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 The action of a muscle may also be impaired by restriction The muscle can become infiltrated with substances that might restrict its action or cause fibrosis Children with hyperthyroid eye disease (e.g., Graves) can have enlarged, tight eye muscles, which along with infiltrate to the periorbital fat, contribute to proptosis An eyeball may be restricted in its movements by tumors or infection in and around the globe Orbital tumors, cellulitis, or abscesses that cause restriction may be associated with proptosis or a displacement of the entire eyeball, either vertically or horizontally With blunt trauma to the eyeball, the globe may be translocated posteriorly, causing increased intraorbital pressure that may result in a “blowout” fracture of the bony orbital wall When an orbital wall fracture occurs, the muscle or surrounding tissues that run along that wall may become entrapped within that fracture, tethering the eyeball so the eye cannot look in the direction opposite the fracture Children with orbital floor or medial orbital wall fractures are prone to entrapment of the inferior or medial rectus muscles, respectively This may not be noticeable until eye movements are attempted For example, fractures of the orbital floor may entrap the inferior rectus muscle, tethering the eye downward and thereby restricting upgaze ( Fig 28.4 ) Less commonly, the eye may have a limitation of movement in the direction of the fracture Orbital wall fractures may also be associated with enophthalmos, in which the eye appears to be sunken in the orbit, or proptosis caused by retroorbital hemorrhage All patients with orbital fractures must receive a complete ophthalmic examination to rule out accompanying ocular injury The most common fracture involves the inferior and/or medial walls of the orbit The lateral wall is rarely fractured Fracture of the superior wall (orbital roof) is particularly worrisome because it may allow communication between the orbit and the subfrontal intracranial space FIGURE 28.1 Normal extraocular muscle anatomy Strabismus With Intact Eye Muscle Function The remaining types of strabismus fall into the category where eye muscle function is normal (i.e., nonparetic and nonrestrictive) These problems are not emergent The eyes may be misaligned as a result of failure of the brain to use both eyes simultaneously in a coordinated fashion (idiopathic), a need for corrective lenses, or the presence of poor vision in one eye Uncorrected farsightedness (hyperopia) can result in accommodative esotropia, which may have an acute onset, usually between the ages of and years, with the misalignment often worse at near viewing Uncorrected nearsightedness (myopia) can result in exotropia, especially when the patient views in the far distance Both types of misalignment may be treated with corrective lenses Checking the vision in both eyes (see Chapter 123 Ophthalmic Emergencies ) is essential in all cases of strabismus along with full examination of eye movements to rule out the presence of uncorrected refractive error or a poorly seeing eye The latter may be due to serious eye problems such as retinoblastoma or cataract Of note, strabismus is the second most common presenting sign of retinoblastoma after leukocoria TABLE 28.1 EXTRAOCULAR MUSCLES Muscle a Medial rectus Cranial nerve III (inferior division) Inferior rectus III (inferior division) Lateral rectus VI Superior rectus III (superior division) Superior oblique IV Levator palpebrae III (superior c division) a Inferior Action b Eye position in palsy Adduction Exotropia Downward gaze Hypertropia Abduction Upward gaze Esotropia Hypotropia Downward gaze Eyelid Hypertropia Ptosis (lid) oblique not included for simplicity Isolated palsy of the inferior oblique is extremely rare in the horizontal or vertical field only Cyclorotatory movements not included c By definition, not truly an extraocular muscle b Action