Ultrastructural alterations in muscle from patients with myasthenia gravis and Eaton-Lumbert syndrome

Role of complement in myasthenia gravis

Myasthenia gravis is a prototypic neuroimmune disorder with autoantibodies targeting the acetylcholine receptor complex at the neuromuscular junction. Patients present with mainly ocular muscle weakness and tend to have a generalized muscle weakness later in the clinical course. The weakness can be severe and fatal when bulbar muscles are heavily involved. Acetylcholine receptor antibodies are present in the majority of patients and are of IgG1 and IgG3 subtypes which can activate the complement system. The complement involvement plays a major role in the neuromuscular junction damage and the supporting evidence in the literature is described in this article. Complement therapies were initially studied and approved for paroxysmal nocturnal hemoglobinuria and in the past decade, those have also been studied in myasthenia gravis. The currently available randomized control trial and real-world data on the efficacy and safety of the approved and investigational complement therapies are summarized in this review.

Complement and myasthenia gravis

Myasthenia gravis is a neuromuscular disease associated with antibodies against components of the neuromuscular junction, most often against the acetylcholine receptor (AChR). Although several mechanisms have been postulated to explain how these autoantibodies can lead to the pathology of the disease, convincing evidence suggests that destruction of the receptor-bearing postsynaptic membrane by complement membrane attack complex is of central importance. In this review, evidence for the importance of complement, and possible relationships between autoantigen, autoantibodies, complement activation, and the destruction of the membrane are discussed. More recent insights from the results of the complement-inhibiting therapeutic antibody eculizumab are also described, and the mechanisms connecting antibody binding to complement activation are considered from a structural viewpoint.

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.

Familial neuromuscular disease with tubular aggregates

We have studied four members of a family in which five individuals of both sexes in three generations have mild weakness and muscle aching. Serum creatine kinase (CK) and/or aldolase were mildly to moderately elevated. Tubular aggregates were present in all patients in all fiber types. Type 1 fiber predominance and type 2 hypotrophy or atrophy was noted in three biopsies. Fiber splitting was prominent in two cases. Electron microscopic studies confirm that the tubular aggregates originate from muscle sarcoplasmic reticulum (SR). The presence of T tubules making contacts with some tubules inside the collection of tubular aggregates is suggestive of their derivation from terminal cisternae.

Effects of myotoxins on skeletal muscle fibers

This review highlights various aspects of a number of experimental myological alterations, induced by different chemical toxicants, including anticholinesterase, colchicine, vincristine, chloroquine, tetanus toxin, botulinum toxin, reserpine and emetine. Despite their chemical diversity and mechanism(s) of action, it is evident from the data discussed here that remarkably different toxic agents exert quite similar effects and induce toxic myopathies. The latter include preferential involvement of slow-twitch red muscle, mitochondrial derangement, denervation-like alterations, formation of membranous whorls, tubular aggregates, autophagic vacuoles and axonal sprouts. The non-invasive experimental models discussed here are valuable in studying various aspects of myopathology in the absence of any mechanical damage to the innervating elements from neurons to axonal terminals.

[Myopathies associated with tubular aggregates]

The authors report the case of a 58-year-old male patient with clinical and electromyographic features of myasthenia. Muscle biopsy with histochemistry and electronic microscopy made it possible to diagnose a myopathy associated with tubular aggregates. Attention is called to the fact that the anatomical pathologic alterations which were found may be present in a heterogenous group of patients showing a great variety of symptoms. Thus, there is no reason to consider the existence of a myopathy associated with tubular aggregates, since the anatomical and pathologic findings are inespecific and do not characterize any specific disease.

Dominantly inherited tubular aggregate myopathy

We report an unusual familial myopathy characterized morphologically by the presence of large tubular aggregates in all fibre types. Two patients, a father and daughter, presented with slowly progressive proximal weakness, limitation of eye movement, and Achilles tendon contractures. Serum creatine kinase was 5-10 times normal. Light microscopy revealed type I fibre predominance. Basophilic accumulations, which stained intensely with the NADH-TR reaction, were present in both fibre types. Electron microscopy revealed that these consisted of tightly packed parallel tubular arrays. These varied somewhat in their ultrastructural appearance and were classified accordingly as type I, II, and III tubular structures. The tubular aggregates appear to be derived from the sarcoplasmic reticulum. This report further supports the evidence of a distinct clinico-pathological entity of genetic origin.

Autosomal dominant neuromuscular disease with cylindrical spirals

Cylindrical spirals (CS) have been reported in muscle biopsies from five individual cases, as well as in two belonging to one family where there was another affected member, clinically associated with cramps, pain, stiffness and/or weakness. Here we studied muscle biopsies of a 70-yr-old mother and her 52-yr-old son, the latter with an associated neuropathy, both with late clinical onset in whose family at least 10 other members, spanning five generations, were diversely affected by muscular weakness, gait disorders, motor impairment and/or scoliosis, featuring an autosomal dominant trait with variable expression. CS as the main pathological findings were observed by light microscopy mostly in type 2 fibres, consisting of subsarcolemmal or intermyofibrillar granular and/or rod-like clusters, bluish with haematoxylin, bright red with Gomori's modified trichrome, non- or lightly reactive with PAS, faintly coloured with NADH-TR, non-reactive with SDH or ATPase, strongly stained with non-specific esterase and myoadenylate deaminase. Ultrastructurally, CS appeared as concentrically wrapped lamellae 1-2 microns in diameter. On occasion CS merged into tubular vesicular structures strongly resembling tubular aggregates (TA). Dilation of terminal cisternae (TC) in their proximity supports an origin from the sarcoplasmic reticulum (SR). Variable gene expression possibly explains both the highly diverse clinical compromise and time of onset.

A case of "myopathy with tubular aggregates" with increased muscle fibre sensitivity to caffeine

A 23-year-old man with "myopathy with tubular aggregates" had suffered from exercise-induced muscle cramps for 1 year. His general and neurological findings were normal. Laboratory investigations were within normal limits except for a slightly elevated serum creatine kinase level. Muscle biopsy showed some small angular fibres and scattered type 2B fibres with prominent tubular aggregates originating from the sarcoplasmic reticulum. Since the muscle fibres contracted at a lower concentration of caffeine, increased muscle fibre sensitivity to caffeine is probably related to muscle cramps in this disorder. Tubular aggregates are then secondarily formed in the muscle fibres.

Ultrastructural study of neuromuscular junctions in experimental autoimmune myasthenia gravis (EAMG) in rabbits

Experimental autoimmune myasthenia gravis (EAMG) was produced in rabbits by the injection of acetylcholine receptor (AChR) protein from electric organ of Narkacion tokyonis with complete Freund's adjuvant, and the ultrastructural alterations of the neuromuscular junctions were studied. A lesion comparable to human myasthenia gravis could be observed in these animals. In acute EAMG, which showed a rapidly progressive severe paralysis 19 to 24 days after the first inoculation, distinct degeneration of the postsynaptic membrane was observed. Sometimes an irregular gap was found between the nerve terminal and postsynaptic sarcoplasm. In chronic EAMG with mild and prolonged muscle weakness, which appeared 23 to 54 days after the first inoculation, poorly developed junctional folds with little degenerative change were seen. By a morphometric analysis, some of the changes were detected even in a subclinical EAMG. In the presynaptic region, there was no ultrastructural alteration except that an increase of nerve terminal area was observed in chronic and subclinical EAMG. The pathogenesis of these alterations was discussed.

Familial spinocerebellar degeneration with corneal dystrophy

We report on two sisters born to normal but consanguineous parents, with the unusual combination of spinocerebellar degeneration and corneal dystrophy. Their manifestations include mental subnormality, bilateral corneal opacification starting in the second year of life and leading to severe visual impairment, and slowly progressive cerebellar abnormalities with variable dorsal column and upper motor neuron involvement. A third sister had only minor spinocerebellar signs but no eye findings, and three other sibs were completely normal. Both affected sisters underwent penetrating keratoplasty and their vision improved. Histologic examination showed findings of corneal dystrophy including corneal edema, thickening of Descemet membrane, and degenerative pannus. High-resolution light and electron microscopy of muscle and sural nerve performed on both patients was abnormal. It is suggested that, in this family, the corneal dystrophy and spinocerebellar degeneration are pleiotropic manifestations of an autosomal-recessive disorder.

The myopathology of Behçet's disease--a histochemical, light-, and electron-microscopic study

The myopathology of Behçet's disease is described in 7 patients using conventional light microscopy, histochemistry, high-resolution light microscopy and electron microscopy. Two patients had clinical evidence of peripheral neuropathy: 5 patients had no clinical evidence of neuromuscular involvement. Histochemical profile of muscle in all patients was normal. Abnormalities in muscle histology using conventional light microscopy were found in only 1 patient. High-resolution light microscopy revealed abnormalities in 3 patients whereas electron microscopy revealed abnormalities in muscle of all 7 patients. The myopathology of Behçet's disease includes thickening of capillary basement membrane, excessive pleating of sarcolemma, subsarcolemmal aggregates of mitochondria and glycogen, disorganization and breakdown of myofibrils, central nucleation and the presence of a variety of cytoplasmic inclusions. No virus particles were present.

Mechanisms of acetylcholine receptor loss in myasthenia gravis

The fundamental abnormality affecting the neuromuscular junctions of myasthenic patients is a reduction of available AChRs, due to an autoimmune attack directed against the receptors. Antibodies to AChR are present in most patients, and there is evidence that they have a predominant pathogenic role in the disease, aided by complement. The mechanism of antibody action involves acceleration of the rate of degradation of AChRs, attributable to cross-linking of the receptors. In addition, antibodies may block AChRs, and may participate in producing destructive changes, perhaps in conjunction with complement. The possibility that cell-mediated mechanisms may play a role in the autoimmune responses of some myasthenic patients remains to be explored. Although the target of the autoimmune attack in myasthenic patients is probably always the acetylcholine receptors, it is not yet clear which of these immune mechanisms are most important. It is likely that the relative role of each mechanism varies from patient to patient. One of the goals of future research will be to identify the relative importance of each of these mechanisms in the individual patient, and to tailor specific immunotherapeutic measures to the abnormalities found.

Myasthenia gravis: a personal view of pathogenesis and mechanism, part 2

A review of our current knowledge of the etiology and pathogenesis of myasthenia gravis is presented, with particular emphasis on the immunological aspects of the disease. Part 1, published in the January/February issue of MUSCLE & NERVE, dealt with the clinical and genetic features of myasthenia gravis which led to the autoimmune theory of the etiology of the disease. Part 2, which appears in this issue, provides a review of the dysfunction of physiology, pharmacology, and structure of the neuromuscular junction in myasthenia gravis, and the part played by the autoimmune process.

Studies of human myasthenia gravis: electrophysiological and ultrastructural evidence compatible with antibody attachment to acetylcholine receptor complex

Neuromuscular junctions from patients with early onset and chronic myasthenia gravis were examined by electrophysiological and ultrastructural techniques. Acetylcholine (AcCh) sensitivities were reduced by 34-63% in early onset myasthenia and 60-80% in chronic myasthenia. Ultrastructural analysis revealed that virtually all junctional folds of the early onset patients were intact but that the AcCh-receptor-rich crests of these folds were uniformly covered by an attached layer of 30 X 70 A particles arranged in small tufts or rosettes. In chronic myasthenic endplates, however, junctional fold crests were destroyed, apparently being replaced by vesicular membrane debris similarly labeled by tufts of 30 X 70 A particles. Thus, the initial reduction in junctional AcCh sensitivity observed in early onset myasthenia gravis may be attributed at least in part to in situ masking or inactivation of AcCh receptors, whereas the marked decrease in AcCh sensitivity observed in the chronic myasthenic patient may represent a combination of two factors: (a) in situ masking of AcCh receptors and (b) destruction of the receptor-containing crests of the junctional folds. These observations are compatible with an autoimmune etiology of myasthenia gravis initially involving an apparent antibody attachment to one or more components of the functional AcCh receptor complex, followed by systematic destruction and removal of junctional folds by both humoral and cell-mediated autoimmune responses.

Immunology and thoracic surgery

This report reviews the current status of three types of diseases in which knowledge of immunology is changing the practice of thoracic surgery. Evidence is presented for myasthenia gravis being an autoimmune disease. Thymectomy is widely considered to be the therapy of choice for patients with generalized myasthenia regardless of age or sex. Opportunistic pulmonary infections of unusual types are being encountered frequently in patients receiving immunosuppressive drugs for organ transplantation or malignancy. The important role of bronchial brushing and open lung biopsy is stressed. Immunotherapy of several types is currently being investigated experimentally and clinically and promises to change therapy for bronchogenic and esophageal neoplasms.

Diagnostic difficulties encountered in the myasthenic syndrome sometimes associated with carcinoma

Six patients with the physiological features of the myasthenic syndrome associated with carcinoma are presented. The clinical features encountered in this group, including age, mode of onset, and duration of symptom indicate that it may escape detection owing to its resemblance to other neuromuscular disorders. Conventional concentric needle electromyography may lead to the disorder being confused with a myopathy. The association with carcinoma is by no means constant. Clinical and physiological improvement with guanidine is described in each case.