Optic Nerve Decompression for Traumatic Optic Neuropathy 

Traumatic optic neuropathy is a devastating potential complication of closed head injury. The hallmark of an optic neuropathy, traumatic or otherwise, is a loss of visual function, which can manifest by subnormal visual acuity, visual field loss, or color vision dysfunction. The presence of an afferent pupillary defect strongly suggests a prechiasmal location for the injury and is necessary to validate the diagnosis of traumatic optic neuropathy. Vision loss associated with traumatic optic neuropathy can be partial or complete and temporary or permanent.

An image depicting successful decompression of the orbit can be seen below.

Hippocrates noted the association of trauma just above the eyebrow and gradual vision loss. By the 18th century, the relationship between frontal trauma and vision loss with an absence of ocular injury was well appreciated. In 1879, Berlin described the first pathologic examination of the optic nerve after head trauma. In 1890, Battle first distinguished penetrating direct from nonpenetrating indirect optic nerve injuries. The 20th century saw significant progress in defining the classification, pathophysiology, and management of traumatic optic nerve injuries.

Historically, observation, medical corticosteroid therapy, or optic canal decompression has been advocated for the treatment of traumatic optic neuropathy. In the early 1900s, transcranial unroofing of the optic canal was the surgical procedure of choice for traumatic optic neuropathy treatment. This procedure was used sparingly because of the inherent risks of intracranial surgery. In the 1920s, Sewell performed a transethmoidal optic canal decompression by removing the lamina papyracea and medial wall of the optic canal. Although his technique was refined by progressive advances in transnasal, transantral, transorbital, and external paranasal sinus surgery, the technique was not performed routinely until the 1960s in Japan and the 1980s in the United States. During this period, systemic corticosteroid treatment was also extended to treatment of traumatic optic neuropathy.

Recent advances in endoscopic instrumentation and intranasal sinus surgical techniques have refined extracranial surgical approaches for traumatic optic neuropathy. Currently, endoscopic optic nerve decompression (OND) via an intranasal and transethmoidal or transsphenoidal approach has gained popular support.

Trauma can precipitate various pathophysiological conditions that ultimately manifest as visual dysfunction. Nonneuropathic ophthalmic injuries should be excluded with a thorough ophthalmic examination, which includes orbital and cranial imaging studies. This examination should clearly delineate the nature of any neuropathic vision loss. Trauma-induced injury to the optic nerve can occur anywhere along the nerve's intraorbital-to-intracranial length.

Direct traumatic optic neuropathy is the term used when the optic nerve is impinged, crushed, or transected. These injuries are usually the result of open craniofacial trauma, such as penetrating wounds (eg, from knives, BBs, pellets) or extensive crush injury with displaced cranio-orbital fractures.

Indirect traumatic optic neuropathy occurs in the absence of direct optic nerve injury and is more common than direct traumatic optic neuropathy.

Frequency

In the United States, incidence of indirect traumatic optic neuropathy is approximately 2.5% in patients with midface trauma and 2-5% in patients with closed head injury.

Internationally, incidence of indirect traumatic optic neuropathy in the Western world is reportedly 0.7-5%. Most clinical series in Western literature involve fewer than 40 patients. A higher incidence of indirect traumatic optic neuropathy is reported in some Japanese studies; however, the reason remains unclear.

Traumatic optic neuropathy is most commonly caused by motor vehicle and bicycle accidents (15-75% of cases, depending on the series). Falls (15-50% of cases) are the next most common cause, followed by physical violence and recreational sports.

The exact pathophysiology of traumatic optic neuropathy is poorly understood. Although optic nerve avulsion and transection, optic nerve sheath hematoma, and optic nerve impingement (from a penetrating foreign body or bony fracture) all reflect traumatic mechanisms of the optic nerve dysfunction, they are frequently considered entities independent of traumatic optic neuropathy. These less common forms of traumatic neuropathic vision loss are covered separately in Traumatic Optic Neuropathy.

Traumatic optic neuropathy, in its most common form, is an indirect event that occurs during or shortly after blunt trauma to the superior orbital rim, lateral orbital rim, frontal area, or cranium. The most widely held belief maintains that compression forces from the trauma are transmitted via the orbital bones to the orbital apex and optic canal. Laser interferometry studies demonstrate that forces applied to the frontal bone are concentrated and transferred to the orbital apex and anterior foramen of the optic canal. Elastic deformation of the sphenoid then allows transfer of the force to the intracanalicular segment of the optic nerve. Contusion of the intracanalicular optic nerve axons and pial microvasculature produces localized optic nerve ischemia and edema. The edematous ischemic axons result in further neural compression within the fixed-diameter bony optic canal, precipitate a positive feedback loop, and trigger the development of an intracanalicular compartment syndrome.

Although ischemia is considered the secondary event that gives rise to the neuropathy, the cellular and subcellular events that constitute the mechanism of neural damage are only now being realized. The roles of oxygen free radicals, enzymes, cytokines, intracellular calcium, and other forms of reperfusion damage are slowly being uncovered through basic science research.

A less common form of traumatic optic neuropathy that involves the intracranial optic nerve results from forces delivered by the brain's shift at the moment of impact. The intracranial optic nerve is sheared as it moves against the falciform dural fold as it overlies the sphenoid plane.

The diagnosis of traumatic optic neuropathy is clinical. Patients with midfacial and cranial trauma should elicit a high index of suspicion for traumatic optic neuropathy. Although patients with traumatic optic neuropathy may have serious and obvious craniofacial, neurosurgical, and other comorbidities, they may also have no visible signs of injury. In addition, although 50% of patients with traumatic optic neuropathy present with a visual acuity of light perception or no light perception, nearly 20% of patients have a visual acuity of 20/200 or better.

Assume optic nerve dysfunction when a loss of best-corrected visual acuity or visual field is accompanied by an ipsilateral afferent pupillary defect (eg, Marcus Gunn pupil). Obtain a detailed medical history and identify premorbid ocular conditions that may limit vision recovery. If the patient's clinical situation limits detailed communication, query the patient's family, paramedics, or witnesses to the trauma about the details of the injury.

Perform a comprehensive ophthalmic examination on all patients in whom traumatic optic neuropathy is suspected and include the following assessments:

• Ocular adnexa: Examination may reveal orbital rim and wall fractures, orbital edema, proptosis or enophthalmos, or extraocular muscle dysfunction. Signs of penetrating injuries, such as protruding foreign bodies, extruding orbital contents, or conjunctival laceration, may range from obvious to subtle.
• Visual acuity: Assess visual acuity immediately upon presentation. Perform a second assessment within 24 hours of the first to discern cases of delayed optic neuropathy (< 10% of traumatic optic neuropathy cases).
• Pupillary reaction: An afferent pupillary defect (APD) is a necessary condition for the diagnosis of traumatic optic neuropathy. Normally, light in one eye causes equal constriction of both pupils (direct and consensual pupillary light reflex). With APD, light in the affected eye causes only mild constriction of both pupils. Light in the unaffected eye causes normal constriction in both pupils. Pupillary reaction is evaluated with the swinging flashlight test (ie, briskly alternating a flashlight beam from one eye to the other). Alterating the light between each eye every 2-3 seconds, the pupil of the affected eye will dilate with direct light and constrict with light in the unaffected eye.
• Intraocular pressure: Increased intraocular pressure may accompany an orbital hematoma, diffuse orbital hemorrhage, orbital emphysema, or soft tissue edema.
• Ophthalmoscopy: Perform ophthalmoscopy with the aid of a short-acting mydriatic agent (pupillary dilation) on all stable patients. Evaluate the retinal and choroidal circulation, optic nerve head morphology, and the presence of ring-shaped hemorrhage adjacent to the optic nerve head.

Medical or surgical intervention or a combination of both may be indicated for patients with indirect traumatic optic neuropathy. Indications for treatment are based on clinical judgment. Absolute indications for intervention, including optic canal decompression, have not been validated by controlled outcome studies; currently, physicians must decide on therapy for traumatic optic neuropathy without a consensus on standard of care.

In the Cochrane Database of Systematic Reviews, no randomized controlled trials were identified for either the use of corticosteroids or surgical treatment for traumatic optic neuropathy. Citing reports of visual recovery rates of 40-60% with conservative management, the authors conclude that the decision to proceed with surgery or high-dose corticosteroids depends on the clinical judgment and surgical skills of the surgeon as well as informed consent of the patient to appreciate the benefits and risks of both treatments.^{[1, 2]}

The International Optic Nerve Trauma Study was organized to compare corticosteroids or surgery and corticosteroids, but after failure to enroll sufficient numbers of patients, the study was transformed into an observational study. Comparing no treatment (observation) versus corticosteroids or surgical decompression, the authors found no difference in the final visual acuity and commented that the decision to treat or not treat should be made on an individual patient basis.^[3] A recently published study from Iran reported a randomized placebo-controlled trial of the use of intravenous high-dose corticosteroids versus saline in 31 patients with traumatic optic neuropathy; the authors found no statistically significant improvement in visual acuity in the 2 groups.^[4]

In one paradigm, treatment is based on visual acuity and response to intravenous corticosteroid therapy. Patients must have a history of traumatic head injury, subnormal visual function, and an afferent pupillary defect. Patients who consent to the risks of treatment are initially started on systemic methylprednisolone therapy (dosage is available in the eMedicine article Traumatic Optic Neuropathy). Optic canal decompression is indicated (1) if visual acuity does not improve to 20/400 or better despite 24-48 hours of steroid therapy or (2) if visual acuity is 20/200 or better but deteriorates during or after completion of steroid therapy.^[5]

The general anatomy of the optic nerve and its surrounding structures is outlined in Traumatic Optic Neuropathy. The features within this section emphasize specific anatomical structures the surgeon must understand to evaluate neuroimaging of the orbital apex and to perform surgical decompression of the optic canal.

If the anterior face of the sphenoid sinus is oriented vertically, the optic canal is likely adjacent to the lateral wall of the sphenoid sinus. If the anterior face is tilted obliquely, the optic canal may be adjacent to either the sphenoid or posterior ethmoid cells or both. When the canal is adjacent to the posterior ethmoid cells, the cells are known as Onodi cells, which may be pneumatized up to and even around the optic canal. In up to 25% of cases, a bony dehiscence may occur along the canal; always use care when working in this anatomic region.

The internal carotid artery is intimately associated with the optic nerve near the posterior foramen of the optic canal. Generally, the artery lies inferolateral to the nerve, away from the area of decompression. However, tortuosity in the carotid siphon, which brings the siphon closer to the zone of surgical decompression, can occur. Study of the carotid artery on preoperative imaging and the use of intraoperative computerized navigation assist appreciation of this anatomic variant.

Within the optic canal, the ophthalmic artery courses along the inferolateral aspect of the optic nerve. The artery does not enter the nerve until both the artery and the nerve are well anterior to the orbital apex. Carefully consider this anatomic relationship when surgically fenestrating the optic nerve sheath. Always perform surgical opening of the sheath in the quadrant medial and superior to the optic nerve.

Another anatomic consideration during decompression is the fused fibrous origin of the 4 rectus muscles (annulus of Zinn, Zinn ring). The optic nerve, ophthalmic artery, and fibers of the sympathetic nervous system emerge within this annulus at the anterior foramen of the optic canal. Some authorities believe that the annulus represents a nonosseous region that restricts optic nerve sheath distention and promotes optic nerve compression. Some proponents of optic canal decompression in traumatic optic neuropathy believe this annulus must be lysed in select clinical cases.

Patients with traumatic optic neuropathy may experience nonocular comorbidities such as closed head injury or multiorgan trauma. Basic and advanced life support is the primary objective until the patient is stabilized. The consultation for visual system evaluation should be prompt but must be triaged among the multiple consultations and ongoing critical care needs required for the individual patient. If neurosurgical concerns prevent pupillary dilation, a comprehensive examination of the posterior sclera, choroid, ciliary body, retina, and optic nerve head may be compromised or delayed. The evaluation and treatment of traumatic optic neuropathy begins after all other life-threatening injuries have been stabilized and basic lifesaving protocols have been fulfilled.

The use of corticosteroids in the treatment of traumatic optic neuropathy should be judicious in patients who are at risk (eg, those with diabetes mellitus, gastric ulcers, osteoporosis). In addition, a randomized controlled trial on the use of corticosteroids in patients with acute traumatic brain injury found a higher risk of death in the steroid group, leading investigators to prematurely terminate the trial.^[6] Although the mechanism of higher mortality in the patients who received steroids remains to be elucidated, this should be considered in the decision to treat patients with traumatic optic neuropathy and head injury with corticosteroids.^[2]