Congenital myasthenic syndromes

Targeted therapies for congenital myasthenic syndromes: systematic review and steps towards a treatabolome

Despite recent scientific advances, most rare genetic diseases — including most neuromuscular diseases — do not currently have curative gene-based therapies available. However, in some cases, such as vitamin, cofactor or enzyme deficiencies, channelopathies and disorders of the neuromuscular junction, a confirmed genetic diagnosis provides guidance on treatment, with drugs available that may significantly alter the disease course, improve functional ability and extend life expectancy. Nevertheless, many treatable patients remain undiagnosed or do not receive treatment even after genetic diagnosis. The growth of computer-aided genetic analysis systems that enable clinicians to diagnose their undiagnosed patients has not yet been matched by genetics-based decision-support systems for treatment guidance. Generating a ‘treatabolome’ of treatable variants and the evidence for the treatment has the potential to increase treatment rates for treatable conditions. Here, we use the congenital myasthenic syndromes (CMS), a group of clinically and genetically heterogeneous but frequently treatable neuromuscular conditions, to illustrate the steps in the creation of a treatabolome for rare inherited diseases. We perform a systematic review of the evidence for pharmacological treatment of each CMS type, gathering evidence from 207 studies of over 1000 patients and stratifying by genetic defect, as treatment varies depending on the underlying cause. We assess the strength and quality of the evidence and create a dataset that provides the foundation for a computer-aided system to enable clinicians to gain easier access to information about treatable variants and the evidence they need to consider.

Chiropractic care of a pediatric patient with myasthenia gravis

OBJECTIVE

To describe the chiropractic care of a pediatric patient with complaints associated with myasthenia gravis. Clinical features A 2-year-old girl was provided chiropractic care at the request and consent of her parents for complaints of ptosis and generalized muscle weakness (ie, lethargy), particularly in the lower extremities. Prior to entry into chiropractic management, magnetic resonance imaging of the brain and acetylcholine receptor antibody tests were performed with negative results. However, the Tensilon test was positive and the diagnosis of myasthenia gravis was made by a pediatrician and seconded by a medical neurologist. Intervention and outcome The patient was cared for with contact-specific, high-velocity, low-amplitude adjustments to sites of vertebral subluxation complexes in the upper cervical and sacral spine. The patient's response to care was positive and after 5 months of regular chiropractic treatment her symptoms abated completely.

CONCLUSION

There are indications that patients suffering from disorders "beyond low back pain" as presented in this case report may derive benefits from chiropractic intervention/management.

The natural history and ophthalmic involvement in childhood myasthenia gravis at the hospital for sick children

OBJECTIVE

To characterize signs, symptoms, and the natural history of myasthenic syndromes in pediatric patients.

DESIGN

Retrospective noncomparative case series.

PARTICIPANTS

Thirty-four patients with a diagnosis of myasthenia were identified from either the hospital's or treating physician's database.

METHODS

Retrospective chart review, clinical examination, and telephone interview.

MAIN OUTCOME MEASURES

Information pertaining to the ophthalmologic and neurologic examination, diagnostic interventions, and treatment was noted. Patients with active disease, attending during the study period, were examined at their outpatient visits. Those who no longer attended the hospital were contacted by means of a telephone interview to complete their follow-up.

RESULTS

Thirty-four children were found to have myasthenia. Two had transient neonatal myasthenia, which resolved quickly. Seven (20.6%) patients had congenital myasthenic syndromes (CMS) and 25 (73.5%, 19 females) were affected with autoimmune myasthenia gravis (AMG). In those patients with severe CMS, three showed signs of generalized weakness, including failure to thrive, frequent apneas, and aspirations. In four patients with mild CMS, eye signs were relatively more prominent. In all patients with CMS, strabismus, ophthalmoplegia, and ptosis were the main ophthalmologic signs and remained relatively constant. Fourteen (56%) patients with AMG had ocular signs and symptoms, and five of them progressed to systemic involvement in 7.8 months on average (range, 1-23). The remaining nine patients with ocular AMG had either strabismus or ptosis and were treated with pyridostigmine (nine patients) and prednisone (two patients). Patients with ocular AMG were seen at 78 months on average, those with systemic AMG at 85.6 months. Systemic AMG was seen in 16 patients. No thymomas were found in 14 patients who underwent thymectomy. Of the 25 patients with AMG, 8 are still being treated, 8 are in remission for an average of 65.2 months and are asymptomatic, 4 patients are receiving long-term immunosuppressants (1 has likely sustained permanent damage to her extraocular muscles with complete ophthalmoplegia and ptosis), and 4 have been lost to follow-up. Finally, one patient died after aspiration because of bulbar weakness.

CONCLUSIONS

Patients with CMS varied in the degree of severity. Apneic attacks, aspiration, and failure to thrive may obscure the diagnosis. Compared with AMG, their ophthalmologic signs and symptoms were usually permanent. Visual signs and symptoms were usually prominent in those patients with active AMG, but those in remission were asymptomatic. More than half of the patients with juvenile AMG had ocular symptoms. Generalization occurred in a minority in an average of 7.8 months. Patients entered remission after approximately 2 years of treatment and were visually asymptomatic. This study suggests that long-term permanent damage to the extraocular muscles as a result of juvenile AMG is rare. Myasthenia gravis is a life-threatening disease as evidenced by the death of one of our patients. Many of these patients are first seen by the ophthalmologist who can aid the diagnosis, screen for amblyopia, and monitor the patient's response to therapy.

Megaoesophagus secondary to acquired myasthenia gravis

Fifteen dogs with confirmed adult onset idiopathic megaoesophagus, in which no generalised muscle weakness was observed, were tested for the presence of acetylcholine receptor antibodies. Of these, six were found to have values greater than 0.6 nmol/litre, previously determined to be diagnostic of acquired myasthenia gravis. The mean serum titre value for these dogs was 5.59 nmol/litre (range 0.78 to 8.72 nmol/litre). It appears that a significant proportion of dogs presenting with megaoesophagus have myasthenia gravis and, if a prompt diagnosis and appropriate treatment can be instituted, clinical signs may improve.

Diagnosis in neuromuscular diseases

The diagnosis of neuromuscular diseases can be challenging and successful in the majority of patients, due to advancements in electrophysiology, muscle and nerve biopsy immunohistochemistry, and cytogenetics. This article reviews diverse topics, highlighting these recent achievements, with an emphasis on how they affect the clinical and laboratory diagnosis of specific neuromuscular disorders.

Arthrogryposis multiplex congenita due to congenital myasthenic syndrome

Two children, now 5 1/2 and 6 years of age, presented as neonates with hypotonia, multiple joint contractures, ptosis, extraocular weakness, bulbar symptoms, and respiratory distress. Fluctuations and episodic exacerbations of weakness necessitated respiratory support. Both children are developmentally delayed and cannot walk independently, although one child underwent bilateral tenotomies. Biochemical investigations and electromyography, including slow-rate, repetitive nerve stimulation, were normal. Acetylcholine receptor antibodies in serum were absent. Single-fiber electromyography with axonal stimulation revealed prolonged mean jitter in the tibialis anterior and extensor digitorum muscles, with more than 2 abnormal individual jitter values in each muscle. Muscle biopsy demonstrated normal pattern and morphology of muscle fibers; immunohistochemical staining for cholinesterase was positive. Electron microscopy revealed abnormalities in motor endplates: atrophy, flattening of primary synaptic clefts, and paucity of side branches. These findings represent one of the postsynaptic abnormalities (i.e., acetylcholine receptor deficiency or paucity of synaptic folds). Both children improved clinically on pyridostigmine therapy. Arthrogryposis congenital multiplex due to congenital myasthenic syndrome, as diagnosed in our patients, has been reported once before. The diagnosis can be established by clinical history, neurologic examination, and electrophysiologic and pathologic findings. Clinical improvement can be achieved with high-dose anticholinesterase therapy.

Congenital myopathies

Several dozen congenital myopathies are defined by clinical and morphological criteria. The application of the current generation of scientific techniques including immunohistochemistry and molecular genetics has resulted in the expansion of our knowledge and understanding of the well-established conditions including central core myopathy and centronuclear/myotubular myopathy and allowed greater understanding of the interrelationships of some of the less common or less well-established conditions. In the near future molecular genetics may allow the identification of the specific gene defect in many of these diseases. This article reviews the major congenital myopathies and presents some of the information gained by application of new technology to these conditions.

Congenital myasthenic syndromes

The Congenital Myasthenic Syndromes (CMS) constitute a group of rare genetic disorders affecting neuromuscular transmission. They differ from myasthenia gravis and the Lambert-Eaton myasthenic syndrome, which are autoimmune antibody-mediated conditions. CMS can present at any time from birth to adulthood, though usually within the first 2 yr of life, and result in a spectrum of diseases ranging from mild weakness to severe disability with life-threatening episodes. Several of these syndromes have been well documented, and in recent years fully investigated using a variety of electrophysiological, histochemical, and morphological techniques. In this review we describe the main results of these investigations, and attempt to classify the disorders into groups that can be recognized by the clinician. They include defects in acetylcholine release, absence of the endplate-specific form of acetylcholinesterase, and alterations in the number or function of postsynaptic acetylcholine receptors. Clinical features are described in detail, and treatment reviewed. These disorders involve a potentially large number of candidate genes. Further elucidation of the underlying abnormalities will not only lead to improved treatment, but should contribute to our understanding of the molecular biology of the neuromuscular junction.

Myasthenia gravis: an autoimmune response against the acetylcholine receptor

Myasthenia gravis (MG) is an organ-specific autoimmune disease caused by an antibody-mediated assault on the muscle nicotinic acetylcholine receptor (AChR) at the neuromuscular junction. Binding of antibodies to the AChR leads to loss of functional AChRs and impairs the neuromuscular signal transmission, resulting in muscular weakness. Although a great deal of information on the immunopathological mechanisms involved in AChR destruction exists due to well-characterized animal models, it is not known which etiological factors determine the susceptibility for the disease. This review gives an overview of the literature on the AChR, MG and experimental models for this autoimmune disease.

Acetylcholine transport, storage, and release

ACh is released from cholinergic nerve terminals under both resting and stimulated conditions. Stimulated release is mediated by exocytosis of synaptic vesicle contents. The structure and function of cholinergic vesicles are becoming known. The concentration of ACh in vesicles is about 100-fold greater than the concentration in the cytoplasm. The AChT exhibits the lowest binding specificity among known ACh-binding proteins. It is driven by efflux of protons pumped into the vesicle by the V-type ATPase. A potent pharmacology of the AChT based on the allosteric VR has been developed. It has promise for clinical applications that include in vivo evaluation of the density of cholinergic innervation in organs based on PET and SPECT. The microscopic kinetics model that has been developed and the very low transport specificity of the vesicular AChT-VR suggest that the transporter has a channel-like or multidrug resistance protein-like structure. The AChT-VR has been shown to be tightly associated with proteoglycan, which is an unexpected macromolecular relationship. Vesamicol and its analogs block evoked release of ACh from cholinergic nerve terminals after a lag period that depends on the rate of release. Recycling quanta of ACh that are sensitive to vesamicol have been identified electrophysiologically, and they constitute a functional correlate of the biochemically identified VP2 synaptic vesicles. The concept of transmitter mobilization, including the observation that the most recently synthesized ACh is the first to be released, has been greatly clarified because of the availability of vesamicol. Differences among different cholinergic nerve terminal types in the sensitivity to vesamicol, the relative amounts of readily and less releasable ACh, and other aspects of the intracellular metabolism of ACh probably are more apparent than real. They easily could arise from differences in the relative rates of competing or sequential steps in the complicated intraterminal metabolism of ACh rather than from fundamental differences among the terminals. Nonquantal release of ACh from motor nerve terminals arises at least in part from the movement of cytoplasmic ACh through the AChT located in the cytoplasmic membrane, and it is blocked by vesamicol. Possibly, the proteoglycan component of the AChT-VR produces long-term residence of the macromolecular complex in the cytoplasmic membrane through interaction with the synaptic matrix. The preponderance of evidence suggests that a significant fraction of what previously, heretofore, had been considered to be nonquantal release from the motor neuron actually is quantal release from the neuron at sites not detected electrophysiologically.(ABSTRACT TRUNCATED AT 400 WORDS)

Transient neonatal myasthenia gravis

Transient neonatal myasthenia gravis is a postsynaptic neuromuscular transmission defect occurring in 21% of infants born to women with active (and, less commonly, in remission) acquired myasthenia gravis. Although passive-transfer acetylcholine receptor (AChR) antibodies are found in the majority of these newborns, their pathogenic role is questionable because only some infants are symptomatic. Pathogenesis in infants without AChR antibodies is unknown. There is still no biologic marker for prenatal identification of this subpopulation of newborns, although HLA typing may be a promising tool. Sucking, swallowing, and respiratory difficulties are the most common presenting signs in the first day of life. Final diagnosis is done when administration of acetylcholinesterase agents transiently corrects the neuromuscular transmission defect. Serum AChR antibody titers follow the same pattern as their mothers. Supportive management and anticholinesterase agents prior to feedings are necessary in about 80% of patients. In the majority of infants the condition resolves spontaneously.

A congenital myasthenic syndrome refractory to acetylcholinesterase inhibitors

We studied 4 siblings (3 men and 1 woman), ages 22 to 43 years, with congenital ptosis, external ophthalmoplegia, proximal muscle weakness and fatigability unresponsive to acetylcholinesterase (AChE) inhibitors. Repetitive nerve stimulation showed a significant compound muscle action potential (CMAP) area decrement at 2 or 3 Hz. Nerve conduction studies and concentric needle electromyography were normal, and repetitive CMAPs to single nerve stimulation were not observed. Voluntary single fiber electromyography (SFEMG) showed increased jitter and blocking. Assessment of individual end-plates using SFEMG with intramuscular axonal microstimulation showed no uniform relationship between jitter and the rate of stimulation, consistent with a postsynaptic defect of neuromuscular transmission. Edrophonium eliminated the decremental response to repetitive nerve stimulation, but caused no significant clinical improvement, suggesting an additional mechanism for weakness in these patients.

Congenital myopathies

About forty different congenital myopathies (CM) are defined by clinical and morphological criteria. Classical types like central core disease, centronuclear myopathy, and nemaline/rod myopathy are now well established and recognized as neuromuscular conditions. Clinical subtypes as infantile, juvenile, and adult forms have been recognized in several CM. Not infrequently, different disease-specific morphological features may occur in muscle tissue of the same patient combined. Other CM are marked by aggregates of desmin filaments indicating the importance of recent immunohistochemical techniques. Modern myopathological techniques enabled nosological separation of CM, immunohistochemistry, actually, may usher in a new period of research in and understanding of CM. However, application of molecular genetic and molecular biological methods to CM may clarify still unsolved aspects of gene localisation for which the hereditary nature of many CM is particularly conducive, aspects of heterogeneity versus homogeneity of certain CM or clinical variants, of prenatal diagnosis of CM, of pathogenetic and nosological significance of muscle fiber proteins in CM, and of a new nosological classification of CM.

Myasthenia gravis in children and its anaesthetic implications

Myasthenia gravis, a rare disease in children, occurs in a variety of forms. The aetiology and clinical presentations are reviewed. Eight patients who were studied with electromyography are presented. The results show that, in general, patients with the usual pattern of disease are resistant to suxamethonium (ED95 3-4 times normal) and are sensitive to nondepolarizing relaxants. When the latter are used it is advisable to administer small increments with neuromuscular monitoring. One patient with the disease localised to the eyelid had normal EMG responses when monitored on the hand with ulnar nerve stimulation.

Acquired myasthenia gravis. Selective involvement of esophageal, pharyngeal, and facial muscles

Serum samples from 152 dogs with a clinical diagnosis of idiopathic megaesophagus without detectable generalized muscle weakness were tested for the presence of antibodies to acetylcholine receptors by immunoprecipitation radioimmunoassay. Positive serum antibody titers (mean, 3.1 nmoL/L; range, 0.77-30 nmoL/L; reference values less than 0.6 nmoL/L) were found in 40 dogs (26%), with German Shepherd dogs (8/25, 32%) and Golden Retrievers (7/20, 35%) having a greater percentage of positive submissions. By immunocytochemical methods, localization of immune complexes at the neuromuscular junction after incubation of serum with normal canine muscle was documented in an additional 17 cases (11% of all samples submitted) that did not have increased antibody titers to acetylcholine receptors. Of the 40 seropositive dogs, 17 (48%) had a clinical improvement or remission of clinical signs associated with decreasing AChR antibody titers. Idiopathic megaesophagus has been associated with a poor prognosis; however, this study demonstrates that a large percentage of the dogs have myasthenia gravis and that with supportive treatment, the clinical signs may improve or resolve.

EMG evaluation of the floppy infant: differential diagnosis and technical aspects

Electromyographic examination of the newborn and young infant provides a relatively uncommon challenge to most electromyographers. The usual reason for referral for electromyographic studies in the newborn and young infant is to evaluate a floppy baby. The electromyographer must not only be aware of important differences in normal physiologic parameters but must also be familiar with a spectrum of diseases that are not typically encountered in the adult. The results of electromyography must also be correlated with the normal maturation of neuromuscular function. Although the most common pathophysiologic mechanisms affecting the peripheral motor unit are infantile motor neuron disease and the congenital myopathies, a large number of other disease entities warrant careful consideration.

Autoimmunity to the voltage-gated calcium channel underlies the Lambert-Eaton myasthenic syndrome, a paraneoplastic disorder

The Lambert-Eaton myasthenic syndrome (LEMS) is a disorder of neuromuscular transmission, often associated with small cell lung carcinoma (SCLC), and characterized by reduced quantal release of acetylcholine from the motor nerve terminals. Another neuromuscular transmission disorder, myasthenia gravis, has a well-understood autoimmunological cause. This review discusses the evidence for a similar autoimmunological effect in the development of LEMS. Injection of LEMS IgG into mice passively transfers the physiological and morphological abnormalities, which include paucity and disorganized arrangement of active zone particles believed to represent the voltage-gated calcium channels (VGCCs). Calcium influx via VGCCs into SCLC cells is reduced by LEMS IgG suggesting that in SCLC-associated LEMS, antibodies may be triggered by VGCCs expressed on these cells; this immunological cross-reactivity may lead to the neurological abnormality. Similar VGCCs on neuronally derived cells may trigger the disorder in those without a tumour. The disorder provides another example of the complicated relationships between the nervous and immune systems and tumorigenic processes.