Seronegative Myasthenia Gravis
Myasthenia Gravis (MG) is an autoimmune disorder affecting the neuromuscular junction resulting in fatigable muscle weakness.  The three antibodies associated with myasthenia gravis include acetylcholine receptor (AchR) antibodies, muscle-specific tyrosine kinase (MuSK) antibodies, and lipoprotein receptor-related protein 4 (LRP4). AchR antibodies are found in approximately 85% of patients. Seronegative MG refers to patients who lack AchR receptors but have MuSK antibodies present which is found in about 5% of patients. Double seronegative MG refers to the absence of both AchR and MuSK antibodies and occurs in about 10% of patients. However, LRP4 antibodies have been noted in some patients to varying degrees.3 
Myasthenia Gravis (MG) is an autoimmune disease most commonly involving IgG1 antibodies against acetylcholine receptors (AChR antibodies) in the neuromuscular junction. Patients with myasthenia gravis who lack AChR antibodies are classified as seronegative MG. As high as 70% of patients with seronegative myasthenia gravis have muscle-specific tyrosine kinase (MuSK) which are typically IgG4.
Antibodies against AChR activate the complement cascade destroying the postsynaptic membrane. Crosslinking of AChRs by antibodies results in internalization and destruction of the receptor, also known as antigenic modulation. Anti-AchR antibodies also bind the AChR ligand binding site, inhibiting receptor activation after acetylcholine binding. MuSK is involved in the organization and maintenance of the neuromuscular junction. Anti-MuSK antibodies do not cause pathogenicity using the same mechanisms as AChR antibodies but rather appear to interfere with cellular signaling pathways rather than causing complement activation. In vivo, LRP4 antibodies block interactions between LRP4 and agrin as well as blocking AChR clustering. Consequently, LRP4 antibodies are pathogenic through their inference with AChR function. 
Seronegative myasthenia gravis presents similar to the classic seropositive disease in terms of fatigable muscle weakness, severity, and response to treatment. Both can have ocular myasthenia gravis, presenting with ptosis and diplopia, and/or generalized myasthenia gravis, presenting with overall use-dependent limb-muscle. In general, seronegative myasthenia gravis has a stronger female predominance and presents at a younger age compared to seropositive disease. Additionally, seronegative myasthenia gravis often has predominant ocular and bulbar weakness rather than widespread limb fatigue. It therefore often manifests as diplopia, ptosis, dysphagia, and dysarthria. This predominance of ocular symptoms over general weakness is seen in all forms of seronegative myasthenia gravis, including both single-seronegative anti-MuSK and double-seronegative disease. Seronegative disease is also typically milder and less commonly becomes generalized myasthenia gravis. The response to treatment varies, with some studies saying seronegative has a poorer response to immunotherapy and others finding its equal or improved. However, seronegative disease often responds well to both immunosuppressive therapy and plasma exchange.
Myasthenia gravis described as variable and fatigable muscle weakness; MG has specific biomarkers that can be used to diagnose a patient. These include the following:
- Acetylcholine receptor (AchR) antibodies
- Muscle specific kinase (MuSK) antibodies
- Lipoprotein receptor related antibody 4 (LRP4)
- Titin (anti-striated muscle) antibodies
Based on these diagnostic criteria, myasthenia gravis can be diagnosed as seropositive for AchR, seen in about 85% of all patients with MG. When patients have a negative titer for AchR (binding, blocking, and modulating), they are termed seronegative, and more testing is often performed with the other antibodies if clinicians have strong suspicion of disease. Patients are termed double seronegative if they are also negative for MuSK antibodies (found in ~6% of all MG patients or 40% of patients AchR-negative) while having MG symptoms. The entire myasthenia gravis panel can be ordered if clinical suspicion is still high (including LRP4 and striated muscle antibodies).
Complete seronegative myasthenia gravis is rare, accounting for <10% of all patients with MG5. There is a possibility that the antibodies have yet to be discovered for these patients, but they remain “seronegative” at this time. Additional testing with repetitive nerve stimulation and single-fiber electromyography (EMG) can be performed to make a diagnosis of myasthenia gravis. Single fiber EMG is a highly sensitive test in both generalized and ocular MG.
Myasthenia gravis can have a multitude of signs and symptoms that can mimic other disease processes (see Myasthenia Gravis). The differential diagnosis for seronegative myasthenia gravis is the same as generalized and ocular myasthenia gravis and includes:
- Thyroid Eye Disease
- Lambert-Eaton Syndrome
- Multiple Sclerosis
- Brainstem Gliomas
- Polymyositis and dermatomyositis
Treatment of myasthenia gravis includes acetylcholine esterase inhibitors such as pyridostigmine as well as immunosuppressive therapy such as corticosteroids. Plasmapheresis and intravenous immunoglobulin (IVIG) can be used to manage acute exacerbations. Azathioprine is often the first choice for long term immunosuppression; however, additional immunosuppressive agents include methotrexate, cyclophosphamide, and cyclosporin. Rituximab, a CD-20 blocking monoclonal antibody, is a treatment option for MG. Emerging therapies include belimumab, eculizumab and the granulocyte- macrophage colony-stimulating factor. Thymectomy is often done in younger patients with MG but additional studies are required to determine the benefit from the procedure.
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- Vincent, A., Bowen, J., Newsom-Davis, J., & McConville, J. (2003). Seronegative generalised myasthenia gravis: clinical features, antibodies, and their targets. Lancet Neurology, 2(2), 99–106. https://doi.org/10.1016/S1474-4422(03)00306-5
- Lazaridis K and Tzartos SJ (2020) Autoantibody Specificities in Myasthenia Gravis; Implications for Improved Diagnostics and Therapeutics. Front. Immunol. 11:212. doi: 10.3389/fimmu.2020.00212
- Rivero A, Crovetto L, Lopez L, et al. Single fiber electromyography of extraocular muscles: A sensitive method for the diagnosis of ocular myasthenia gravis. Muscle Nerve 1995;18:943–947.