Paucity of secondary synaptic clefts in a case of congenital myasthenia with multiple contractures: ultrastructural morphology of a developmental disorder

Clinical and neurophysiological characteristics of congenital myasthenic syndromes presenting in early infancy

The congenital myasthenic syndromes (CMS) constitute a group of genetic disorders, which affect neuromuscular transmission, presenting usually within the first years of life and with a clinical spectrum ranging from mild muscle weakness to severe disability with life-threatening episodes. We present clinical and neurophysiological data of 11 patients (four males, seven females) with CMS diagnosed during the last 5 years. Eight of the 11 patients presented immediately after birth and the remainder by 10 months of age; eight patients had contractures at birth and seven of them required assisted ventilation either immediately in the neonatal period, or at some point afterwards due to respiratory distress or recurrent apnoeas. Neurological signs at presentation were: in nine patients profound hypotonia, in five absent tendon reflexes, in seven ptosis and in eight bulbar signs. In six patients an edrophonium test was performed: only three of them had a positive response; however, eight out of 11 patients responded at least partially at some point in their illness to pyridostigmine. Diagnosis of CMS was confirmed either by demonstration of a decrement after repetitive nerve stimulation or by increased instability and jitter after stimulated single fibre EMG. In five patients, there was a positive family history with death of at least one previous sibling with an undiagnosed neuromuscular disorder. As regards final outcome, five patients died at ages ranging from 1 to 17 months, two patients are still ventilator-dependent at 3 and 5 months of life, respectively, and four patients still survive with either a mild or a moderate motor delay (follow-up range 8-38 months). None of the clinical or neurophysiological characteristics were correlated with outcome (Fisher's exact test). We conclude that a significant number of CMS patients may present in the neonatal period with a variable clinical expression and usually with a poor prognosis. The recognition of specific clinical constellations combined with a search for aetiology at a molecular level will enable the further characterisation of subgroups of CMS.

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.

Aberrant arrested in maturation neuromuscular junctions in centronuclear myopathy

Unusual ultrastructural changes of the nerve terminals have been found in an infant born with severe, fatal XLR form of centronuclear myopathy. Aberrant neuromuscular junctions in myotubes decorated by N-CAM were observed. The junction changes were manifested by simplification of postsynaptic membrane and paucity of secondary synaptic clefts. These resembles fetal neuromuscular junctions. The findings suggest that the expression of N-CAM by arrested myotubes may be promoted by abnormal nerve-muscle cell interactions, induced by motor endplate immaturity.

Familial infantile myasthenia: a diagnostic problem

The authors report the diagnostic difficulties experienced with two siblings presenting with recurrent apnoeic attacks caused by the rare condition, familial infantile myasthenia. Standard repetitive nerve stimulation studies were normal in both cases, while changes typical of a subacute neurogenic condition were noted on concentric needle electromyography in one. A 14-day course of pyridostigmine did not elicit any clinical improvement in the elder sibling, in whom tracheomalacia was diagnosed by chest fluoroscopy and bronchoscopy. This infant died at the age of 11 months with hypoxic brain-damage after a severe apnoeic episode. For the second sibling, a positive decremental response was obtained in the hypothenar muscles only after two minutes continuous 10 Hz stimulation of the ulnar nerve. This child has responded well to treatment with pyridostigmine.

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.

Genetic forms of myasthenia gravis

Myasthenia in newborns and infants is usually genetic and not mediated by antibodies to the acetylcholine receptor. Several different pre- and postsynaptic defects are responsible for early-onset myasthenia. This review presents a classification of these syndromes based upon knowledge of their pathophysiologic bases and a plan for the evaluation of new patients.

Congenital myasthenic syndromes

By the mid-1970s the autoimmune origin of myasthenia gravis had been well established. Once this feat had been accomplished, it also became apparent that myasthenic disorders occurring in a genetic or congenital setting had a different etiology. As a result, a number of distinct myasthenic syndromes have been recognized and investigated by electrophysiological and ultrastructural methods. The newly recognized disorders are conditioned by divergent causes, such as a failure of acetylcholine resynthesis or packaging, absence of acetylcholinesterase from the neuromuscular junction, abnormal gating properties of the acetylcholine receptor-associated ion channel, or an abnormality in the regulation of the density of acetylcholine receptor molecules in the postsynaptic membrane. These genetic defects either impair neuromuscular transmission directly or result in secondary derangements that eventually compromise the safety margin of neuromuscular transmission.

A myasthenic syndrome with congenital paucity of secondary synaptic clefts: CPSC syndrome

Two cases of a newly recognized myasthenic syndrome were investigated (CPSC syndrome). The course of the disease was observed for periods of 6 and 3 years. In infancy, exacerbations of the symptoms occurred during febrile illness, but thereafter the clinical course was stable and the children appeared to be only slightly handicapped. Biopsies were taken from the intercostal muscle in both patients. Microelectrode studies revealed small Mepp amplitudes. Light microscopy demonstrated predominance of type I fibers and focal type-grouping. There was a lowered cholinesterase activity and frequent branching of preterminal axons. Electron microscopy revealed that there were few, if any, folds of the postsynaptic membrane and that there were no signs of degeneration. Methods for localization of acetylcholine receptors (AChR) revealed a deficiency and altered distribution of AChRs at these postsynaptic membranes and the occurrence of extrajunctional AChRs in some muscle fibers. It is concluded that the syndrome is a clinicopathological entity, characterized morphologically by a congenital paucity of secondary synaptic clefts (CPSC syndrome).

A congenital myasthenic disorder with paucity of secondary synaptic clefts: deficiency and altered distribution of acetylcholine receptors

Congenital myasthenia (CM) constitutes a heterogeneous group of disorders with different underlying defects. The authors investigated a case of CM, presenting with congenital contractures. Endplate studies in the first year of life showed a developmental disorder of postsynaptic membranes. Clinical follow-up demonstrated a beneficial effect of pyridostigmine, resulting in normal motor development. Results of a second biopsy at age 4 are reported in this paper. Microelectrode study showed small Mepp amplitudes, which returned to nearly normal in the presence of neostigmine. In the electronmicroscope the postsynaptic membranes showed a paucity of infoldings, as in the first biopsy. These membranes showed only scanty, patchy enhancement with two different methods for localization of AChR. The extrajunctional membranes showed evidence of local presence of AChR. Our results show a developmental disorder of postsynaptic membranes with a deficiency and altered distribution of AChRs.

Ultrastructure of motor endplates in canine congenital myasthenia gravis

Morphometric analysis was carried out on electron micrographs of motor endplates from three Jack Russell terriers affected with congenital myasthenia gravis (CMG) aged 10 weeks, 12 weeks and 22 weeks, respectively. Control endplates from age-matched pups and an adult Jack Russell were also examined. The results showed that postsynaptic membrane density was significantly increased in affected animals and secondary fold length was decreased. The ratio of postsynaptic to presynaptic membrane length was normal in the 10 and 12-week-old pups, but reduced in the 22-week-old CMG animal. These changes were unrelated to muscle fibre diameter and there was no evidence of a destructive process. It is suggested that the alteration in membrane folding pattern in this condition may be related to abnormal trophic influences during synaptogenesis.