MRI evidence of early muscle atrophy in MuSK positive myasthenia gravis

Myasthenia gravis with tongue muscle atrophy: A case series

Here, we presented 6 patients who were admitted to our institution and diagnosed as myasthenia gravis (MG) with tongue muscle atrophy. All these 6 patients developed symptoms of bulbar muscle weakness in acetylcholine receptor antibodies positive MG (AChR-MG) (3/6), muscle-specific receptor tyrosine kinase antibodies positive MG (MuSK-MG) (1/6), and sero-negative MG (2/6). Most of patients had "triple-furrowed" tongue except for patient 2 with irregular atrophy of tongue muscle. Tongue muscle atrophy occurs in patients with MuSK-MG, AChR-MG, and sero-negative MG. Atrophied tongue muscles of five patients with MG were reversible after immunotherapy.

MRI evidence of extraocular muscle atrophy and fatty replacement in myasthenia gravis

PURPOSE

The aim of the study was to evaluate extraocular muscle (EOM) atrophy and fatty replacement in ocular myasthenia gravis (OMG) and generalized myasthenia gravis (GMG) patients with chronic and untreated ocular symptoms or with inadequate response to immunotherapy and unprovoked ocular exacerbations despite chronic immunotherapy.

METHODS

Nineteen patients with either OMG or GMG and 19 healthy age-matched controls underwent an orbital MRI. Visually obvious muscle atrophy and muscle fatty replacement were evaluated by two raters independently. Maximum thickness of EOM was measured. Measurements of the muscles of each participant were added up, in order to calculate the total thickness.

RESULTS

Eleven patients suffered from AChR-positive GMG, and 8 patients from OMG. All patients had chronic ocular symptoms or inadequate response to corticosteroids and unprovoked ocular exacerbations in spite of immunotherapy. Fatty replacement was reported in 6/19 (31.6%) patients and 0/19 (0%) controls (p = 0.02). Obvious atrophy in at least one muscle was reported in 8/19 (42.1%) patients and 1/19 (5.3%) controls (p = 0.019). Statistically significant differences between the two groups were also found in the mean total thickness, as well as in the thickness of superior recti, levator palpebrae, inferior recti, and superior oblique muscles.

CONCLUSION

EOM atrophy and fatty replacement were seen frequently in our series of MG patients with treatment difficulties and frequent relapses of ocular involvement.

[Myasthenia gravis and muscle atrophy]

Muscle atrophy is uncommon in myasthenia gravis. There are some reports in the literature of muscle atrophy in MUSK-positive myasthenia gravis. The authors present eight cases of AChR-positive myasthenia gravis associated with muscle atrophy. Symmetrical atrophy of the forearms with preserved hand muscles was identified in six cases and scapular winging in two cases. Atrophy appeared 3-18 years after the onset of myasthenia gravis and after a period of progression, the condition was stabilized. In all cases, there were no pain syndrome, no sensory loss, normal creatine phosphokinase (CPK) levels, no significant abnormality on MRI of the cervical spine. Concomitant neuropathies, amyotrophic lateral sclerosis and other neuromuscular diseases were excluded. All patients underwent quantitative electromyography and repetitive nerve stimulation. The authors suggest that the atrophy is not the sign of myasthenia gravis in this case and is caused by other neuromuscular diseases.

MuSK-Associated Myasthenia Gravis: Clinical Features and Management

Muscle-specific tyrosine kinase (MuSK) myasthenia gravis (MG) is a rare, frequently more severe, subtype of MG with different pathogenesis, and peculiar clinical features. The prevalence varies among countries and ethnic groups, affecting 5–8% of all MG patients. MuSK-MG usually has an acute onset affecting mainly the facial-bulbar muscles. The symptoms usually progress rapidly, within a few weeks. Early respiratory crises are frequent. The disease may lead to generalized muscle weakness up to muscle atrophy. The main bulbar involvement, the absence of significant thymus alterations, and the association with HLA class II DR14, DR16, and DQ5 alleles have been confirmed. Atypical onset, such as ocular involvement, lack of symptom fluctuations, acetylcholinesterase inhibitors failure, and negative results of electrophysiologic testing, if not specifically performed in the mainly involved muscle groups, makes MuSK-MG diagnosis challenging. In most cases, steroids are effective. Conventional immunosuppressants are not commonly able to replace steroids in maintaining a satisfactory long-term control of symptoms. However, the majority of MuSK-MG patients are refractory to treatment. In these cases, the use of rituximab showed promising results, resulting in sustained symptom control.

Muscle-Specific Tyrosine Kinase and Myasthenia Gravis Owing to Other Antibodies

Around 20% of patients with myasthenia gravis are acetylcholine receptor antibody negative; muscle-specific tyrosine kinase antibodies (MuSK) were identified as the cause of myasthenia gravis in 30% to 40% of these cases. Anti MuSK myasthenia gravis is associated with specific clinical phenotypes. One is a bulbar form with fewer ocular symptoms. Others show an isolated head drop or symptoms indistinguishable from acetylcholine receptor-positive myasthenia gravis. These patients usually respond well to immunosuppressive therapy, but not as well to cholinesterase inhibitors. Other antibodies associated with myasthenia gravis, including low-density lipoprotein receptor-related protein 4, are discussed.

Myasthenia gravis with antibodies to MuSK: an update

Myasthenia gravis with antibodies to the muscle-specific tyrosine kinase (MuSK⁺ MG) is a rare disease with distinctive pathogenic mechanisms and clinical features. An acute onset and predominant bulbar muscle weakness are very common and highly suggestive of the disease. On the other hand, a more indolent course, atypical ocular presentation, and signs of cholinergic hyperactivity may complicate the diagnosis. Though MuSK⁺ MG is still a severe disease, over the years we have observed a steady reduction in the rate of respiratory crisis and a significant improvement in the clinical outcome, both likely related to earlier diagnosis and timely treatment. Despite the improved management, MuSK⁺ MG patients tend to remain dependent on long-term immunosuppressive treatment and may develop permanent disabling weakness. In uncontrolled studies, B cell depletion with rituximab proved effective in most patients with refractory disease, inducing prolonged clinical responses associated with a sustained reduction of serum antibody levels. Promising results from experimental studies and case reports suggest that both 3,4-diaminopyridine and albuterol may be effective as symptomatic agents.

Myopathy, muscle atrophy and tongue lipid composition in MuSK myasthenia gravis

Myasthenia gravis (MG) associated with anti-muscle-specific tyrosine kinase (MuSK) antibodies differs in many aspects from typical presentation of acetylcholine receptor (AChR)-positive MG. Myopathy and muscle atrophy are observed in MuSK-positive MG patients, unlike AChR-positive patients with MG. That is why the aim of this study was to assess the presence of myopathy and muscle atrophy as well as the tongue lipid composition in our cohort of MuSK-positive MG patients. Clinical examination, electromyography (EMG) and proton magnetic resonance spectroscopy were performed in 31 MuSK-positive and 28 AChR-positive MG patients. Myopathic EMG was more frequent in MuSK compared to AChR MG patients. In AChR MG patients, myopathic EMG in facial muscles was more frequent after long-term corticosteroid treatment, which was not the case with MuSK-positive MG patients. Facial and/or tongue muscle atrophy was registered in 23 % of MuSK MG patients. Longer disease duration was observed in patients with clinical signs of tongue and/or facial muscle atrophy compared to those with normal tongue muscle. Intramyocellular lipid deposition in the tongue was present in 85.2 % of MuSK and 20 % of AChR MG patients. Female MuSK MG patients had more frequently electrophysiological signs of myopathy on the facial muscles and signs of intramyocellular lipid deposition in the tongue, compared to male patients with MuSK-positive MG. Myopathy, muscle atrophy and intramyocellular lipid deposition in the tongue are more frequent in MuSK-positive compared to AChR-positive MG patients.

Muscle-specific regulation of the mTOR signaling pathway in MuSK antibody seropositive (MuSK+) experimental autoimmune Myasthenia gravis (EAMG)

Myasthenia gravis (MG) patients with antibodies against muscle specific tyrosine kinase (MuSK+) typically present focal fatigue and atrophy of the facial and bulbar muscles, including the masseter muscle, whereas leg muscles often are clinically spared. This study addresses the regulation of the mTOR signaling pathway in the masseter muscle versus the leg muscle tibialis anterior (TA). We analyzed muscle morphology, protein levels of mTOR components as well as atrogenes and mitochondrial markers in these muscles of healthy control mice and mice with different clinical severity grades of MuSK+ experimental autoimmune MG (EAMG). Protein levels of mTOR components were reduced in the atrophic masseter muscle of MuSK+ EAMG mice, whereas enhanced accumulation of mTOR components was observed in the TA muscles. Two other muscles: omohyoid and soleus showed intermediate spectra. In conclusion, the anabolic mTOR signaling pathway is differentially regulated even in muscles with the same activity pattern in the same neuromuscular disease. This could in part explain the clinical phenotype in MuSK+ EAMG as well as in muscular dystrophies.

Animal models of antimuscle-specific kinase myasthenia

Antimuscle-specific kinase (anti-MuSK) myasthenia (AMM) differs from antiacetylcholine receptor myasthenia gravis in exhibiting more focal muscle involvement (neck, shoulder, facial, and bulbar muscles) with wasting of the involved, primarily axial, muscles. AMM is not associated with thymic hyperplasia and responds poorly to anticholinesterase treatment. Animal models of AMM have been induced in rabbits, mice, and rats by immunization with purified xenogeneic MuSK ectodomain, and by passive transfer of large quantities of purified serum IgG from AMM patients into mice. The models have confirmed the pathogenic role of the MuSK antibodies in AMM and have demonstrated the involvement of both the presynaptic and postsynaptic components of the neuromuscular junction. The observations in this human disease and its animal models demonstrate the role of MuSK not only in the formation of this synapse but also in its maintenance.

Management challenges in muscle-specific tyrosine kinase myasthenia gravis

Myasthenia gravis with antibodies to muscle-specific tyrosine kinase (MuSK-MG) is generally considered a severe disease because of the associated weakness distribution with prevalent involvement of bulbar muscles and a rapidly progressive course and early respiratory crises. Its treatment can be unrewarding, owing to poor response to acetylcholinesterase inhibitors in most patients, disease relapses in spite of high-dose immunosuppression, and development of permanent bulbar weakness. High-dose prednisone plus plasma exchange is the recommended approach for treating rapidly progressive bulbar weakness. In the disease management, oral steroids proved effective, plasma exchange produced marked, albeit short-term, improvement, while conventional immunosuppressants were comparatively less effective. Rituximab is a promising treatment for refractory MuSK-MG; in uncontrolled studies, nearly all treated patients achieved significant improvement with substantial decrease of medication. It is yet to be clarified whether the early use of rituximab could prevent the permanent bulbar weakness, which constitutes a relevant disability in these patients.

Diagnosis and therapy of myasthenia gravis with antibodies to muscle-specific kinase

Myasthenia gravis (MG) with antibodies to the muscle-specific receptor tyrosine kinase (MuSK-MG) is a rare disease which covers 5-8% of all MG patients. Symptoms are nearly always generalized, though more focal than in MG with anti-acetylcholine receptor antibodies, with predominant involvement of cranial, bulbar and axial muscles; early respiratory crises are frequent. Focal atrophy, mostly of facial, masseter and tongue muscles, occurs in a proportion of patients. Diagnosis is often challenging on account of atypical presentation with little or no symptom fluctuations, lack of response to acetylcholinesterase inhibitors in a high proportion of patients and negative results of electrodiagnostic studies when performed on limb muscles. Immunosuppression is the mainstay of treatment, since the response to acetylcholinesterase inhibitors is generally unsatisfactory and thymectomy does not appear to improve the course of the disease. Although corticosteroids result in marked improvement, disease flares are frequent during prednisone dosage tapering and most patients remain dependent on treatment. Since treatment with rituximab, in uncontrolled studies, induced sustained benefit in patients with refractory disease, B cell depletion is an attractive option for MuSK-MG patients unresponsive to conventional immunosuppressants.

Videofluorographic detection of anti-muscle-specific kinase-positive myasthenia gravis

A 47-year-old woman with dysphagia and ptosis gradually developed dysarthria and muscular weakness. Magnetic resonance imaging, testing for anti-acetylcholine receptor antibodies, edrophonium chloride (EC) test, and electrophysiologic test revealed no abnormalities. A psychogenic reaction was suspected. Four months after disease onset, the patient presented to our hospital. In videofluoroscopic examination of swallowing (VF), there was no aspiration for swallowing of either liquid or soft food. It revealed, however, poor pharyngeal constriction, no epiglottis inversion, repeated swallowing movements, and large amounts of pharyngeal residue. Videofluoroscopic examination of swallowing after an intravenous injection of 10 mg EC showed improvements in all above observations; particularly, it was clear when swallowing soft food. Furthermore, the anti-muscle-specific kinase (MuSK) antibody titer was elevated, and anti-MuSK antibody-positive myasthenia gravis (MuSK-MG) was diagnosed. Thus VF during EC test may be helpful in diagnosing MuSK-MG in patients with dysphagia.

AchR-positive myasthenia gravis with MRI evidence of early muscle atrophy

Muscle atrophy, when it occurs in myasthenia gravis (MG), is usually associated with long-standing disease or with chronic corticosteroid treatment. Early muscle atrophy in a steroid-naive patient has been reported previously only in muscle-specific tyrosine kinase (MuSK)-MG. We report a 63-year-old male patient with acetylcholine receptor (AchR)-positive MG with a short duration of disease, no steroid treatment and MRI evidence of muscle atrophy.

Neurophysiological characteristics of MuSK antibody positive myasthenia gravis mice: focal denervation and hypersensitivity to acetylcholinesterase inhibitors

Myasthenia Gravis (MG) patients with antibodies against the muscle specific tyrosine kinase (MuSK+) typically present with focal fatigue and atrophy of the facial and bulbar muscles, along with unbeneficial reactions upon administration of acetylcholinesterase inhibitors (AChEIs). This study addresses the neurophysiological characteristics in facial versus limb muscles, before and after intraperitoneal injection of AChEIs, in mice immunized with MuSK. We performed in-vivo neurophysiological examinations in the masseter and gastrocnemius muscles of mice with MuSK+experimental autoimmune MG (EAMG) and in healthy control mice before and after administration of AChEIs. Abnormal spontaneous activity (fibrillations) was observed in the masseter muscle of MuSK+mice. Furthermore, 94% of MuSK-immunized mice displayed so called extra discharges (EDs) upon administration of a therapeutic AChEI dose, in contrast to 22% of the control mice, indicating neuromuscular hyperactivity. These findings support functional denervation in the masseter muscle and neuromuscular hypersensitivity already at a standard dose of AChEIs in MuSK+EAMG.