Pretutorial
Tutorial
Introduction
Misha Pless, MD
Introduction
Any patient with a disorder of afferent visual function after trauma, characterized by loss of visual acuity, color perception, visual field defect, or loss of contrast sensitivity in one or two eyes, should be suspected of having traumatic optic neuropathy (TON). Generally, TON occurs in two different ways — directly and indirectly. Direct TON involves actual disruption of anatomical structures along any of the paths of the optic nerve, from the scleral canal to the chiasm. Traumatic optic chiasmopathy is a rare entity, but it has been reported. Presence of a relative afferent pupillary defect is critical but not indispensable in the diagnosis of TON, given occasional balanced, bilateral optic nerve injuries. Indirect TON occurs without disruption of anatomical structures, such as the optic nerve proper or the tissues around it, along its long course to the chiasm. In indirect TON percussion energy is transmitted to the axons of retinal ganglion cells disrupting axoplasmic flow and in some instances causing actual microscopic interruption of nerve fibers. Patients suffering from direct TON tend to have a poorer prognosis than those with indirect TON, and they generally respond less to treatment. Distinguishing indirect from direct TON is occasionally difficult. An example of such a clinical scenario is when head trauma causes nondisplaced fracture of a bone involving the optic canal. In such cases, the optic nerve is believed to be uninterrupted at the macroscopic level, thus fooling neuroimaging, but microscopic analysis, if possible, would plausibly show interruption of neuronal continuity.
TON can occur with even mild head injury. Loss of consciousness is not an obligatory feature for the diagnosis, but it is said to occur in 50% to 72% of patient with TON. Visual acuity ranges from normal to no light perception. Severe visual loss (hand motion to no light perception) occurs in approximately half of all patients with TON. Displaced fracture of the optic canal is justifiably associated with severe visual loss. Motor vehicle accidents, bicycle falls, blunt trauma after falls, and bullet-associated injuries are the most common causes of TON. Individuals involved in motorcycle accidents are particularly prone to TON as well as severe intracranial pathology elsewhere. In addition to direct vs. indirect TON, one can classify TON in terms of the location of pathology. Namely, anterior TON is usually related to trauma of the optic nerve at the scleral canal or even avulsion in the worst cases. Trauma to the optic nerve in its intraorbital course may be iatrogenic, associated with needle puncture. Hemorrhage related to surgical procedures is not uncommon and it may be under-reported. In such instances, the cause is usually identifiable by history and neuroimaging. Retrobulbar TON at the optic canal is typically related to transmission of percussion energy through the orbital apex, or fracture. In fact, the most common site of indirect optic nerve injury is the optic canal. Intracranial optic nerve injuries are less common and usually bilateral. Vertical displacement of the suppler subarachnoid optic nerves against the rigid optic canal-clinoid process complex causes blunt injury to the nerves.
Diagnosis
Obtaining a history is critical to the diagnosis of TON. Careful neuro-ophthalmic examination of a patient with suspected TON confirms the diagnosis already suspected by history alone. Neuroimaging studies complement a diagnosis already made clinically and should not be employed in isolation to make the diagnosis unless history is unobtainable and the examination is equivocal. In unconscious patients, examination of all cranial structures is essential. However, watchfulness for the diagnosis is fundamental since blunt head trauma can be associated with TON with little or no external injury. In unconscious patients, presence of a relative afferent pupillary defect in the setting of otherwise normal funduscopic examination is virtually pathognomic of the diagnosis of TON, particularly if cranio-orbital imaging is negative. Occasionally, the diagnosis is made only after radiological studies suggest a fracture of the sphenoid bone at the optic canal. An outpatient may present with visual symptoms after having suffered moderate head trauma. In this setting, subtle abnormalities of color perception or contrast sensitivity may be the only indication of TON to an otherwise unsuspecting ophthalmologist, who has not checked visual fields. Visual field testing is thus fundamental in the diagnosis of TON though there is no pathognomic defect or pattern of defects to assist in the diagnosis. Confrontational visual field testing may miss a great majority of defects in patients with TON who have only mild to moderate symptoms. Occasionally, visual evoked potential testing is necessary and it may be a useful adjunct technique.
Neurodiagnostic imaging is essential in the diagnosis and management of TON. Computed tomography (CT) scanning is ostensibly superior to magnetic resonance imaging (MRI) in the acute evaluation of patients suspected of having TON. Evaluation of the bone in cranio-orbital sequences is achieved with greater accuracy by CT scanning and is of paramount significance, particularly in evaluating the integrity of the bones making up the optic canal. MRI scanning can better evaluate the optic nerve proper from scleral canal to optic chiasm. MRI scanning can detect presence and longevity of hemorrhage within the optic nerve sheath, swelling of the optic nerve in the tight optic canal, and thickening of the optic nerve sheath; these may all accompany TON. When possible a combination of orbital CT scanning with thin coronal slices through the optic canal along with brain MRI scanning is the optimal radiological approach in patients with TON. Indirect TON is generally not associated with pathology identifiable by most common neuroimaging available today with the exception of non-displaced fractures and perhaps mild signal change in the optic nerve. High resolution MRI, which includes diffusion-weighted sequences able to detect subtle changes in hydration properties of the optic nerve, will surely blur the distinction between direct and indirect TON in the future.
Delayed TON has been known to occur and the entity was first described in the medical literature. It is thought to take place when the optic nerve is traumatized by indirect (energy transmission or percussion) forces, not significant enough to cause clinically apparent symptoms initially. Swelling of the nerve against a particularly narrow optic canal then follows causing neurotoxicity and failure of axoplasmic flow, thus leading to delayed symptoms of optic neuropathy. In fact, an optic neuropathy can occur up to 48 hours after initial injury.
Management
The management of TON is controversial but some generalizations can be made. Cooperative efforts displayed by numerous authors in a variety of medical centers in the past two decades have aimed at establishing comprehensive guidelines for surgical versus medical management versus both. No single study has come up with a coherent algorithm accepted by all treating physicians. Some management guidelines are, however, available after at least two decades of scientific study of treatments for TON. TON is a neuro-ophthalmic emergency and there is good evidence that early intervention is crucial. There is substantiation from several studies in the U.S., European, and Japanese literature that early treatment with corticosteroids yields discernible improvement in visual function. Most recent information about the treatment of TON with high-dose corticosteroids has been derived from spinal cord injury studies such as NASCIS 1 & 2. Various doses of corticosteroids were given to patients with spinal cord injury, whose clinical outcomes were compared to placebo-treated patients in multicenter, randomized, double-masked, placebo control studies in the 1980s and 1990s. Significant improvement in motor and sensory function was noted in the patient who received methylprednisolone at 30 mg/kg initial bolus, followed by a continuous infusion of 5.4 mg/kg/hr for 24 hours. Currently, decompression of the optic canal through various surgical techniques continues to be performed empirically. Studies in the Japanese literature have suggested that trans-ethmoidal or trans-sphenoidal optic canal decompression can be safe, at least in their patient population. Japanese authors have reported improved prognosis in TON comparing surgical vs. medical intervention. However, this contention has not been universally accepted. Surgical intervention for indirect TON compared to placebo and/or high-dose corticosteroids has not proven to be significantly better than doing nothing and so far continues to be an empirical treatment. However, surgical decompression can be attempted by an experienced surgeon in cases of optic nerve sheath hematoma, subperiosteal hematoma with optic nerve compression, or intraorbital hematoma.
Broad guidelines for the treatment and management of patients with TON are feasible. TON due to displaced fracture of the optic canal is associated with poor prognosis of visual recovery regardless of treatment. Surgical repair of the fracture and decompression of the optic canal should be attempted only by experienced surgical hands and generally if vision is poor to begin with, since the procedure itself may cause visual loss. Orbital or canalicular hemorrhages causing optic nerve compression may be evacuated along with optic canal decompression and/or optic nerve sheath fenestration, carefully weighing the risk-benefit equation for the patient. Unroofing of the optic canal through a craniotomy approach is sporadically attempted in cases of clinoid process fracture with optic nerve displacement. Bony fragment extraction should be attempted only if there is uncontroversial radiographic evidence that the fragment is extraneural and intracanalicular or clearly within the nerve sheath but outside the optic nerve proper. Optic nerve microsection for the removal of intraneural foreign bodies or osseous fragments has been met with poor visual outcome. Current wisdom recommends the treatment of indirect TON with corticosteroids, if tolerated by the patient. The dosage to be employed is controversial but, as mentioned above, practice in this area of neuro-ophthalmology has been influenced by spinal cord injury research.
In my practice, I treat cases of acute indirect TON with a 30 mg/kg loading dose of intravenous methylprednisolone followed by a 5.4 mg/kg/hr drip for 2 to 3 days. Lack of visual improvement after this initial period and visual acuity worse than 20/400 prompt a discussion with the family or patient whereupon surgical decompression of the optic canal is offered, highlighting the empirical nature of the procedure. If vision improves with surgical decompression, an oral prednisone taper is begun. Lack of change or worsening in visual function after surgery prompts further treatment with intravenous methylprednisolone at 3.5 mg/kg IV four times a day for 2 to 3 more days, followed by an oral prednisone taper
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