Optic Neuropathies: Ischemic, Inflammatory, Infectious, or Compressive
Mitchell B. Strominger
Victoria M. Hammond
THE CLINICAL CHALLENGE
Optic neuropathy is the general term that describes vision loss secondary to a disease process involving the prelateral geniculate optic nerves. Patients present with the primary complaint of acute or chronic vision loss in one or both eyes depending on the primary diagnosis. The visual defect occurs either centrally or peripherally secondary to a visual field abnormality. Occasionally, patients might also notice a decrease in color vision and have nonocular symptoms such as pain on eye movement or headaches. Clinically, it is challenging because the differential for causes of optic neuropathy is broad, taking into account age, systemic complaints, and other medical disorders.
PATHOPHYSIOLOGY
Optic neuropathies can be ischemic, inflammatory, infectious, or compressive.
Ischemic Optic Neuropathy
Ischemic optic neuropathy can be separated into its two most common forms: arteritic anterior ischemic optic neuropathy (AAION) and nonarteritic anterior ischemic optic neuropathy (NAAION).
Nonarteritic Anterior Ischemic Optic Neuropathy
NAAION is by far the most common form of an acute ischemic optic neuropathy that presents for acute evaluation in the emergency and urgent care settings. It is typically seen in patients older than 50, most of whom have underlying vascular risk factors, including hypertension and diabetes. The onset of vision loss is sudden and classically altitudinal, meaning the patient’s visual field loss respects the horizontal meridian. The inferior field is more commonly affected than the superior field.
Patients often notice vision loss when wakening in the morning, which may be related to the normal diurnal fluctuation in blood pressure and potentially exacerbated in those who take antihypertensives at bedtime. Other risk factors include the use of amiodarone, phosphodiesterase (PDE) inhibitors, and sleep apnea.1,2,3 NAAION can also be seen perioperatively, most notably after coronary artery bypass grafting and prolonged spinal-fusion surgery, with the patient being in the prone position.
Arteritic Anterior Ischemic Optic Neuropathy
AAION is less frequent than NAAION but more visually devastating if not recognized and treated emergently. Most cases occur secondary to giant cell arteritis (GCA) and should not be missed. GCA is an inflammatory vasculitis most commonly occurring in the arteries of the head, scalp, and arms. The mean age of patients with AAION is 70 years and rarely presents in patients under 50. Thus, for patients with acute vision loss, it is imperative to take a comprehensive history for other signs of GCA, including temporal headache, scalp tenderness, jaw claudication, anorexia, weight loss, fever, and associated polymyalgia rheumatica. Workup for GCA includes inflammatory markers (erythrocyte sedimentation rate [ESR] and C-reactive protein [CRP]) and referral for temporal artery biopsy.
Infectious Optic Neuropathy
Infections can directly involve the optic nerve, leading to an optic neuropathy. Vision loss occurs as a result of optic nerve head swelling and often hemorrhage. Primary infection of the optic nerve may lead to exudation into the macula in a star pattern. Retinal vasculitis can also occur, causing exudation, flame hemorrhages, and ischemic cotton wool spots. This combined disorder of the optic nerve and retinal vasculature is called neuroretinitis.4 Inflammation as well can occur in the vitreous (vitritis), and, depending on the organism (eg, tuberculosis), choroidal lesions can be seen. Common causes of infectious optic neuropathy include Bartonella and toxoplasmosis, but syphilis, Lyme, tuberculosis, cytomegalovirus, and herpes simplex virus must also be considered. Each of these has characteristic involvement of the optic nerve, retina, and vitreous, which is beyond the scope of this chapter. However, primary optic nerve involvement can occur without retina involvement in Bartonella and syphilis. In addition, optic nerve swelling from meningitis may cause an optic perineuritis.
Inflammatory Optic Neuropathy
Optic neuritis broadly describes any inflammatory condition involving the optic nerve, although it most commonly refers to inflammation caused by a demyelinating disease. Neuromyelitis optica spectrum disorder (NMOSD) is a form of optic neuritis associated with transverse myelitis and other brain stem lesions. Symptoms include unexplained hiccups, nausea or vomiting, and diencephalic syndrome. Unfortunately, the visual prognosis is poorer in NMOSD and tends to recur, leading to severe visual impairment. Myelin oligodendrocyte glycoprotein antibody disease (MOGAD) is another more recently described demyelinating central nervous system disorder that appears to be different than MS. Its presentation, however, is similar to NMOSD, with optic neuritis, transverse myelitis, or both. Magnetic resonance imaging (MRI) findings are similar, especially involving the brainstem, but can have dramatic improvement or resolution over time. Compared to NMOSD, MOGAD patients tend to be younger, lack female predominance, can present with acute disseminated encephalomyelitis (ADEM), and have a better outcome.
Other causes that overlap in the ischemic or infiltrative category include autoimmune processes such as systemic lupus erythematous, associations with inflammatory bowel disease, sarcoidosis, and Wegener.5
Compressive Optic Neuropathy
Compressive lesions of the optic nerve can occur anywhere from behind the globe to the chiasm and optic tract. The degree and location of compression correlates with the symptoms. The most common causes include optic nerve sheath and intracranial meningioma, anterior communicating artery aneurysm, and pituitary adenoma. Other infiltrative tumors should also be considered, such as optic nerve glioma in neurofibromatosis type 1 and metastatic carcinoma. Classically, patients describe a more chronic progressive peripheral vision loss. In the cases of central fiber compression, or with an acutely expanding lesion such as compressive thyroid orbitopathy, an expanding intracranial aneurysm, or pituitary apoplexy, patients may have a more acute vision loss.
APPROACH/THE FOCUSED EXAM
Nonarteritic Anterior Ischemic Optic Neuropathy
Unless bilateral, the examination of a patient with NAAION reveals a relative afferent pupillary defect. Funduscopic exam demonstrates an edematous optic nerve with areas of hemorrhage (Figure 50.1). On direct ophthalmoscopy, the optic nerve head may appear small and crowded with no cup. Owing to acute disc edema, this trait may be appreciated only in the uninvolved eye and is known as a “disc at risk.” The diagnosis is essentially a clinical diagnosis after other inflammatory and infectious etiologies have been ruled out. These cases can be bilateral and more severe and occur in the retrobulbar portion of the optic nerve. Therefore, these cases present a clinical challenge because exam and/or photography of the optic nerve head may appear normal. Depending on the location within the nerve, MRI with gadolinium may or may not show enhancement.
Arteritic Anterior Ischemic Optic Neuropathy
Vision loss is typically severe and central, with acuities worse than 20/200 in most patients. On funduscopic examination, the disc has “pallid” swelling (Figure 50.2), which is whiter and more ischemic, appearing in distinction to the hemorrhagic findings in NAAION. Other areas of retinal vascular ischemia can be seen as cotton wool spots.
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