FIGURE 28.7 Evaluation of horizontal strabismus a With head held in straight ahead position EOM, extraocular muscle movement; MR, medial rectus; LW, lateral orbital wall; MW, medial orbital wall; fx, fracture; LR, lateral rectus; ICP, intracranial pressure; INO, internuclear ophthalmoplegia Lateral rectus palsy (sixth cranial nerve palsy) occurs most commonly secondary to head trauma (see Chapter 122 Neurosurgical Emergencies ), increased ICP, neoplasm, or following viral illness or immunizations Other CNS signs, such as papilledema, may be present A CT of the brain may be the first imaging choice to evaluate for intracranial hemorrhage given its rapid availability, however magnetic resonance imaging (MRI) of the brain is the imaging study of choice to further investigate the ophthalmologic findings Sixth cranial nerve palsy can also occur rather precipitously after the placement of ventricular shunts designed to relieve increased ICP even if the palsy was not obvious preoperatively Sixth cranial nerve palsy may be bilateral, resulting in bilateral esotropia with reduced ability to abduct bilaterally which requires careful examination to detect Asymmetric presentations are more likely to be readily identified EXOTROPIA EMERGENCIES