Neuromyelitis Optica, IgG Autoantibodies

Neuromyelitis optica (NMO), also known as Devic's disease, is an immune-mediated neurologic disorder that causes very severe, often bilateral, optic neuritis and longitudinally extensive spinal cord lesions.^2,3 Historically, NMO was thought to be a form of multiple sclerosis;³ however, the nerve damage suffered by patients with NMO has been shown to be distinct from that found in patients with multiple sclerosis.^2-4 Patients with NMO can present with a variety of symptoms, including visual impairment, muscle weakness, reduced peripheral sensation, and loss of bladder and/or bowel control.⁵ NMO typically follows a relapsing course without marked remission between flare-ups. Neural damage tends to be cumulative, resulting in rapid progression of disability and the development of irreversible deficits.⁶

Recently, antibodies to the most abundant water channel in the central nervous system, aquaporin-4 (AQP4), have been found in patients with NMO.¹ These antibodies, referred to as AQP4-Ab or NMO-Ab, have been detected in 60% to 90% of patients with NMO and have been shown to correlate with NMO disease activity.^1,3 NMO-Ab have also been observed in some patients with limited forms of the disease (NMO spectrum disorders), including longitudinally extensive transverse myelitis (LETM) and isolated optic neuritis. Studies have demonstrated that injection of NMO-Ab-positive serum into animals or cultured cells expressing AQP4 produces a disruption of the blood-brain barrier, impairment of glutamate homeostasis, and induction of necrotic cell death.³ These studies suggest a direct role of NMO-Ab in the pathogenesis of NMO.

Immunoassays employing a variety of different immunological techniques for the detection of NMO-Ab in patients with NMO have been described.³ Wingerchuk and associates have proposed a set of criteria for the diagnosis of neuromyelitis optica that incorporate the assessment of NMO-Ab status.⁷ This guideline suggests that the diagnosis of NMO requires two absolute criteria (optic neuritis and acute myelitis) plus at least two of three supportive criteria, one of which is NMO-Ab seropositive status. Measurement of NMO-Ab can be of value in distinguishing NMO from MS when full clinical features may not be apparent and early intervention may prevent or delay disability. Early diagnosis of NMO is critical for avoiding the cumulative effects that result in progressive disability. A positive value is consistent with NMO or a related disorder and helps justify initiation of appropriate therapy.

Four hundred thirty-three samples routinely submitted for NMO-Ab testing were tested by both the aquaporin-4 autoantibody ELISA and indirect immunofluorescence assay (IFA) performed at Mayo Clinic Laboratory.⁸ 98.1% of results were in agreement with a positive result agreement of 78.6% and negative result agreement of 98.8% (IFA as reference method). These results are comparable to those observed in a previously published study comparing an ELISA to IFA for diagnosing NMO.⁹