Chapter 194 – Traumatic Optic Neuropathies
THOMAS C. SPOOR
• Optic nerve damage after cranio-orbital trauma.
• Decreased visual acuity.
• Relative afferent pupillary defect.
• Visual field defects.
• Optic nerve infarction.
• Orbital or optic nerve sheath hematoma.
• Central retinal artery occlusion.
• Orbital and optic canal fractures.
The optic nerve may be damaged directly or indirectly after cranio-orbital trauma. Normal visual acuity and normal visual field and pupillary function are not compatible with the diagnosis of optic nerve injury. Both direct and indirect injury may damage the optic nerve as a result of transection of nerve fibers, interruption of blood supply, or secondary hemorrhage and edema. Primary injury to the optic nerve fibers by transection or infarction at the time of injury results in permanent damage. Neural dysfunction secondary to compression within the optic canal, as a result of edema and hemorrhage, may respond to medical or surgical intervention.
EPIDEMIOLOGY AND PATHOGENESIS
Indirect injury to the optic nerve occurs in 0.5–5% of patients who suffer closed head trauma. The optic nerve is enclosed tightly within the bony optic canal; it may be damaged by shearing and avulsion of its nutrient vessels ( Fig. 194-1 ) or by pressure transmitted along the bone to the optic canal ( Fig. 194-2 ).
Patients who have optic neuropathy have decreased visual acuity or visual field defects. A relative afferent pupillary defect—the sine qua non—of an optic neuropathy often is the ocular abnormality evident. If a relative afferent pupillary defect is not evident, the patient does not have a traumatic optic neuropathy, unless it is bilateral. Patients who have bilateral optic nerve dysfunction demonstrate light–near dissociation of their pupillary reactions. The near response is brisker than the pupillary response to light.
If the anterior optic nerve is injured, infarction, hemorrhage, or a central retinal artery occlusion are evident on ophthalmoscopy. Patients who have a more posterior injury to the nerve may be found to have a normal fundus on examination but have an afferent pupillary defect and visual loss.
Figure 194-1 Acceleration–deceleration injury to the optic nerve results in avulsion of its nutrient vessels.
Associated ocular injuries should be documented and treated, if possible. If the ocular injuries do not account for the degree of visual loss, a traumatic optic neuropathy should be suspected.
DIAGNOSIS AND ANCILLARY TESTING
High-resolution computed tomography (CT) is the diagnostic procedure of choice for patients who have suspected traumatic optic neuropathy. Treatable causes of optic nerve compression, such as orbital and optic nerve sheath hemorrhages, are detected using CT scans ; also, orbital and optic canal fractures may be detected using these scans. The presence of an optic canal fracture is not necessary for the diagnosis of traumatic optic neuropathy. Prior to transethmoidal optic canal decompression, CT scans show the information necessary to plan surgery.
Visual loss and an afferent pupillary defect result from optic nerve dysfunction. Traumatic optic nerve injury may occur even after a seemingly trivial head injury. Detection of visual loss may be coincident with the traumatic event. The differential diagnosis should include other causes of optic neuropathies, as well as causes of obviously treatable optic nerve compression ( Box 194-1 ).
The optic nerve can be injured anywhere along its course, most commonly at the intracanalicular and intracranial portion. Forces applied to the frontal bone may be transmitted and concentrated at the optic canal. Acceleration and deceleration forces may cause a partial or total avulsion of the retrobulbar optic nerve, or contusion
Figure 194-2 Pressure transmitted through the sphenoid bone to the optic canal may cause traumatic optic nerve injury.
DIFFERENTIAL DIAGNOSIS OF TRAUMATIC OPTIC NEUROPATHY
Optic nerve sheath hematoma
Optic nerve inflammation
Sinusitis with orbital involvement
Coincident optic neuropathies
Ischemic optic neuropathy
Compression by tumor or aneurysm
necrosis and avulsion of the vascular supply of the intracanalicular optic nerve (see Fig. 194-1 ). Orbital hemorrhage or hemorrhage into the optic nerve sheath also may cause progressive visual loss. The intracranial optic nerve may be injured by the falciform dural fold as a result of the forces of a shifting brain at the moment of impact. Swelling of the intracanalicular optic nerve causes delayed, progressive visual loss through exacerbation of the ischemic effects of the original injury. Any or all of these mechanisms may be responsible for optic nerve injury.
Optic nerve injuries may be caused by primary and secondary mechanisms. The primary mechanisms cause permanent, irreparable damage to the optic nerve. Treatment modalities are effective only for secondary mechanisms of injury. These may be at the cellular level or from intracanalicular optic nerve swelling that further compromises an injured optic nerve. A review of such mechanisms was published recently. Treatment consists of attempts to limit secondary injury and salvage axons that survive the initial trauma.
Visual loss and a relative afferent pupillary defect that accompany an orbital hemorrhage warrant immediate decompression by either drainage of the hemorrhage or lateral canthotomy or cantholysis, or both. A much less common occurrence is an intraoptic nerve sheath hemorrhage, which results in compression of the optic nerve and compromise of visual function. Prompt surgical decompression of the optic nerve sheath may restore visual function. The optic nerve may be injured anywhere along its length. These primary injuries are not treatable. The secondary effects of the primary injury, edema and hemorrhage, may be treatable. Secondary compression of the intracanalicular optic nerve may appear as progressive visual dysfunction after the initial loss. Treatment of traumatic optic nerve injuries with megadose corticosteroids is based, in part, upon the success attained in treatment of spinal cord injuries. Administered within 8 hours after injury, megadose corticosteroids have an antioxidant and membrane-stabilizing effect that limits secondary cell damage and increases microcirculatory perfusion. After 8 hours, corticosteroids decrease edema and swelling but have little effect on the biochemical dynamics of the injured axon. Reduced swelling by removal of the bony wall of the optic canal relieves compression of the optic nerve and allows it to regain function. Unfortunately, these proposed treatments limit secondary injuries only. Axons damaged terminally do not regenerate—their visual function is irretrievably lost.
The effects of all the treatment options have not yet been clarified by a randomized, double-blind study; however, initial treatment with megadose corticosteroids is reasonably safe and possibly effective. If possible, treatment should begin within 8 hours of injury; regardless, it should be initiated as soon as possible. Methylprednisolone 30?mg/kg is administered intravenously over 30 minutes, followed by 15?mg/kg 2 hours later. This compensates for the rapid serum half-life of methylprednisolone. Treatment is continued with 15?mg/kg every 6 hours for 24–48 hours. If visual function improves, the corticosteroids are tapered rapidly. If visual function deteriorates as corticosteroids are tapered, optic canal decompression should be offered. Patients who do not respond to megadose corticosteroids may be considered candidates for optic canal decompression. The rare patient who has a bona fide history of progressive visual loss after closed head injury is a certain candidate for megadose corticosteroids treatment and extracranial optic canal decompression. As for any surgical procedure, potential risks must be weighed against possible benefits. A steep learning curve exists for extracranial optic canal decompression, being a far safer operation in the hands of an experienced surgeon than in those of an occasional exponent.
COURSE AND OUTCOME
Unfortunately, no treatment for primary, traumatic optic neuropathies has been proved effective in a randomized, controlled clinical trial. Besides, no standard of care exists for the management of patients who have traumatic optic neuropathies.   Several studies show that patients treated with corticosteroids or a combination of corticosteroids and extracranial optic canal decompression seem to have better visual prognosis than untreated patients.  Treatment with megadose corticosteroids seems to improve vision more quickly than treatment with high-dose intravenous corticosteroids, but there is no significant difference in the final visual outcome.   Patients may improve spontaneously without treatment, but treated patients appear to have a better visual prognosis. 
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