Neuromyelitis optica spectrum disorder

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

AQP4 is concentrated on astrocytic end-feet at the blood-brain barrier. Pathogenic AQP4-IgG activates complement and recruits inflammatory cells, producing astrocyte injury, secondary demyelination and necrosis. This explains the predilection for optic nerves, central spinal cord, medulla around the area postrema and other AQP4-rich sites. Seronegative NMOSD is a more heterogeneous clinical category and requires stricter imaging criteria and careful exclusion of mimics.

Attacks, rather than slow progression between attacks, drive most disability. Optic neuritis is often posterior, long-segment, chiasmal or bilateral and may leave poor acuity or colour vision. Cord disease can be extensive, centrally placed and swollen, causing weakness, a defined sensory level, neuropathic pain and sphincter dysfunction. Brainstem disease can compromise swallowing or respiration. A normal brain MRI does not exclude the diagnosis.

Diagnosis and treatment should sit with an experienced neuroinflammatory service. AQP4 and MOG assays are most informative in serum using validated cell-based methods. Immunotherapy can reduce attack risk but brings infection, haematological, hepatic, reproductive and vaccine considerations. Product choice depends on serostatus, previous attacks, comorbidity, pregnancy plans, access and NHS commissioning, with shared decisions documented.

Key points

Neuromyelitis optica spectrum disorder is an autoimmune astrocytopathy, most often associated with serum aquaporin-4 immunoglobulin G; it is not simply a severe form of multiple sclerosis.
Core presentations are severe or bilateral optic neuritis, acute myelitis, area-postrema syndrome, an acute brainstem syndrome, symptomatic diencephalic disease and characteristic cerebral syndromes.
Longitudinally extensive transverse myelitis spans three or more vertebral segments, although an early or treated lesion can be shorter and a normal first scan does not settle a convincing case.
Area-postrema inflammation causes otherwise unexplained intractable hiccups, nausea or vomiting and may precede optic-nerve or cord involvement by weeks.
Request a serum AQP4-IgG cell-based assay before immunotherapy when feasible; test for MOG-IgG in an appropriate AQP4-negative phenotype because MOG-associated disease has different implications.
A severe attack is treated urgently with high-dose intravenous corticosteroid and early plasma exchange when response is incomplete or the deficit is profound; arrangements vary between specialist centres.
AQP4-positive disease is usually relapsing, so one confirmed attack commonly prompts discussion of relapse-prevention immunotherapy rather than watchful waiting for a second event.
Some multiple-sclerosis disease-modifying treatments are ineffective or potentially harmful in NMOSD; diagnostic precision must precede long-term treatment selection.
Recovery, pain, spasticity, bladder dysfunction, vision rehabilitation, mood, pregnancy and vaccination all need planned multidisciplinary follow-up alongside immunosuppression safety.
The regimens below describe common UK specialist practice; acute dosing, exchange schedules and maintenance biologics require local neuroimmunology protocols.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Aquaporin-four autoimmunity

Most NMOSD is associated with pathogenic serum AQP4 immunoglobulin targeting astrocytes, often alongside other autoimmune susceptibility.

Seronegative NMOSD

A compatible core syndrome may meet specialist criteria without detectable AQP4 antibody after careful assay, timing and mimic review.

Associated systemic disease

Systemic autoimmunity and rarely neoplasia may coexist, prompting phenotype-led assessment without assuming that every positive antibody explains the neurological syndrome.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Astrocyte antibody binding
AQP4 antibodies cross a disrupted blood–brain barrier and bind water channels concentrated on astrocyte endfeet during inflammatory attacks.
2
Complement-mediated inflammation
Antibody binding activates complement and recruits inflammatory cells, producing astrocyte injury, oedema and blood–brain barrier breakdown.
3
Secondary demyelination and necrosis
Severe astrocytic injury damages oligodendrocytes, myelin and axons in optic nerves, spinal cord, area postrema and selected brain regions.
4
Attack-driven disability
Each relapse can destroy substantial tissue, so incomplete recovery rather than slow independent progression usually drives accumulating disability.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Severe optic neuritisRed flag

Eye pain is followed by marked monocular or bilateral visual loss, impaired colour perception and a relative afferent pupillary defect. Chiasmal involvement, poor recovery or recurrent attacks should raise NMOSD above typical MS-associated optic neuritis.

Long cord syndromeRed flag

Subacute bilateral weakness, a sensory level, band-like pain and bladder or bowel dysfunction evolve over hours to days. High cervical lesions may cause diaphragmatic weakness and demand respiratory monitoring.

Area-postrema syndromeRed flag

Persistent hiccups or vomiting has no adequate gastrointestinal explanation and may coexist with vertigo, dysphagia or an upper-cervical sensory disturbance. Recognising this pattern prevents repeated abdominal investigations while inflammation advances.

Relapsing phenotype

Neurological episodes separated in time affect optic nerve, cord or brainstem, with incomplete recovery and stepwise disability. Previous diagnoses of atypical MS or unexplained recurrent myelitis should be revisited.

Autoimmune context

Women are affected more often, and systemic autoimmunity such as lupus, Sjogren syndrome or autoimmune thyroid disease may coexist. Their presence supports immune assessment but does not establish the neurological diagnosis.

Important alternatives

MOG-associated disease, MS, sarcoidosis, infection, B12 or copper deficiency, vascular myelopathy, malignancy and structural compression can mimic components of NMOSD and require targeted exclusion.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
MRI brain, orbits and whole spine with contrastFirst step
Why
Locate active optic-nerve, medullary, cerebral and cord inflammation while excluding compression and vascular or neoplastic alternatives.
Interpretation and limitations
Look for longitudinal central cord lesions, posterior or chiasmal optic-neuritis patterns and dorsal medullary area-postrema lesions. Imaging supports a syndrome but cannot replace serology and expert clinical correlation.
02
Serum AQP4-IgG cell-based assay
Why
Identify the pathogenic biomarker that strongly supports AQP4-positive NMOSD and determines relapse-prevention choices.
Interpretation and limitations
Take serum before plasma exchange or prolonged immunosuppression when this does not delay care. A negative result is not an invitation to use a low-specificity assay; reconsider timing, phenotype and specialist retesting.
03
Serum MOG-IgG and selected mimic screen
Why
Separate MOG-associated disease and test plausible inflammatory, nutritional, infectious or systemic autoimmune explanations.
Interpretation and limitations
Order MOG-IgG only with a compatible syndrome and interpret low-positive results cautiously. Tailor B12, copper, HIV, syphilis, ANA/ENA, ACE or malignancy tests to history and imaging rather than using an indiscriminate panel.
04
Lumbar puncture
Why
Assess inflammation and exclude infection or malignant infiltration when safe after appropriate imaging.
Interpretation and limitations
Cell count and protein may rise during an attack; neutrophils or eosinophils can occur. CSF-restricted oligoclonal bands are less persistently typical than in MS, but neither their presence nor absence is decisive.
05
Visual and physiological assessment
Why
Quantify baseline injury, functional recovery and complications that may not be captured by routine MRI.
Interpretation and limitations
Record high- and low-contrast acuity, colour vision, fields and OCT where available. Respiratory observations, forced vital capacity, post-void residual and mobility measures are added according to lesion level.
06
Pretreatment immune-safety work-up
Why
Make urgent and maintenance immunotherapy safer without obscuring the need for timely attack treatment.
Interpretation and limitations
Use local protocols for full blood count, renal and liver profile, immunoglobulins, hepatitis, HIV, tuberculosis, pregnancy testing and vaccination review; particular biologics require additional labelled screening.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Multiple sclerosis

Shorter cord lesions, typical brain plaques and less severe optic neuritis favour MS; some MS treatments are unsuitable in NMOSD.

MOG-associated disease

Marked disc swelling, bilateral optic neuritis and different relapse patterns support MOG disease, confirmed by an appropriate serum cell-based assay.

Compressive or vascular myelopathy

Mechanical compression or maximal-at-onset cord deficit favours structural or ischaemic injury and should be excluded before immune treatment.

Sarcoid, infection or malignancy

Persistent enhancement, systemic inflammation, marked cerebrospinal-fluid pleocytosis or infiltrative imaging should broaden assessment beyond antibody-mediated disease.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01Acute attackTreat threatened function promptlyFirst stepNew objective optic, spinal, brainstem or cerebral deficit compatible with inflammatory relapse.

1Admit when vision, walking, respiration, swallowing or sphincter function is threatened; document a neurological baseline and arrange urgent MRI while excluding compression, vascular disease and infection.
2After senior neuroinflammatory agreement, give high-dose intravenous methylprednisolone, commonly 1 g daily for three to five days in adults, with glucose, blood-pressure, gastric and infection precautions under local policy.
3EscalationEscalate early to therapeutic plasma exchange, often five to seven exchanges, if the attack is severe or steroid response is insufficient; do not postpone referral until the steroid course is long finished.
4Begin rehabilitation, thromboprophylaxis, pressure-area care and management of pain, spasticity, bladder and bowel function in parallel rather than after immunotherapy ends.

02Secure diagnosisClassify the inflammatory syndromeOptic neuritis, longitudinal myelitis or area-postrema symptoms raise NMOSD suspicion.

1Send validated serum AQP4-IgG and, when phenotypically justified, MOG-IgG before exchange; retain the exact assay, timing and treatment context in the record.
2Review brain, orbital and whole-spine MRI with neuroradiology, analyse CSF when safe, and investigate infection, systemic inflammation, nutritional disease, malignancy and structural mimics selectively.
3Apply current criteria through an appropriate specialist, using the stricter seronegative requirements and avoiding an MS label based solely on scattered white-matter lesions.

03Prevent relapseBuild a durable specialist planAQP4-positive NMOSD or a recurrent/high-risk seronegative syndrome is confirmed.

1Discuss relapse severity, serostatus, disability, age, infection history, fertility and pregnancy intentions, vaccination status and individual NHS access before choosing maintenance therapy.
2Select and initiate a commissioned or locally approved immunotherapy such as B-cell depletion, conventional immunosuppression or a licensed targeted agent through the neuroimmunology service.
3Provide written relapse instructions, monitoring dates and named contact routes, and coordinate ophthalmology, rehabilitation, continence, pain, mental-health and primary-care support.

Key medicines and prescribing safety5 treatments · regimens, roles and cautions

Suppresses acute inflammation and should be started rapidly when a disabling NMOSD attack is judged likely.

Intravenous methylprednisolone

A common specialist attack regimen is 1 g intravenously once daily for three to five days.

Exclude or cover serious infection as appropriate; monitor glucose, blood pressure, mood, sleep, gastrointestinal risk and electrolytes, following the treating centre's protocol.

Removes pathogenic antibody and complement and is particularly valuable for severe steroid-incomplete optic or spinal attacks.

Therapeutic plasma exchange

Specialist services commonly deliver five to seven exchanges over about one to two weeks.

Requires apheresis expertise and vascular-access planning; monitor haemodynamics, calcium, fibrinogen, infection, bleeding and interactions with medicines removed by exchange.

B-cell depletion is used for relapse prevention in selected NMOSD, particularly AQP4-positive disease.

Rituximab

Induction and redosing schedules vary; prescribe only through the specialist NHS protocol and commissioning route.

Screen for hepatitis B and other infection risks, review vaccines, immunoglobulins and blood counts, and plan around pregnancy; infusion reactions and late hypogammaglobulinaemia require surveillance.

Conventional steroid-sparing immunosuppression may be selected when biologic treatment is unsuitable or unavailable.

Azathioprine or mycophenolate mofetil

Start low and titrate by body size, response and laboratory monitoring under neuroimmunology supervision.

Check TPMT or NUDT15 where relevant before azathioprine, monitor blood count and liver function, manage infection and skin-cancer risk, and apply drug-specific contraception and pregnancy advice.

Complement, interleukin-6 receptor or B-cell directed drugs may reduce relapses in eligible AQP4-positive disease.

Licensed targeted biologic therapy

Agent-specific loading and maintenance regimens must follow the current product licence and NHS commissioning criteria.

Eligibility and infection hazards differ substantially; meningococcal protection may be critical with complement inhibition, and urgent specialist safeguards cannot be substituted by a generic dose summary.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Permanent blindness

Severe or recurrent optic neuritis can cause profound bilateral axonal loss and loss of independent vision.

Paralysis and autonomic dysfunction

Extensive myelitis leaves weakness, spasticity, neuropathic pain, respiratory risk and bladder, bowel or sexual dysfunction after severe attacks.

Area-postrema morbidity

Intractable vomiting and hiccups cause dehydration, electrolyte disturbance, weight loss and aspiration before the neurological cause is recognised.

Relapse and immunotherapy harm

Untreated AQP4-positive disease commonly relapses, while prevention treatment adds infection, blood, reproductive and organ-monitoring burdens during long-term care.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Track each suspected attack with dated objective measures of acuity, power, sensation, gait, respiratory function and sphincter status so true relapse is distinguished from fluctuation or infection-related worsening.
Perform treatment-specific blood counts, liver and renal profiles, immunoglobulins, B-cell markers or infection surveillance at the intervals set by the neuroimmunology service.
Review vaccinations before sustained immunosuppression and provide clear advice on fever, exposure and live vaccines; timing around B-cell depletion needs specialist planning.
Use ophthalmic acuity, colour testing, fields and OCT, together with mobility, falls, spasticity, pain, fatigue and continence review, to measure disability beyond the conventional examination.
Revisit reproductive intentions and pregnancy plans before every material therapy change because relapse risk and fetal or neonatal exposure differ between medicines and disease phases.
Safety-net new eye pain, loss of colour or acuity, ascending numbness, weakness, retention, persistent hiccups or vomiting for same-day neurological contact.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Serum comes first

AQP4-IgG is generally more sensitive in serum than CSF, and exchange or immunotherapy can lower the measured signal; record when the sample was obtained.

Short lesions can mislead

A cord lesion may look short very early, late or after treatment, so lesion length must be interpreted against timing and the whole phenotype.

MOGAD is separate

MOG antibody-associated disease can resemble NMOSD but differs in relapse probability, imaging, prognosis and maintenance decisions; it should not be renamed seronegative NMO automatically.

Hiccups are neurological

Persistent hiccups with unexplained vomiting can localise to the dorsal medulla and may be the first actionable clue to an AQP4-mediated attack.

Disability is attack-led

Preventing the next relapse is central because recovery from each optic or cord event may be incomplete even when interval progression is absent.

Pregnancy needs anticipation

Disease behaviour, placental drug transfer, postpartum relapse and breastfeeding compatibility require preconception neuroimmunology and obstetric planning rather than abrupt self-discontinuation.

11Common pitfallsFrequent interpretation and management errors.

01
Calling a severe optic neuritis episode multiple sclerosis before checking AQP4-IgG, MOG-IgG and the orbital MRI pattern.
02
Waiting several days after an incomplete steroid response before discussing plasma exchange for profound visual or motor loss.
03
Using low-specificity antibody results without the clinical syndrome, assay method or specialist laboratory confirmation.
04
Treating persistent vomiting only as gastrointestinal disease when a dorsal medullary syndrome or previous myelitis is present.
05
Starting an MS-specific disease-modifying medicine in an unresolved NMOSD phenotype.
06
Focusing on relapse count while overlooking neuropathic pain, vision loss, continence, mood, fertility and rehabilitation needs.