Mitochondrial dysfunction in myasthenia gravis. Report of a case

The Impact of Mitochondrial Deficiencies in Neuromuscular Diseases

Neuromuscular diseases (NMDs) are a heterogeneous group of acquired or inherited rare disorders caused by injury or dysfunction of the anterior horn cells of the spinal cord (lower motor neurons), peripheral nerves, neuromuscular junctions, or skeletal muscles leading to muscle weakness and waste. Unfortunately, most of them entail serious or even fatal consequences. The prevalence rates among NMDs range between 1 and 10 per 100,000 population, but their rarity and diversity pose difficulties for healthcare and research. Some molecular hallmarks are being explored to elucidate the mechanisms triggering disease, to set the path for further advances. In fact, in the present review we outline the metabolic alterations of NMDs, mainly focusing on the role of mitochondria. The aim of the review is to discuss the mechanisms underlying energy production, oxidative stress generation, cell signaling, autophagy, and inflammation triggered or conditioned by the mitochondria. Briefly, increased levels of inflammation have been linked to reactive oxygen species (ROS) accumulation, which is key in mitochondrial genomic instability and mitochondrial respiratory chain (MRC) dysfunction. ROS burst, impaired autophagy, and increased inflammation are observed in many NMDs. Increasing knowledge of the etiology of NMDs will help to develop better diagnosis and treatments, eventually reducing the health and economic burden of NMDs for patients and healthcare systems.

A review of the histopathological findings in myasthenia gravis: Clues to the pathogenesis of treatment-resistance in extraocular muscles

In myasthenia gravis autoantibodies target components of the neuromuscular junction causing variable degrees of weakness. In most cases, autoantibodies trigger complement-mediated endplate damage and extraocular muscles may be most susceptible. A proportion of MG cases develop treatment-resistant ophthalmoplegia. We reviewed publications spanning 65 years reporting the histopathological findings in the muscles and extraocular muscles of myasthenic patients to determine whether pathological changes in extraocular muscles differ from non-ocular muscles. As extraocular muscles represent a unique muscle allotype we also compared their histopathology in myasthenia to those in strabismus. We found that in myasthenia gravis, the non-ocular muscles frequently demonstrate neurogenic changes regardless of myasthenic serotype. Mitochondrial stress/damage was also frequent in myasthenic muscles and possibly more evident in muscle-specific kinase antibody-positive MG. Although myasthenia-associated paralysed extraocular muscles demonstrated prominent fibro-fatty replacement and mitochondrial alterations, these features appeared commonly in paralysed extraocular muscles of any cause. We postulate that extraocular muscles may be more susceptible than limb muscles to poor contractility as a consequence of myasthenia, resulting in a cascade of atrophy signaling pathways and altered mitochondrial homeostasis which contribute to the tipping point in developing treatment-resistant myasthenic ophthalmoplegia. Early strategies to improve force generation in extraocular muscles are critical.

31P MR spectroscopic assessment of muscle in patients with myasthenia gravis before and after thymectomy: initial experience

PURPOSE

To prospectively assess muscle metabolism in myasthenia gravis (MG) patients before and after thymectomy by using phosphorus 31 (31P) magnetic resonance (MR) spectroscopy.

MATERIALS AND METHODS

With institutional review board approval and informed consent, resting and dynamic (31)P MR spectroscopy were performed in 14 healthy volunteers (five men, nine women; mean age, 33 years; range, 23-48 years) and 16 MG patients (six men, 10 women; mean age, 37 years; range 18-50 years) before and after thymectomy. Patients were stratified into groups according to the modified Osserman classification: mild-MG group (classes I-IIA) and moderate-to-severe-MG group (classes IIB-IV). Variables compared among the three groups (Kruskal-Wallis test) included the inorganic phosphate (P(i))-adenosine triphosphate (ATP) (P(i)/ATP) ratio, phosphocreatine (PCr)-ATP (PCr/ATP) ratio, P(i)/PCr ratio, muscle pH at resting and at end-exercise ( 31)P MR spectroscopy, rate constant for PCr recovery (k(PCr)), and maximum oxidative capacity (V(max)). These variables were also compared in MG patients before and after thymectomy (Wilcoxon signed rank test).

RESULTS

There were no significant differences in resting P(i)/ATP, PCr/ATP, and P(i)/PCr ratios and resting muscle pH among the three groups (control group, 14; mild-MG group, nine; moderate-to-severe-MG group, seven). Comparison of the control group with the mild-MG group and comparison of the mild-MG group before thymectomy with the mild-MG group after thymectomy showed no significant differences in end-exercise P(i)/ATP, PCr/ATP, and P(i)/PCr ratios; end-exercise muscle pH; k(PCr); and V(max). Compared with the control and mild-MG groups, the moderate-to-severe-MG group had significantly higher end-exercise P(i)/ATP and P(i)/PCr ratios and significantly lower end-exercise muscle pH, k(PCr), and V(max) before thymectomy (P < or = .001), but these values showed significant restoration to normal after thymectomy (P = .018).

CONCLUSION

Mild-MG group patients have muscle oxidative metabolism similar to that of healthy control subjects, whereas moderate-to-severe-MG group patients have impaired V(max) during exercise and a noticeable shift to glycolytic metabolism, but these abnormalities are reversible after thymectomy.

Mitochondrial myopathy associated with myasthenia gravis in a young man

An 18-year-old man presented with progressive weakness of proximal muscles with prominent diurnal variation for 3 months. He had bilateral ptosis since his childhood without diurnal variation or double vision. Neurological examination showed involvement of levator palpebrae superioris and lateral rectus muscles bilaterally. The plasma glucose after 75 gm glucose load was 302 mg/dL. The electrophysiological study revealed myopathic pattern and a decremental response in repetitive nerve stimulation. The plasma lactate was elevated and the muscle biopsy showed numerous ragged-red fibers. Serum acetylcholine receptor antibody assay was positive. We diagnosed myasthenia gravis with mitochondrial myopathy.