Optic neuritis (ON) is a debilitating condition that is defined as inflammation of cranial nerve II which results in disruption of the neurologic pathways that allow visual sensory information received by the retina to be able to be transmitted to the visual cortex of the brain. This disorder of the optic nerve may arise through various pathophysiologic mechanisms, such as through demyelination or inflammation, leading to partial or total loss of vision. Optic neuritis may be a result of standalone idiopathic disease, but is often a manifestation that occurs secondary to an underlying disease. Signs of ON classically present as sudden-onset visual impairment in one or both eyes that can range in severity from mild visual blurring to complete blindness in the affected eye(s). Although pain is typically considered a hallmark feature of optic neuritis, the absence of pain does not preclude a diagnosis or consideration of ON as some patients may not report any pain. ON is typically subtyped into "typical" ON and "atypical" ON. The most commonly considered etiologies are multiple sclerosis (MS), neuromyelitis optica (NMO) / neuromyelitis optica spectrum disorder (NMOSD), and myelin oligodendrocyte glycoprotein-antibody-associated disease (MOGAD). Other etiologies include idiopathic ON, infections (e.g., syphilis, Lyme disease, and viral infections such as herpes simplex and varicella-zoster), and systemic autoimmune diseases (e.g., systemic lupus erythematosus and sarcoidosis). Diagnosis of ON can be made with a combination of symptom manifestation, clinical exam findings, imaging findings, and serologic studies. Modern medical practice employs high-dose steroids, such as IV methylprednisolone, as the first-line treatment for optic neuritis. Optic neuritis should not be confused with optic neuropathy, which is a condition manifesting as visual impairment that occurs as a result of damage to the optic nerve from any cause – one of those causes being optic neuritis.
Classification
Definition The optic nerve comprises axons that emerge from the retina of the eye and carry visual information to the primary visual nuclei, most of which is relayed to the occipital cortex of the brain to be processed into vision. The phrase optic neuritis is derived from the pathophysiologic changes observed in this disorder whereby the optic nerve (optic) becomes inflamed (neuritis). As such, optic neuritis is typically a manifestation of an underlying, causative etiology. Because of the breadth of underlying causes of optic neuritis, this disorder is typically classified into the subtypes of "typical" ON and "atypical" ON. Typical ON refers to a demyelinating etiology which most commonly stems from multiple sclerosis or standalone idiopathic disease. Atypical ON essentially refers to optic neuritis from any other cause.
Typical Optic Neuritis ON is classified as typical optic neuritis when demyelination is the causative pathophysiologic aberrancy. This classification attributes the manifestation of ON primarily to multiple sclerosis or standalone idiopathic disease, of which multiple sclerosis is the most common underlying etiology of all subtypes of optic neuritis.
Multiple Sclerosis Multiple sclerosis, in brief, is an autoimmune disease that results in demyelination of the nerves in the central nervous system (CNS). Demyelination in MS can be diffusely affect the CNS leading to a multitude of physical manifestations. Optic neuritis is acquired this way when MS spreads to the optic nerve. Optic neuritis is often among the first, if not the foremost, manifestation of MS. One review found that inflammatory demyelinating optic neuritic (IDON) is the initial presentation of MS in approximately 20% of studied patients, while another review found that up to 50% of MS patients will develop optic neuritis during their disease course.
Idiopathic Disease Idiopathic optic neuritis (ION) generally refers to optic neuritis that does not present with serologic markers attributable to a known underlying cause (e.g., NMOSD, MOGAD, etc.). Opinions among specialists seem to differ as to whether idiopathic optic neuritis should be classified as MS-associated ON or if it should not be exclusively associated with MS. A group of reviewers from China propose that ION should not be associated with MS and hence defined with the literal meaning of "idiopathic" as ON with an unknown cause. On the other hand, multiple groups of Western reviewers propose that ION should be considered a preceding manifestation, among a collection of manifestations, that precedes the development of MS in the absence of other serologic findings.
Atypical Optic Neuritis ON is classified as atypical optic neuritis when the underlying cause is an etiology other than multiple sclerosis or standalone idiopathic disease. Atypical ON is most frequently seen as an early manifestation of neuromyelitis optica spectrum disorder (NMOSD), formerly known as neuromyelitis optica (NMO). Other causes of atypical ON include myelin oligodendrocyte glycoprotein-antibody-associated disease (MOGAD), other autoimmune disorders (e.g., Sarcoidosis, Sjogren syndrome, rheumatoid arthritis, systemic lupus erythematosus), and infections (Bacterial [Tuberculosis, syphilis, meningitis, Lyme's disease, Bartonella] or Viral [measles, mumps, rubella, chicken pox, herpes]).
NMOSD (Formerly NMO) Neuromyelitis optica spectrum disorder (NMOSD) is a disorder consisting of six syndromes that cause inflammation and demyelination of the CNS. The hallmark diagnostic criteria of NMOSD is the presence of the aquaporin-4 immunoglobulin G antibodies (AQP4-IgG) found on serology, which is the underlying aberrancy of this autoimmune condition that engenders its distinction from multiple sclerosis. Clinical manifestations of NMOSD, such as optic neuritis, occur due to antibody-mediated damage against the aquaporin-4 transmembrane water channels which are found in the foot processes of astrocytes at high concentrations within the optic nerve, brainstem, and spinal cord.
MOGAD Myelin oligodendrocyte glycoprotein-antibody-associated disease (MOGAD) is an autoimmune condition against the myelin oligodendrocyte glycoprotein (MOG) located on both the myelin sheathe and oligodendrocyte cell surfaces of the CNS. Cell-mediated and complement-mediated inflammation results hallmark findings of perivenous and confluent white matter demyelination.
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