The optic nerve head (ONH) can be subdivided into four regions: intra-ocular, prelaminar region, lamina cribrosa, and retrolaminar regions.
The pathophysiology of NAION is not completely understood, but it is accepted that relative hypoperfusion of the optic nerve head, as well as structural or other factors, lead to edema and infarction of optic nerve fibers, most often in the superior half of the ONH.
NAION is the most common cause of optic neuropathy in adults over 50 years of age. The prevalence of NAION in the US has been estimated to be anywhere between 2.3 to 10.2 per 100000. It is less common in blacks and is most common in Caucasians presumably because blacks tend to have a larger cup to disc ratio and are thus less likely to have small optic nerve cups, which is the biggest risk factor for developing NAION.
As mentioned above, the exact pathophysiology of NAION; however, the main theory is that the hypoperfusion of short posterior ciliary arteries supplying the optic nerve head leads to localized edema of the involved axons. As the scleral canal through which optic nerve passes into its intra-orbital compartment is small, in predisposed individuals (those with small cup-to-disc ratio) localized optic nerve head edema leads to a compartment syndrome where the swelling is propagated by affecting the neighboring axons. This eventually leads to severe axonal swelling/ischemia and apoptosis with loss of function of the involved axons.
Complete neuro-ophthalmological history should be obtained emphasizing the onset of visual loss (typically sudden in NAION and semi-acute in optic neuritis) and any other associated symptoms (about 10-15% of patients with NAION experience pain in and around the eye but not with eye movements as is typical in optic neuritis).
Typically there usually aren't any other accompanying neurological symptoms.
Medications history with specific attention to the use of PD-5 inhibitors and anti-hypertensive medications, especially the use of these medications at night, as well as the use of amiodarone, which can be associated with the anterior optic neuropathy whose presentation can be similar to NAION.
A complete examination should be performed.
The diagnosis of NAION is a clinical one: all patients must have optic nerve edema at the time of presentation and over 97% of patients would have a disc-at-risk in the fellow eye with the cup to disc ratio of 0.2 or less.
While many medications and treatment strategies have been tried over the years, none have been proven to be effective. One of the very few randomized controlled clinical trials in neuro-ophthalmology was done evaluating whether patients with NAION will benefit from optic nerve head decompression via vitrectomy and demonstrated that surgery was not beneficial and potentially harmful.
Intravitreal injections of bevacizumab, as well as triamcinolone, have been tried with disappointing results as well.
Recently, a clinical trial evaluating intravitreal injection of QRK207, a caspase 2 inhibitor preventing apoptosis, in patients with recent (within 14 days from onset) onset of NAION did not demonstrate its efficacy and was stopped early.
Currently, the only clinical trial that is ongoing includes subcutaneous injections of RPh201, an isolated botanical extract of gum mastic, for patients who had an onset of NAION within 1-5 years prior to enrollment.
The natural course of the disease in NAION has been elucidated by the results of the optic nerve decompression treatment trial which demonstrated that 1/3 of patients will re-gain 3 or more lines of vision at 2 years follow up, 30% will lose 3 or more lines at 2 years, and the rest will demonstrate unchanged visual acuity. In reality, most patients likely maintain the vision they have after the resolution of acute optic nerve head edema, and the improvement seen was secondary to their learned ability to fixate around their scotomas.
Ischemic optic neuropathies lead to atrophy to the optic nerve in one or both the eyes and can lead to permanent blindness.
There are several risk factors of anterior ischemic optic neuropathy, which can be modified. Patients are advised to have a sleep study, and if diagnosed with sleep apnea to start treatment for this condition, modifications of vascular risk factors should be recommended to all patients, avoidance of nocturnal hypotension has been suggested by many clinicians to be an important strategy to prevent NAION in the fellow eye as well. The use of phosphodiesterase inhibitors should also be discussed.
The correct diagnosis and management of ischemic optic neuropathies are essential to ensure that all potential risk factors are modified in order to decrease the chance of a patient developing this condition in the fellow eye. The possible associations like diabetes, hypertension, hyperlipidemia, when present, should be managed by the interprofessional team, including endocrinologists and general physicians. The team can also include optometrists, neurologists, and ophthalmologists. The nurses participate in patient education and follow up, informing the ophthalmologist of any new issues.
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