Idiopathic demyelinating optic neuritis is the most common optic neuropathy under the age of 40 years. It may occur as an initial clinically isolated demyelinating event in the absence of a diagnosis of MS or in
a patient with an established diagnosis of MS (Case 1). Typical presentation is vision loss in one eye that progresses over 1 to 2 weeks and is accompanied by eye pain that is typically worse with eye movement.⁴ Bilateral simultaneous optic neuritis can occur, especially in children,⁵ but it is uncommon in adults and should generate a broader differential diagnosis.⁶

Visual acuity, color vision, low contrast vision, pupils, visual fields, and funduscopic examination are the key elements of the examination to perform in the patient with suspected optic neuritis. Optic neuritis is a clinical diagnosis, and it is important to keep in mind a list of red flags (Table 8.1) that would be atypical for idiopathic demyelinating optic neuritis and may suggest an alternative cause of optic neuropathy. The degree of vision loss is highly variable in optic neuritis and can range from mild to severe, with initial visual acuity typically between 20/25 and 20/200.⁴ However, the presence of no light perception vision is a red flag and should suggest other potential causes of an acute optic neuropathy, including ischemic and systemic causes. The patient’s best visual acuity should always be assessed using corrective lenses or pinhole correction. Color vision is typically formally tested with Ishihara or Hardy Rand Rittler (HRR) color plates that display differentially colored numbers or geometric shapes. The advantage of the HRR plates is that they contain blue and purple shapes, which can sometimes be more affected than the red-green plates contained in the Ishihara series. A simple beside assessment of color vision can be performed by comparing the brightness of a red object between the affected and unaffected eyes, seeking “red desaturation” in the affected eye. Other color tops can also be tried. Detection of an afferent pupillary defect is critical in determining that the vision loss is attributable to an optic nerve process. The afferent pupillary defect may be detected by moving a light back and forth rhythmically between the two pupils. With an optic neuropathy, there will be an asymmetric response and the affected pupil will often paradoxically dilate to the light stimulus.⁷ Central visual field loss, called a central scotoma, can often be found on confrontation or automated visual field examination. On ophthalmoscopy, most adults with optic neuritis will have a normal-appearing fundus, suggesting that most of the swelling is retrobulbar or behind the optic nerve head.⁴ However, OCT may reveal subclinical optic disc swelling in some of these patients (Case 1). The optic disc will demonstrate mild swelling on ophthalmoscopy in about one-third of
patients. Terms to describe this optic nerve head swelling include anterior optic neuritis or papillitis. Children are more likely than adults to have anterior optic neuritis.⁸ The presence of severe optic disc swelling in an adult with vision loss in one eye is unlikely to be idiopathic demyelinating optic neuritis, and other etiologies should be considered. Bilateral optic neuropathy is also unusual for typical optic neuritis, and other causes, such as sarcoid, syphilis, vasculitis, and viral processes, should be sought. It should be remembered that papilledema, which is the term used to describe optic nerve head swelling from raised intracranial pressure, usually does not affect the visual acuity like a case of optic neuritis.