Miastenia gravis «seronegativa» y anticuerpos antiMuSK positivos: descripción de una serie española

Differential response to rituximab in anti-AChR and anti-MuSK positive myasthenia gravis patients: a single-center retrospective study

BACKGROUND

B-cell targeted therapy with rituximab has shown durable response in treating refractory myasthenia gravis (MG). This study compares the response to rituximab between patients with acetylcholine receptor autoantibody positive (AChR+) and muscle-specific kinase autoantibody positive (MuSK+) MG.

METHODS

This retrospective study included 33 patients with either AChR+ or MuSK+ MG who were treated with rituximab from 05/31/2003 to 05/31/2017. Pretreatment and post-treatment immunotherapy regimens, clinical symptoms, and examination findings were evaluated.

RESULTS

Median MGFA Class of II at baseline improved to an asymptomatic median classification at 12-months and last follow-up (p-values <.001) post-rituximab. Improvement in MGFA class was not significantly different between the groups. Twenty-one patients achieved clinical remission (12/17 AChR+, 9/16 MuSK+) with time to remission of 441.4 ± 336.6 days for AChR+ versus 230 ± 180.8 days for MuSK+ patients (p-value 0.049). The mean prednisone dosage requirement decreased significantly in both groups post-rituximab. AChR+ patients required more hospitalizations for exacerbation post-rituximab (p-value 0.046).

CONCLUSION

Rituximab treatment response is observed in both AChR+ and MuSK+ patients supporting the role of B cell depletion in the management of MG. While there was no significant difference between these groups in terms of clinical improvement, symptom-free state, and prednisone burden, MuSK+ MG patients may experience greater benefits, including earlier time to remission, fewer exacerbations and hospitalizations post-treatment.

Clinical Features and Diagnostic Usefulness of Antibodies to Clustered Acetylcholine Receptors in the Diagnosis of Seronegative Myasthenia Gravis

IMPORTANCE

Cell-based assays (CBAs) were shown to improve detection of acetylcholine receptor (AChR) antibodies in patients with myasthenia gravis (MG). Herein, we asked whether these assays were able to help determine the diagnosis in patients studied in routine clinical practice.

OBJECTIVES

To determine the diagnostic usefulness of CBAs in the diagnosis of MG and to compare the clinical features of patients with antibodies only to clustered AChRs with those of patients with seronegative MG (SNMG).

DESIGN, SETTING, AND PARTICIPANTS

All patients with clinical suspicion of MG who were seen within the Division of Clinical Neurology at the John Radcliffe Hospital in Oxford, England, between November 1, 2009, and November 30, 2013. Their serum antibodies and clinical features were studied.

EXPOSURES

Radioimmunoprecipitation assay (RIPA) and CBA were used to test for standard AChR antibodies and antibodies to clustered AChRs in 138 patients. All available samples from patients with SNMG were retrospectively tested for lipoprotein receptor-related protein 4 (LRP4) antibodies.

MAIN OUTCOMES AND MEASURES

Demographic, clinical, neurophysiological, and laboratory data.

RESULTS

In total, 138 patients were tested for antibodies to clustered AChRs, and 42 had a final diagnosis of MG. The clustered AChR CBA detected antibodies in 38.1% (16 of 42) of RIPA-negative patients with MG with 100% specificity. All patients with SNMG who were tested for LRP4 antibodies (21 of 26) were negative by CBA. Compared with patients with SNMG, patients with antibodies only to clustered AChRs had frequent prepubertal onset (62.5% [median age, 6 years; age range, 1-52 years] vs 11.5% [median age, 38 years; age range, 2-72 years], P ≤ .05), high prevalence of ocular MG (62.5% vs 42.3%), milder disease severity with less bulbar involvement (25.0% vs 46.2%), and absence of respiratory symptoms (0% vs 23.1%). Response to treatment and prognosis was good, with a reduced need for thymectomy (6.3% vs 19.2%) and a high proportion of patients going into remission (50.0% vs 8.3%, P ≤ .05). These observations also apply to the classic AChR MG phenotype seen in large series.

CONCLUSIONS AND RELEVANCE

Cell-based assay is a useful procedure in the routine diagnosis of RIPA-negative MG, particularly in children. Patients with antibodies only to clustered AChRs appear to be younger and have milder disease than other patients with MG. These observations will have implications in planning treatment.

The role of muscle-specific tyrosine kinase (MuSK) and mystery of MuSK myasthenia gravis

MuSK myasthenia gravis is a rare, severe autoimmune disease of the neuromuscular junction, only identified in 2001, with unclear pathogenic mechanisms. In this review we describe the clinical aspects that distinguish MuSK MG from AChR MG, review what is known about the role of MuSK in the development and function of the neuromuscular junction, and discuss the data that address how the antibodies to MuSK lead to neuromuscular transmission failure.

Sustained response to Rituximab in anti-AChR and anti-MuSK positive Myasthenia Gravis patients

We report the results of treatment with Rituximab in six severe, non-responder MG patients. We treated three AChR+MG and three MuSK+MG patients, representing 2% and 20% of the respective groups of our series. Patients were assessed according to the Myasthenia Gravis Foundation of America (MGFA) recommendations. Antibody titers to AChR and MuSK, Ig levels, and IgG subclasses, were tested before treatment and during a follow-up of 9-22 months. All patients, one class V and five class IVB, improved dramatically, with no side effects. Antibody titers declined in all patients (p=0.006). The decline was significantly better in MuSK+MG patients at 9 months (p=0.046) and correlated with a more sustained clinical improvement. We did not find any significant changes in IgG4 that could explain the different outcome observed between these two groups.

Antibodies to AChR, MuSK and VGKC in a patient with myasthenia gravis and Morvan's syndrome

BACKGROUND

A 46-year-old woman presented to a local hospital with acute respiratory failure and a 2-year progressive history of fatigue, personality changes, increased sweating, dysphagia with substantial weight loss, dysarthria, and intermittent ptosis and diplopia. Neurological examination showed facial weakness, lingual atrophy and bulbar palsy, which necessitated the use of a feeding tube and ventilatory support. Mild limb weakness with severe muscle atrophy and diffuse muscle twitches were observed. The patient had also developed visual hallucinations and persecutory delusions. Her personal and family medical histories were unremarkable.

INVESTIGATIONS

Sensory and motor nerve conduction studies, repetitive nerve stimulation, electromyogram, blood-cell counts, general chemistry and metabolic function tests, a CT scan, an [(18)F]fluorodeoxyglucose-PET scan, and tests for serum antibodies to acetylcholine receptors, muscle-specific tyrosine kinase, voltage-gated potassium channels, P/Q-type voltage-gated calcium channels, and paraneoplastic antigens, were carried out.

DIAGNOSIS

Myasthenia gravis associated with antibodies to acetylcholine receptor and muscle-specific tyrosine kinase, and Morvan's syndrome associated with antibodies to voltage-gated potassium channels in the absence of thymoma.

MANAGEMENT

Combined treatment with prednisone, intravenous immunoglobulin, ciclosporin, and rituximab.

Clinical and experimental features of MuSK antibody positive MG in Japan

We investigated the presence of antibodies (Abs) against muscle-specific tyrosine kinase (MuSK) in Japanese myasthenia gravis (MG) patients. MuSK Abs were found in 23 (27%) of 85 generalized seronegative MG (SNMG) patients but not in any of the ocular MG patients. MuSK Ab-positive patients were characterized as having female dominance (M:F, 5:18), age range at onset 18 to 72 (median 45) years old, and prominent oculobulbar symptoms (100%) with neck (57%) or respiratory (35%) muscle weakness. Limb muscle weakness was comparatively less severe (52%), thymoma absent. Most patients had good responses to simple plasma exchange and steroid therapy. MuSK IgG from all 18 patients was exclusively the IgG 4 subclass and bound mainly with the MuSK Ig 1-2 domain. Serial studies of 12 individuals showed a close correlation between the variation in MuSK Ab titers and MG clinical severity (P = 0.01 by Kruskal-Wallis). MuSK Ab titers were sharply decreased in patients who had a good response to early steroid therapy or simple plasma exchange, but there was no change, or a rapid increase on exacerbation after thymectomy. Measurement of MuSK Ab titers aids in the diagnosis of MG and the monitoring of clinical courses after treatment.