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Optic nerve. The optic nerve is tethered in the optic canal and is subject to stretch injury during brain shifts.
The bony architecture of the orbit directly transfers forces from the superolateral orbital rim to the optic nerve canal (Gross et al 1981). A quarter of these injuries result from penetrating trauma, mostly gunshot wounds. Frontal blows to the orbit along the superior lateral rim also often damage the optic nerve, whereas occipital injuries rarely cause optic nerve injury (Lindenberg et al 1973). Most traumatic optic neuropathies are the result of severe head trauma, and altered consciousness in the patient delays the diagnosis. Traumatic optic neuropathy has been estimated to occur in 0.5% to 5% of patients with head trauma (Wu et al 2008). Severe maxillofacial trauma may also result in traumatic optic neuropathy in the absence of brain injury up to 2.25% of the time (Urolagin et al 2012). An afferent pupillary defect in an unconscious patient is useful in the detection of optic nerve injury, whereas testing for visual fields and acuity usually establishes the diagnosis in cooperative patients.
About 10% of patients show signs of bilateral optic nerve injury or chiasmal damage, and the majority of chiasmal injuries are incomplete (Savino et al 1980). Chiasmal injury is often associated with diabetes insipidus and decreased olfaction. The pupillary reflex is also useful in confirming nonorganic visual loss (Mavrakanas and Schutz 2009).