Acetylcholine Receptor Antibody

Acetylcholine receptor (AChR) antibodies are IgG autoantibodies against the nicotinic acetylcholine receptor at the postsynaptic neuromuscular junction and cause myasthenia gravis. They are detected in serum by radioimmunoassay (binding antibodies, the main method), cell based assay, ELISA or fluorescence assay, and further classed as binding, blocking and modulating antibodies. Binding antibodies are the main first test. Results are reported in nmol/L.

Myasthenia gravis has a prevalence of about 15 to 25 in 100 000 people, with an annual incidence of 1 to 2 in 100 000, a bimodal age distribution (women in the third decade and men in the sixth to seventh decade), and rising frequency in older adults. About 85% of generalised myasthenia gravis has AChR antibodies, and 50 to 60% of purely ocular disease.

Another 6% have anti-MuSK antibodies (mostly young women with bulbar disease), 2% anti-LRP4, and the rest are seronegative. Thymic abnormality is present in 75% of AChR-positive patients (hyperplasia in 65%, thymoma in 10 to 15%).

Indications. Fluctuating, fatigable weakness: ptosis, diplopia, dysphagia, dysarthria, neck extensor and proximal limb weakness, worse at the end of the day. Respiratory weakness of uncertain cause. Suspected myasthenic crisis. Thymoma on imaging. Neonatal hypotonia in the infant of a myasthenic mother. Drug induced myasthenic syndromes (penicillamine, checkpoint inhibitors). Before thymectomy. Not for monitoring routine disease activity.

Specimen. Serum 1 to 2 mL, no fasting. Stable for days at 2 to 8 °C and longer frozen. Collected before plasma exchange or immunoglobulin therapy, since these lower titres.

Reference values (radioimmunoassay).

• Binding antibody: negative under 0.25 nmol/L, equivocal 0.25 to 0.40 nmol/L, positive over 0.40 nmol/L.
• Modulating antibody: negative under 20% modulation, positive over 20 to 30%.
• Blocking antibody: negative under 25%.

Interpretation.

• Positive AChR binding antibody with fatigable weakness: myasthenia gravis is confirmed. Specificity is over 99%, so a positive result is diagnostic in the right clinical setting.
• Negative AChR binding antibody: does not exclude disease (generalised 15%, ocular 40 to 50%), and anti-MuSK, anti-LRP4 and clustered AChR cell based assays are requested, with repetitive nerve stimulation and single fibre electromyography.
• Titre does not correlate with severity between patients, although within a patient a fall of titre after treatment or thymectomy tracks improvement.
• Anti-MuSK positive: bulbar, neck and respiratory predominant, young women, poor response to anticholinesterases and thymectomy not useful, with good response to rituximab.
• Anti-LRP4 positive: usually mild.
• Positive AChR antibody with thymoma: very high titres are common, and anti-titin and anti-ryanodine receptor antibodies suggest thymoma and more severe disease.
• Positive binding antibody with no myasthenic features: consider thymoma, Lambert-Eaton overlap, drug induced autoimmunity (penicillamine), graft-versus-host disease, or hereditary and acquired conditions with low titre positivity (lung cancer, primary biliary cholangitis, amyotrophic lateral sclerosis).

Critical values and findings needing immediate action.

• Myasthenic crisis (dyspnoea, weak cough, bulbar weakness, forced vital capacity under 20 mL/kg or falling, negative inspiratory force worse than minus 30 cmH2O): intensive care, ventilatory support, plasma exchange or intravenous immunoglobulin, and withdrawal of provoking drugs.
• Positive antibody with bulbar symptoms, aspiration risk and weight loss: admission, swallow assessment and early immunotherapy.
• Thymoma on CT chest: surgical referral for thymectomy and tumour staging.
• Drugs that worsen myasthenia (fluoroquinolones, aminoglycosides, macrolides, magnesium, beta blockers, botulinum toxin, checkpoint inhibitors): stop or replace.
• Neonate of a myasthenic mother with weak cry and poor feeding: neonatal myasthenia, neonatal unit monitoring.

Interferences and pitfalls.

• A negative test does not exclude myasthenia. Seronegative disease requires neurophysiology, a bedside ice pack or edrophonium test, and anti-MuSK and anti-LRP4 testing.
• Low positive results (0.25 to 0.40 nmol/L) have low predictive value and need clinical correlation and repeat testing.
• Titres can be falsely raised by a prior radioisotope study (a recent nuclear medicine scan with isotopes such as iodine-125 interferes with radioimmunoassay).
• Immunosuppression and plasma exchange lower titres, and a negative sample after treatment may reflect therapy.
• Antibody concentration cannot be used to adjust therapy, and clinical scales (myasthenia gravis activities of daily living, quantitative myasthenia gravis score) are used.
• Other causes of ptosis and diplopia are excluded: thyroid eye disease, cranial nerve palsy, mitochondrial disease, botulism, Lambert-Eaton syndrome, brainstem lesions, motor neurone disease.
• Thymic imaging (CT or MRI chest) is mandatory in all patients with AChR antibodies, and not only those with generalised disease.

Special groups.

• Children: congenital myasthenic syndromes (seronegative, genetic) differ from juvenile autoimmune myasthenia. Transient neonatal myasthenia from transplacental antibody lasts 2 to 4 weeks.
• Pregnancy: disease may worsen or improve, and magnesium sulphate is avoided. Neonatal myasthenia occurs in 10 to 20% of babies.
• Older adults: late onset myasthenia (over 50 years) is common, often AChR-positive without thymoma, with ocular onset and frequent comorbidity.
• Regions: access to the antibody assay is limited, and diagnosis often rests on clinical findings, the neostigmine or edrophonium response and electrophysiology.

Follow up tests. Anti-MuSK, anti-LRP4, anti-striated muscle (titin, ryanodine) antibodies, CT or MRI of the mediastinum, repetitive nerve stimulation, single fibre electromyography, spirometry (forced vital capacity), thyroid function and thyroid antibodies, ANA, B12, HbA1c, screening for hepatitis B, tuberculosis and HIV before immunosuppression, bone density scan with long term steroids, and thiopurine methyltransferase before azathioprine.