------------------------------------------------------------------------------- TITLE: Traumatic Optic Neuropathy SOURCE: Dept. of Otolaryngology, UTMB, Grand Rounds DATE: October 11, 1995 RESIDENT PHYSICIAN: John Yoo, M.D. FACULTY: Brian Driscoll, M.D. SERIES EDITOR: Francis B. Quinn, Jr., M.D. ------------------------------------------------------------------------------- "This material was prepared by resident physicians in partial fulfillment of educational requirements established for the Postgraduate Training Program of the UTMB Department of Otolaryngology/Head and Neck Surgery and was not intended for clinical use in its present form. It was prepared for the purpose of stimulating group discussion in a conference setting. No warranties, either express or implied, are made with respect to its accuracy, completeness, or timeliness. The material does not necessarily reflect the current or past opinions of members of the UTMB faculty and should not be used for purposes of diagnosis or treatment without consulting appropriate literature sources and informed professional opinion." Traumatic Optic Neuropathy: Introduction Traumatic optic neuropathy is an uncommon, but serious ophthalmologic sequealae of trauma to the head. The uncommon nature of this condition contributes to the lack of knowledge regarding its epidemiology, pathophysiology, and natural history, and to the controversy regarding its treatment.4 Hippocrates was credited with the first description of this injury when he wrote "dimness of vision occurs in injuries of the brow and those placed slightly above." This condition can be divided into direct and indirect traumatic optic neuropathies. Direct traumatic optic neuropathies are due to injury or compression of the optic nerve by bony fragments, hematomas, foreign bodies, and optic canal fractures that can be readily identified by CT scan. In indirect traumatic optic neuropathy, there is no fracture or radiographic evidence of abnormalities contiguous to the course of the optic nerve. Indirect optic neuropathies can be further subdivided into anterior injuries which involve the intraocular nerve and posterior injuries which involve the retrobulbar optic nerve. Anterior injuries show ophthalmologic signs on funduscopic exam mimicking central retinal artery occlusion.4,5 Walsh and Hoyt describe indirect traumatic optic neuropathy as a "traumatic loss of vision which occurs without external or initial ophthalmoscopic evidence of injury to the eye or its nerve," in which they are most likely describing an indirect posterior traumatic optic neuropathy, the topic of the remaining discussion.6 Incidence The incidence of traumatic optic neuropathy is about 2% of closed head injuries and about 2.5% of midfacial fractures.5 Anatomy The optic nerve can be arbitrarily divided into four segments. The intraocular segment measures about 1 mm and is supplied by the choroidal and ciliary vessels. The intraorbital segment measures 23 to 30 mm and assumes an "S" shape allowing slack for the movement of the globe. The nerve is also buffered by orbital fat and extraocular muscles from injury. This segment is nourished by the dural and central branches of the ophthalmic artery. Hematomas within the nerve sheath or within the orbit can cause compressive damage to the optic nerve. The intracanalicular segment is about 8-10 mm long and is immobilized within the bony canal by its dura. This segment is nourished by the pial branches of the ophthalmic and carotid arteries. The optic canal transmits the optic nerve, meninges, ophthalmic artery, and sympathetic fibers. Indirect traumatic optic neuropathies are most likely to occur to the intracanalicular segment of the optic nerve, because of the firm attachment of the dural sheath to the optic nerve at this location, the small enclosed space, and the vulnerability of the blood supply. The intracranial segement is about 15 mm and is supplied by branches from the major intracranial arteries.4,5,7 Within the cranial cavity, the optic nerve is surrounded by only the pia mater, but at the entrance to the optic canal, the arachnoid and dura are added to the covering. At the exit of the optic canal, the dura splits into two layers, with the outer layer continuous with the periorbita and the inner layer remaining with the optic nerve. The ophthalmic artery branches from the internal carotid artery and travels with the optic nerve inside the sheath before exiting the dura at the anterior end of the optic canal.1 Pathophysiology The intraorbital segment is usually spared injury due to its laxity and buffering by the surrounding fat and extraocular muscles. The intracranial segment is protected by the surrounding brain and bone as well as the fact that the shearing forces are absorbed by the intracanalicular segment and as a result do not reach the intracranial segment. Holographic studies by Anderson, et al, showed that blows to the malar and frontal areas were transmitted mostly to the optic foramen.8 Even without fracture, these forces can cause compression, shearing, contusion, and stretching injuries to the optic nerve. In addition, the sheath of the optic nerve is firmly attached to the bony canal, and the canal itself is a closed space unforgiving to any degree of edema or hemorrhage.4,5,7 The exact sequence of events surrounding traumatic optic neuropathy is not known for sure, but vascular insufficiency is felt to play a key role. Initially, hemorrhage into the nerve or any layer of the sheath, laceration, contusion necrosis may occur due to shearing forces. As a result, edema and compression within the narrow canal causes further necrosis and infarction of the nerve. It is also possible that vascular insufficiency may play only a minor role. In this scenario, shearing forces cause edema which compresses the nerve and causes necrosis.7 In all likelihood, the mechanisms are multifactorial. Clinical features The pertinent findings on physical exam include loss of vision, sluggish pupil, and afferent pupillary defect on swinging flashlight test, on an eye that otherwise looks normal.7,9 Ophthalmology consultation should be obtained in all cases of this condition. A thorough exam should be documented which includes evaluation of visual acuity, pupillary reactivity, and visual field defects, as well as direct and indirect ophthalmoscopy. Visual evoked response (VER) may be useful to document nerve conduction and is particularly helpful if the patient is unresponsive. CT scan can better define any fractures, reveal any bony fragment or hematoma directly impinging on the optic nerve, or uncover any unsuspected injuries. An ultrasound should be considered for the globe if anterior optic nerve injuries are suspected.7 Natural History Due to the rarity of this condition, there are few studies regarding the natural history. Spontaneous improvement occurs in about 20-40% of patients who are left untreated.4 Treatment Most studies regarding treatment for traumatic optic neuropathy are small, uncontrolled and nonrandomized. As a result, the treatment for this uncommon malady is controversial. However, the use of high dose steroids early in the course of treatment is being widely accepted. The rationale for steroid use comes from the above studies as well as the benefits shown in the use of megadose methylprednisolone for acute spinal injury (NASCIS II). The mechanism of action of these megadose steroids is not exactly known, but the steroids are felt to decrease microvascular spasm and soft tissue edema, thereby improving blood flow, and reducing cell death. Most protocols use dexamethasone or methylprednisolone: dexamethasone methylprednisolone loading dose 1.0-6.0 mg/kg 30 mg/kg two hours later 1/2 loading dose 1/2 loading dose and every 6 hours If the vision improves steadily, the steroids are continued until no further improvement occurs. The steroids are discontinued after 7 days if no improvement occurs.4,5,7,9 Patients with obvious compressive lesions or bony fragments should be decompressed immediately.4 The use of surgical decompression is highly controversial, however. Studies are divided in regards to the efficacy of decompression. Recently, Joseph, et al, showed that 11 out of 14 patients showed improvement in vision following extracranial optic nerve decompression for traumatic optic neuropathy.10 Earlier approaches to decompression of the optic nerve used a subfrontal approach to transcranial unroofing. However, all the risks of intracranial surgery as well as possible lack of exposure caused by edematous frontal lobe detracted from its effectiveness.5 Newer extracranial approaches such as the external, trans-antral, sublabial, and trans-nasal ethmoidectomies have gained more favor due to their reduced morbidity.7 Walsh in 1966 proposed that decompression not be performed on unconscious patients, that decompression is less likely to be successful in patients who lose their vision at the time of impact, that surgery is more likely to be helpful in patients who lose their vision after the time of impact, and if time of visual loss is unknown, then it is prudent to wait 4-6 days.11 Gossman, et al, recommend trans-ethmoid decompression for patients who do not improve after 5-7 days of steroids, those who worsen on steroids, and those who cannot take steroids for whatever reason.9 Volpe, et al, propose the following guidelines for treating this condition: 1. If indirect optic nerve trauma is diagnosed in the setting of blunt head trauma and there are no medical contraindications, megadose steroids should be given as soon as possible. 2. Once on steroids, if there is continued deterioration, or improvement and then deterioration, optic nerve decompression should be performed. 3. Once on steroids, if there is no improvement after 12-24 hours, decompression may be indicated but must be individualized by the treating physicians. 4. If the patient relapses on discontinuation of systemic steroids, surgical decompression should be considered. 5. Decompression should not be used as an elective procedure in unconscious patients who are unable to cooperate with examination and unable to understand preoperatively the risks and benefits of surgery. 6. Decompression may be used primarily in a comatose patient with indirect optic nerve trauma who is undergoing other craniofacial surgery. 7. If an obvious compressive lesion (such as hematoma or bone fragment) is present, surgical decompression should be performed. 8. Recovery from no light perception can occur, and because it is difficult to determine if this loss occurred at the moment of impact, patients with no light perception should be treated as others. 9. Transethmoid surgery is as effective as transcranial surgery and is probably associated with less morbidity."7 The timing of surgery is also quite controversial. Some advocate performing surgery within the 7 days of the injury, but good results have been obtained anecdotally up to 6 months after the inciting event.4,7 ------------------------------------------------------------------------- BIBLIOGRAPHY 1. Warner JEA and Lessell S. Traumatic optic neuropathy. International Ophthalmology Clinics. 35(1):57-62, 1995. 2. Beretska JS and Rizzo JF. Controversy in the management of traumatic optic neuropathy. Int Ophthalmol Clin 34(3):87-96, 1994. 3. Walsh F, Hoyt W. Clinical neuro-ophthalmology. Baltimore: Williams and Wilkins, 1969:2375-2380. 4. Volpe N, Lessell S, Kline L. Traumatic optic neuroipathy: diagnosis and management. Int Ophthalmol Clin 1991; 31; 142-156. 5. Anderson RL, Panje WR, Gross CE. Optic nerve blindness following blunt forehead trauma. Ophthalmology 1982;89(5):445-455. 6. Gossman MD, Roberts DM, Barr CC. Ophthalmic aspects of orbital injury: a comprehensive diagnostic and management approach. Clinics in Plastic Surgery. 19(1):71-85, 1992 June. 7. Joseph MP, et al. Extracranial optic nerve decompression for traumatic optic neuropathy. Arch Ophthalmol 1990; 108:1091-1093. 8. Walsh FB: Pathological-clinical correlations. I.Indirect trauma to the optic nerves and chiasm. Invest Ophthalmol 5:433-449, 1966. -----------------------------END---------------------------------------