the patient with complex strabismus, which is best left to the ophthalmology consultant The differential diagnosis of third cranial nerve palsies is summarized in Chapter 29 Eye: Unequal Pupils Muscle Restriction