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Genetic forms of myasthenia gravis
Review Article
Genetic Forms of Myasthenia Gravis Karl E. Misulis, MD, PhD and Gerald M. Fenichel, MD
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. Misulis KE, Fenichel GM. Genetic forms of myasthenia gravis. Pediatr Neurol 1989;5:205-10.
Introduction In a previous report, Fenichel provided a classification of myasthenic syndromes in children by clinical presentation [1]. This clinical approach was useful at the time but now should be replaced by a nosology that incorporates pathophysiology (Table 1). This approach is especially useful with syndromes in which the pathophysiology is genetic rather than acquired. Juvenile- and adult-onset acquired myasthenia gravis is due to antibodies that bind to the acetylcholine receptor (AChR) which causes increased turnover and destruction of the receptor, thereby decreasing the number of receptors available for interaction with acetylcholine [2]. Transient neonatal myasthenia occurs in infants of myasthenic mothers and is caused by either passive transfer or fetal synthesis of antibodies [3,4]. In contrast, the genetic myasthenias discussed in this review are not antibody-mediated. The clinical syndromes of congenital and familial-infantile myasthenia now are recognized as the phenotypic expression of several different defects in neuromuscular transmission [5]. Genetic heterogeneity appears likely for each phenotype. Sufficient phenotypic variability occurs among siblings that if the previous classification were used, 1 child may appear to have familial-infantile myasthenia, while a sibling may appear to have congenital myasthenia [6]. A nosology that allows such an outcome is unsuitable for understanding or managing disease. The expected onset of symptoms in most genetic myasthenias occurs in the neonatal period or early infancy. Not withstanding expectations, symptoms in some patients may not appear until childhood or even as late as adult-
.From the Department of Neurology; Vanderbilt University Medical Center; Nashville, Tennessee.
hood. This review presents the syndromes of early-onset, genetic myasthenia gravis within a pathophysiologic classification.
Defects in Neuromuscular Transmission Presynaptic Defects. Two presynaptic defects have been described: disturbances in ACh resynthesis or mobilization and disturbances in ACh release. The distinction between these defects can be made by special electrophysiologic testing in viol); however, this test has not been performed in all patients, thus no complete contrast of clinical features is possible. The clinical syndromes associated with both presynaptic defects are relatively homogeneous and are suggestive of the previously described syndrome of familial-infantile myasthenia. Respiratory and/or feeding disturbances are present at birth or shortly thereafter. Ptosis and generalized weakness may be present at birth or develop during childhood; however, ophthalmoparesis is not present. Anticholinesterase medication was useful in only a few patients. Clinical features could not predict the response to treatment [7-10].
Abnormal A Ch Resynthesis or Mobilization One male infant [7] and a brother-sister pair [8,9] were described with defects in ACh resynthesis or mobilization without abnormalities in ACh release. The brother-sister pair had fluctuating ptosis from birth, feeding difficulty during infancy, and easy fatiguability on exertion. Apneic episodes triggered crying, vomiting, or febrile illnesses. These symptoms responded to prostigmine, but the route of administration was not reported. The parents were normal, but 3 other siblings with similar symptoms died suddenly in infancy. Autosomal recessive inheritance appears likely. When the brother-sister pair were 18 and 6 years of age, respectively, repetitive stimulation at 2 Hz produced an abnormal decremental response after exercise but not at rest. The AChRs were normal in number and distribution and the morphology and size of nerve terminals and endplates were normal. Synaptic vesicles were of normal size but 60% greater than normal in electron density. Miniature
Communications should be addressed to: Dr. Fenichel; Vanderbilt University Medical Center; 2100 Pierce Avenue; Nashville, TN 37212. Received February 22, 1989; accepted April 5, 1989.
Table I. --:::
Myasthenic syndromes in children :
:
:::-_
:
::
:
::::
::
Genetic I. Presynaptic defects a. Abnormal ACh resynthesis or mobilization b. Abnormal ACh release 2. Postsynaptic defects a. End-plate AChE deficiency b. Reduced number of AChRs c. Impaired function of AChRs d. Slow channel syndrome
Acquired 1. AChR antibody positive a. Transitory neonatal b. Juvenile myasthenia* 1. Generalized 2. Mainly ocular 2.
AChR antibody negative a. Juvenile myasthenia 1. Mainly ocular 2. Relapsing ocular
* Antibody-negative juvenile myasthenia is classified as acquired, although a genetic basis or predisposition has not been definitively eliminated.
end-plate potentials (MEPP) amplitude decreased only after prolonged stimulation. The male infant reported by Albers et al. was the product of a consanguineous marriage [7]. Apnea was observed immediately after birth and intubation was required. He was diffusely weak and areflexic but had normal extraocular motility. Only transitory improvement was observed during the next 3 months. Unassisted ventilation could only be tolerated for intervals up to 4 hours. Prostigmine did not improve strength and guanidine made symptoms worse. The child died at 8 months of age. Repetitive stimulation at all rates from 1-50 Hz produced a profoundly abnormal decremental response. Facilitation of the initial compound action potential was demonstrated 15 sec after 5 sec of 50 Hz stimulation. AChRs appeared normal in size and distribution. The morphology of the end-plate was normal and AChE was abundant. The facilitation following high-frequency stimulation suggested a defect in mobilization of the transmitter. Three patients presented by Mora et al. had synaptic vesicles that were significantly smaller than normal controis [10]. All of these patients carried the clinical diagnosis of familial-infantile myasthenia with early onset of weakness. All responded to pyridostigmine. MEPP amplitudes were normal, but with repetitive stimulation at 10 Hz, MEPP amplitude and compound motor action potential amplitude were reduced. The normal resting MEPP amplitude suggested that release was normal, but the decrement with repetitive stimulation suggested that resynthesis or mobilization was impaired. Abnormal ACh Release
Two unrelated males with almost identical clinical syndromes were described by separate a u t h o r s - - V i n c e n t
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el al., Patient 1 161; Lecky el al., Patient I I Ii] Both patients had respiratory problems innnedialel 5' after birth and one had feeding difficulty as well. Motor development was slow during the first year of life, but both walked at 14 months of age. They fell more often than other children their age and were identified as being weak. During childhood they developed ptosis and weakness of the face, neck, and limbs. Ophthalmoparesis did not develop. Tendon reflexes were obtainable. An abnormal decremental response was observed at 5 Hz in 1 boy and at l Hz and at 3 Hz in the other. One had a positive response to edrophonium chloride and was treated unsuccessfully with pyridostigmine, the other did not respond to edrophonium chloride and was not treated. The number of AChRs as determined by ~-bungarotoxin binding and the morphology of the end-plate were normal, but MEPP amplitude was reduced in both patients. Tubocurarine affinity, ionchannel properties, and passive membrane properties were measured in 1 patient and were normal I 11 ].
Postsynaptic Defects Postsynaptic defects may be caused by AChE deficiency or by abnormalities of the AChR. Three different AChR abnormalities have been described 112]: (1) Abnormal numbers; (2) Impaired function; and, (3) Prolonged open time of the ACh-induced channel (slow channel syndrome). Despite considerable phenotypic diversity, patients with postsynaptic defects tend to be affected to a milder degree than those with presynaptic defects and respond more often to parenteral and oral administration of anticholinesterase medications, Some of these patients fit the previous classifications of congenital myasthenia or familial infantile, while others fit neither.
End-plate AChE Deficiency A male with this condition was 16 years of age when evaluated [13]. No other family member was similarly affected. Ptosis was observed 5 days after birth. At 6 months of age, a prostigmine test and several subsequent edrophonium tests were negative. Motor milestones were markedly delayed and he did not walk until 3 years of age. Weakness worsened with exertion but did not respond to pyridostigmine. At 14 years of age, he had small muscle bulk, lordosis, kyphoscoliosis, ophthalmoparesis, and weakness of the face, chest, and limbs. Tendon reflexes were absent in the arms and difficult to elicit in the legs. Single nerve stimulation produced repetitive muscle action potential. Such a response usually is observed in patients taking anticholinesterase medication, but this patient was taking no medication. An abnormal decremental response was recorded during repetitive stimulation at 2 and 40 Hz. Guanidine did not improve strength or electrophysiologic abnormalities. Prednisone improved strength but not electrophysiologic abnormalities.
MEPP amplitude was normal, but MEPP frequency was decreased; duration and half-decay time was prolonged. The store of quanta immediately available for release was reduced. Nerve terminals were reduced 3- to 4-fold in size. The postsynaptic membrane was reduced in size. The postsynaptic folds revealed focal degeneration and many were distended by labyrinthine membranes. The AChR was normal, but AChE was absent from the end-plate. A congenital defect in the molecular assembly of AChE or in its attachment to the postsynaptic membrane was suggested as the primary defect.
Reduced Number of AChRs Two unrelated boys were reported several times by a single group of investigators; the primary defect was a congenital paucity of secondary synaptic clefts [14-16]. Decreased fetal movements were observed during both pregnancies and arthrogryposis was present at birth in both. Both newborns had respiratory distress, feeding difficulties, facial weakness, hypotonia, and mild ptosis without ophthalmoplegia. Early mental development was normal, but motor development was delayed and weakness was exacerbated by intercurrent illness. Edrophonium tests were positive. Single nerve stimulation was normal, but repetitive stimulation at 3-5/sec produced an abnormal decremental response. Oral pyridostigmine improved function, decreased limb deformities, and improved motor development. MEPP amplitudes were reduced in size. There was a predominance of type I fibers and some type grouping. AChE activity was reduced and preterminal axons had increased branching. The postsynaptic membrane had few, if any, folds. There was no sign, however, of degeneration. AChRs were deficient in number and abnormal in distribution. Extrajunctional AChRs were present in some muscle fibers.
Impaired Function of AChRs Five patients with impaired function of AChRs have been reported by 2 different groups [6,11]. The defects were not the same. The first patient, a male, presented at birth with ptosis, feeding difficulty, and a weak c r y Vincent et al., Patient 3 [6]. At age 3 years, he was easily fatigued; edrophonium was administered to increase strength. Neostigmine treatment was initiated but the results of treatment were not reported. By 13 years of age, he had ptosis, almost complete ophthalmoplegia, and slight weakness and fatiguability of the face and limb muscles. Physical examination was not substantially different when studies of neuromuscular function were conducted at 16 years of age. Repetitive stimulation at 3 Hz produced an abnormal decremental response. The amplitude of MEPPs was reduced, ~-bungarotoxin binding was slowly reversible, and some muscle fibers had multiple end-plate regions. Ultrastructural studies of end-plate morphology were not performed. ACh content of the muscle was normal. These
investigations suggested an abnormality of the AChR macromolecule, the most prominent finding being the slow reversibility of c~-bungarotoxin binding. The second patient, an 18-year-old woman whose parents were cousins, had appeared normal at birth but developed bilateral ptosis at 7 months of age - - Lecky et al., Patient 2 [11]. The parents, related by a common maternal grandfather, both had congenital bilateral ptosis. Subsequent motor development was normal. At 16 years of age, she developed proximal limb weakness and abnormal fatiguability. Examination revealed ptosis, ophthalmoplegia, and limb weakness; all of which were increased by exercise. Tendon reflexes were normal. Edrophonium reversed the ocular and limb weakness; pyridostigmine therapy provided moderate improvement. Repetitive nerve stimulation at I, 3, and 5 Hz demonstrated neither an abnormal decremental nor incremental response and an abnormal decrement could not be provoked at 3 Hz after maximal voluntary contraction. Type 2 muscle fiber atrophy was observed in both the intercostal and triceps muscles. AChE activity was abundant in most intercostal muscle fibers, but the end-plates were elongated and multiple. Ultrastructure examination revealed a normal number of synaptic vesicles and a normal configuration of postsynaptic folds. Postsynaptic c~bungarotoxin binding was markedly reduced but without extrajunctional binding, suggesting that binding was very slow or rapidly reversible. This finding suggested a defect in the AChR macromolecule, although different from that reported by Vincent et al. (Patient 3) [61. Three additional patients reported by Vincent et al. (Patients 2,4,5) [6] revealed abnormalities of the AChR similar to those found in the patient reported by Lecky et al. [ l 1]. Patient 2 was the son of second cousins. He had ptosis and hypotonia at birth and developed generalized weakness by 4 years of age. There was some limitation of extraocular movements. Treatment with prednisone and plasma exchange had no beneficial effect. Although facial weakness was present, there were no respiratory or feeding difficulties. Repetitive stimulation at 3 Hz did not provoke a decremental response. Patient 4 was normal at birth but had slow gross motor development and ptosis which was evident by 18 months of age. Limitation of extraocular movements developed later. Repetitive stimulation at 3 Hz produced a decremental response. Patient 5 was the brother of Patient 4. Feeding difficulty, ptosis, and impaired extraocular motility were prominent shortly after birth. Generalized weakness and fatiguability were observed by 1 year of age. Repetitive stimulation at 3 Hz did not produce a decremental response. These 3 patients demonstrated reduced MEPP amplitude and reduced binding of c~-bungarotoxin. Similar abnormalities are found in patients with autoimmune myasthenia gravis. Although the findings in these patients could be explained by a reduction in the number of AChRs, the most likely explanation is an alteration in the receptor, whereby the affinity for ~-bungarotoxin is reduced or the binding is readily revers-
Misulis and Fenichel: MyastheniaGravis 207
ible. A definitive determination cannot be made on the basis of available data.
Slow Channel Syndrome The slow channel syndrome has been described in 2 male siblings from 1 family, 7 individuals from 3 generations of a second family, 3 individuals from 2 generations of a third family, and 2 sporadic patients [17,18[. The defect is transmitted by autosomal-dominant inheritance with incomplete penetrance; some individuals have electromyographic (EMG) evidence of disease without clinical symptoms. Onset occurred during infancy in 4 patients, at 12 years of age in 1, and during the third decade of life in 3. None was symptomatic at birth. Weakness of the cervical and scapular muscles often is the initial feature. Other features present in most patients include exercise intolerance, ophthalmoparesis, and muscle atrophy. Ptosis, bulbar dysfunction, and leg weakness are less common. The rate of progression is measured in decades and patients were not evaluated until 12-34 years after onset. None of the patients responded to injection or oral administration of anticholinesterase medication. Two appeared to be hypersensitive to the injection and responded with muscarinic side effects. Repetitive nerve stimulation at 3/sec produced an abnormal decremental response and single nerve stimuli produced a repetitive muscle potential in all patients tested. Light microscopy demonstrated type 1 fiber predominance. Some specimens also demonstrated group atrophy, tubular aggregates, and an abnormal endplate configuration. AChE activity at the end-plate was abundant. The EPP was markedly prolonged and the amplitude of the MEPPs was reduced. Fibers with the lowest MEPP amplitude demonstrated focal degeneration of junctional folds with loss of AChR. The basic abnormality is believed to be a prolonged open time of the ion channel associated with AChR.
Discussion Genetic forms of myasthenia gravis are generally distinguished from acquired forms by early onset and absence of circulating antibodies directed against AChR; however, these criteria are not absolute because both acquired and genetic forms of myasthenia gravis may manifest during the neonatal period, infancy, or early childhood, and acquired myasthenia also may be antibody-negative. Studies of human leukocyte antigen (HLA) suggest that a genetically determined breakdown in immune tolerance may be responsible for acquired myasthenia gravis in young females, as well as other autoimmune disorders [19-21]. Generalized myasthenia gravis in young females is associated with an increased frequency of HLA-B8 [1921], B8, and/or DRw3 [21]. Ocular myasthenia and myasthenia gravis associated with thymoma are not associated with a specific HLA type. Seybold and Lindstrom described 3 siblings, 1 of whom had clinical myasthenia gravis and elevated concentrations
208 PEDIATRICNEUROLOGY Vo|. 5 No. 4
of antibody to the acetylcholine receptor protei~ tAChRP): 2 were asymptomatic but had detectable AChRP antibodies at lower concentrations 1221. All 3 carried the tt I.,AB8 allele, as well as 4 other family members. The most perplexing patients are those presumed to have acquired myasthenia who are antibody-negative. In most patients, onset occurs during childhood, usually alter age 5 years, and the only symptoms are ptosis and diplopia. The response to AChE inhibitors and corticosteroids is variable and often difficult to assess because the natural history is characterized by spontaneous remissions and exacerbations [23]. The pathophysiology of such disorders is unknown and although they have been designated as acquired in Table 1, a genetic basis cannot be excluded. The nosology offered in Table 1 is tentative and reflects an incomplete state of knowledge concerning bx~lh acquired and genetic forms of myasthenia gravis. Its main purpose is to direct attention away from pure clinical classifications which have served a useful purpose in the past but now are misleading.
Approach to Patients with Possible Genetic Myasthenia The major difficulty in classifying the genetic myasthenic syndromes by their pathophysiologic defect is lack of information. Clinical descriptions are meager in some reports and laboratory evaluations are not equivalent. It is probably not possible to perform all tests needed for accurate characterization of the neuromuscular defect on 1 infant, especially when the infant is ill; however, a pathophysiologic determination would be helpful not only for evaluation of other potentially treatable diseases, but would also add to our understanding of congenital myasthenia. An ideal evaluation would include all of the following elements: The following features of the history and physical examination should be documented." (I) The onset and severity of feeding difficulty, respiratory dysfunction, ophthalmoparesis, ptosis, hypotonia, and limb fatiguability; (2) The absence of signs in (1) as important negative features: (3) All developmental milestones and progression or regression of symptoms during infancy and childhood; (4) The response to parenteral and oral anticholinesterase medications and to guanidine and corticosteroids; and, (5) A complete family pedigree that indicates which members were examined or studied electrophysiologically, those with any neuromuscular disease, and fetal or neonatal deaths. A complete laboratory evaluation shmdd include: (1) Needle EMG of weak muscles, motor and sensory nerve conduction velocities, and nerve repetitive stimulation at 3 and 10 Hz before and after exercise; (2) Serum concentrations of antibody directed against AChR and skeletal muscle;
Neonatal Respiratory I or Feeding Difficulty I Yes I
No
V
,
I
IOphthalmoparesisl Yes I No AChR
I Ophthalmoparesisl Yes I No
|
toxin Binding
Yes
I
Reduced AChR Number
No
to Single Stimulus
ACh Release
Yesl
No
Presynaptic [ACHE Deficiency I Defect Yes I No AChE Deficiency
Defective AChR
Slow Channel Syndrome
Figure 1. Flow chart for the diagnosis of the genetic myasthenias, developed from data presented in this review. The flow indicates the minimum information required to establish a provisional diagnosis. Confirmation of the defect (e.g., defective AChR) requires additional laboratory testing.
(3) Skeletal muscle biopsy for light microscopy and intercostal muscle biopsy for electron microscopy, immunohistochemistry, receptor binding studies, ACh content, and AChE assay. This latter portion often is performed in conjunction with in vitro electrophysiology. (4) In vitro electrophysiology for determination of MEPP amplitude, frequency, and quantal content which is typically performed on intercostal muscle. Briefly, intracellular recordings are obtained using a glass microelectrode with stimulation of nerve fibers facilitated by suction
electrodes. Potentials measured are the spontaneously occurring MEPPs and EPPs evoked by stimulation of a nerve branch. Quantal content is calculated from knowledge of the MEPP and EPP characteristics. The most important of the measurements is MEPP amplitude. Figure 1 is a flow chart for diagnosis of genetic myasthenias which was constructed from patient data presented above. Table 2 summarizes these data. As additional patients with these syndromes are reported, Table 2 and Figure 1 will undoubtedly be modified.
Table 2. Summary of distinguishing features in genetic myasthenia Presynaptic Defects Abnormal Abnormal ACh ACh RM Release
End-plate AChE
Deficiency
Postsynaptic Defects Reduced Impaired Number
AChR
of AChRs
Function
Slow Channel Syndrome
History Neonatal respiratory difficulty Neonatal feeding difficulty etosis Ophthalmoparesis Easy fatigability
+ + + + + + + + .9
+
+ + +
+ + + -+
+ ? ?
+ + - + +
+ + -
_ _ ? - - ?
_ _ - -
_ +
+ + +
+ + + -+
+ +
+ + +
+ + + +
? ? + + +
++++ ++++ ++++
+ + - + +
+ - - - ? ? ? - + + +
++++ ++++ +++--
+ + _+ ?
+ _ ? .9 - ? ? ?
+
+ + +
+ ? + + +
Electrophysiology Decrement Repetitive Decreased
at 3/sec discharge MEPP amplitude
+ + +
Biochemistry Decreased AChR AChE deficiency
number
* Data for this table are from the references therefore
not clearly
a genetic
cited in this review.
Not all patients
from all papers
were tabulated
because
some
had late onset and
etiology.
Abbreviations: +
= Finding
reported
-
was absent
Finding
in 1 patient
_+
= Finding
was equivocal
?
= Presence
not reported
RM
= Resynthesis
or mobilization
Misulis
and Fenichel:
Myasthenia
Gravis
209
References [1] Fenichel GM. Clinical syndromes of myasthenia m infancy and childhood. Arch Neurol 1978;35:97-103. [21 Lindstrom JM, Seybold ME, Lennon VA, Whittingham S, Duane DD. Antibody to acetylcholine receptor in myasthenia gravis. Prevalence, clinical correlates, and diagnostic value. Neurology 1976: 26:1054-9. [3] Lefvert AK, Osterman PO. Newborn infants to myasthenic mothers: A clinical study and an investigation of acetylcholine-receptor antibodies in 17 children. Neurology 1983;33:133-8. [4] Morel E, Eymard B, Vernet-der Garabedian B, Pannier C, Dulac O, Bach JF. Neonatal myasthenia gravis: A new clinical and immunological appraisal on 30 cases. Neurology 1988;38:138-42. [5] Gordon N. Congenital myasthenia. Dev Med Child Neurol 1986;28:810-3. [6] Vincent A, Cull-Candy SG, Newsom-Davis J, Trautman A, Molenaar PC, Polak RL. Congenital myasthenia: End-plate acetylcholine receptors and electrophysiology in five cases. Mumle Nerve 1981;4:306-18. [71 AIbers JW, Faulkner JA, Dorovini-Zis K, Barald KF, Must RE, Ball RD. Abnormal neuromuscular transmission in an infantile myasthenic syndrome. Ann Neurol 1984; 16:28-34. [8] Engel AG. Morphologic and immunopathologic findings in myasthenia gravis and in congenital myasthenic syndromes. 1 Neurol Neurosurg Psychiatry 1980;43:577-89. [9] Hart ZH, Sahsashi K, Lambert EH, Engel AG, Lindstrom JM. A congenital familial myasthenic syndrome caused by a presynaptic defect of transmitter resynthesis or mobilization. Neurology 1979;29: 556-7. [10] Mora M, Lambert EH, Engel A. Synaptic vesicle abnormality in familial infantile myasthenia. Neurology 1987;37:206-14. [11] Lecky BRF, Morgan-Hughes JA, Murray NMF, Landon DN, Wray D, Prior C. Congenital myasthenia: Further evidence of disease heterogeneity. Muscle Nerve 1986;9:233-42. [12] Vincent A. Disorders affecting the acetylcholine receptor: Myasthenia gravis and congenital myasthenia. J Recept Res 1987; 7:599-616.
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[13] Engel AG, Lambert EH, Gomez MR. A mv~ m?a~theni~ syndrome with end-plate acetylcholinestera~ deficiency, small nerve terminals, and reduced acetylcholine release. Ann Nctm~! 1977: 1:315-30. [14] Hageman G, Smit LME, Ftoogland R..Icnm:kens [~'(il, Willemse J. Muscle weakness and congenital contractures m :~ case of congenital myasthenia. J Pediatr Orthop 1986:6:227-3I. [15] Smit LME, Hageman G, Veldman H, Molenaar P(', {)eh BS. Jennekens FGI. A myasthenic syndrome with congenita! paucity of secondary synaptic clefts: CPSC syndrome. Muscle Nerve 1988"11: 377-48. [161 Smit LME, .lennekens FGI, Veldman H, Barth PG. Paucity of secondary synaptic clefts in a case of congenital myasthenia with mul-. tiple contractures: Ultrastructural morphology of a developmental disorder. J Neurol Neurosurg Psychiatry 1984;47:1091-7. [17] Engel AG, Lambert EH, Mulder DM, et al. A newly recognized congenital myasthenic syndrome attributed to a prohmged open time of the acetylcholine-induced ion channel. Ann Neurol 1982; 11:553-69. [18] Oosterhuis HJGH, Newsom-Davis J, Wokke JHJ, el al. The slow channel syndrome. Two new cases. Brain 1987;1 Ill: 1061-79. [19] Fritze D, Hermann C Jr, Naeim F, Smith GS, Zeller E, Walford RL. The biologic significance of HL-A antigen markers in myasthenia gravis. Ann NY Acad Sci 1976;274:440-50. [20] Pirskanen R. Genetic aspects in mya~sthenia gravis. A family study of 264 Finnish patients. Acta Neurol Scand 1977;56:365-88. [21] Compston DAS, Vincent A, Newsom-Davis J, Batchelor JR. Clinical, pathological, HLA antigen and immunological evidence for disease heterogeneity in myasthenia gravis. Brain 1980; 103:579-601. [22] Seybold ME, Lindstrom JM. Antiacetylcholine receptor antibody and its relationship to HLA type in asymptomatic siblings of a patient with myasthenia gravis. Neurology 1981:31:778-80. [23] Rollinson RD, Fenichel GM. Relapsing ocular myasthenia. Neurology 1981;31:325-6.
Genetic Forms of Myasthenia Gravis Karl E. Misulis, MD, PhD and Gerald M. Fenichel, MD
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. Misulis KE, Fenichel GM. Genetic forms of myasthenia gravis. Pediatr Neurol 1989;5:205-10.
Introduction In a previous report, Fenichel provided a classification of myasthenic syndromes in children by clinical presentation [1]. This clinical approach was useful at the time but now should be replaced by a nosology that incorporates pathophysiology (Table 1). This approach is especially useful with syndromes in which the pathophysiology is genetic rather than acquired. Juvenile- and adult-onset acquired myasthenia gravis is due to antibodies that bind to the acetylcholine receptor (AChR) which causes increased turnover and destruction of the receptor, thereby decreasing the number of receptors available for interaction with acetylcholine [2]. Transient neonatal myasthenia occurs in infants of myasthenic mothers and is caused by either passive transfer or fetal synthesis of antibodies [3,4]. In contrast, the genetic myasthenias discussed in this review are not antibody-mediated. The clinical syndromes of congenital and familial-infantile myasthenia now are recognized as the phenotypic expression of several different defects in neuromuscular transmission [5]. Genetic heterogeneity appears likely for each phenotype. Sufficient phenotypic variability occurs among siblings that if the previous classification were used, 1 child may appear to have familial-infantile myasthenia, while a sibling may appear to have congenital myasthenia [6]. A nosology that allows such an outcome is unsuitable for understanding or managing disease. The expected onset of symptoms in most genetic myasthenias occurs in the neonatal period or early infancy. Not withstanding expectations, symptoms in some patients may not appear until childhood or even as late as adult-
.From the Department of Neurology; Vanderbilt University Medical Center; Nashville, Tennessee.
hood. This review presents the syndromes of early-onset, genetic myasthenia gravis within a pathophysiologic classification.
Defects in Neuromuscular Transmission Presynaptic Defects. Two presynaptic defects have been described: disturbances in ACh resynthesis or mobilization and disturbances in ACh release. The distinction between these defects can be made by special electrophysiologic testing in viol); however, this test has not been performed in all patients, thus no complete contrast of clinical features is possible. The clinical syndromes associated with both presynaptic defects are relatively homogeneous and are suggestive of the previously described syndrome of familial-infantile myasthenia. Respiratory and/or feeding disturbances are present at birth or shortly thereafter. Ptosis and generalized weakness may be present at birth or develop during childhood; however, ophthalmoparesis is not present. Anticholinesterase medication was useful in only a few patients. Clinical features could not predict the response to treatment [7-10].
Abnormal A Ch Resynthesis or Mobilization One male infant [7] and a brother-sister pair [8,9] were described with defects in ACh resynthesis or mobilization without abnormalities in ACh release. The brother-sister pair had fluctuating ptosis from birth, feeding difficulty during infancy, and easy fatiguability on exertion. Apneic episodes triggered crying, vomiting, or febrile illnesses. These symptoms responded to prostigmine, but the route of administration was not reported. The parents were normal, but 3 other siblings with similar symptoms died suddenly in infancy. Autosomal recessive inheritance appears likely. When the brother-sister pair were 18 and 6 years of age, respectively, repetitive stimulation at 2 Hz produced an abnormal decremental response after exercise but not at rest. The AChRs were normal in number and distribution and the morphology and size of nerve terminals and endplates were normal. Synaptic vesicles were of normal size but 60% greater than normal in electron density. Miniature
Communications should be addressed to: Dr. Fenichel; Vanderbilt University Medical Center; 2100 Pierce Avenue; Nashville, TN 37212. Received February 22, 1989; accepted April 5, 1989.
Table I. --:::
Myasthenic syndromes in children :
:
:::-_
:
::
:
::::
::
Genetic I. Presynaptic defects a. Abnormal ACh resynthesis or mobilization b. Abnormal ACh release 2. Postsynaptic defects a. End-plate AChE deficiency b. Reduced number of AChRs c. Impaired function of AChRs d. Slow channel syndrome
Acquired 1. AChR antibody positive a. Transitory neonatal b. Juvenile myasthenia* 1. Generalized 2. Mainly ocular 2.
AChR antibody negative a. Juvenile myasthenia 1. Mainly ocular 2. Relapsing ocular
* Antibody-negative juvenile myasthenia is classified as acquired, although a genetic basis or predisposition has not been definitively eliminated.
end-plate potentials (MEPP) amplitude decreased only after prolonged stimulation. The male infant reported by Albers et al. was the product of a consanguineous marriage [7]. Apnea was observed immediately after birth and intubation was required. He was diffusely weak and areflexic but had normal extraocular motility. Only transitory improvement was observed during the next 3 months. Unassisted ventilation could only be tolerated for intervals up to 4 hours. Prostigmine did not improve strength and guanidine made symptoms worse. The child died at 8 months of age. Repetitive stimulation at all rates from 1-50 Hz produced a profoundly abnormal decremental response. Facilitation of the initial compound action potential was demonstrated 15 sec after 5 sec of 50 Hz stimulation. AChRs appeared normal in size and distribution. The morphology of the end-plate was normal and AChE was abundant. The facilitation following high-frequency stimulation suggested a defect in mobilization of the transmitter. Three patients presented by Mora et al. had synaptic vesicles that were significantly smaller than normal controis [10]. All of these patients carried the clinical diagnosis of familial-infantile myasthenia with early onset of weakness. All responded to pyridostigmine. MEPP amplitudes were normal, but with repetitive stimulation at 10 Hz, MEPP amplitude and compound motor action potential amplitude were reduced. The normal resting MEPP amplitude suggested that release was normal, but the decrement with repetitive stimulation suggested that resynthesis or mobilization was impaired. Abnormal ACh Release
Two unrelated males with almost identical clinical syndromes were described by separate a u t h o r s - - V i n c e n t
206
PEDIATRIC NEUROLOGY
Vol. 5 No. 4
el al., Patient 1 161; Lecky el al., Patient I I Ii] Both patients had respiratory problems innnedialel 5' after birth and one had feeding difficulty as well. Motor development was slow during the first year of life, but both walked at 14 months of age. They fell more often than other children their age and were identified as being weak. During childhood they developed ptosis and weakness of the face, neck, and limbs. Ophthalmoparesis did not develop. Tendon reflexes were obtainable. An abnormal decremental response was observed at 5 Hz in 1 boy and at l Hz and at 3 Hz in the other. One had a positive response to edrophonium chloride and was treated unsuccessfully with pyridostigmine, the other did not respond to edrophonium chloride and was not treated. The number of AChRs as determined by ~-bungarotoxin binding and the morphology of the end-plate were normal, but MEPP amplitude was reduced in both patients. Tubocurarine affinity, ionchannel properties, and passive membrane properties were measured in 1 patient and were normal I 11 ].
Postsynaptic Defects Postsynaptic defects may be caused by AChE deficiency or by abnormalities of the AChR. Three different AChR abnormalities have been described 112]: (1) Abnormal numbers; (2) Impaired function; and, (3) Prolonged open time of the ACh-induced channel (slow channel syndrome). Despite considerable phenotypic diversity, patients with postsynaptic defects tend to be affected to a milder degree than those with presynaptic defects and respond more often to parenteral and oral administration of anticholinesterase medications, Some of these patients fit the previous classifications of congenital myasthenia or familial infantile, while others fit neither.
End-plate AChE Deficiency A male with this condition was 16 years of age when evaluated [13]. No other family member was similarly affected. Ptosis was observed 5 days after birth. At 6 months of age, a prostigmine test and several subsequent edrophonium tests were negative. Motor milestones were markedly delayed and he did not walk until 3 years of age. Weakness worsened with exertion but did not respond to pyridostigmine. At 14 years of age, he had small muscle bulk, lordosis, kyphoscoliosis, ophthalmoparesis, and weakness of the face, chest, and limbs. Tendon reflexes were absent in the arms and difficult to elicit in the legs. Single nerve stimulation produced repetitive muscle action potential. Such a response usually is observed in patients taking anticholinesterase medication, but this patient was taking no medication. An abnormal decremental response was recorded during repetitive stimulation at 2 and 40 Hz. Guanidine did not improve strength or electrophysiologic abnormalities. Prednisone improved strength but not electrophysiologic abnormalities.
MEPP amplitude was normal, but MEPP frequency was decreased; duration and half-decay time was prolonged. The store of quanta immediately available for release was reduced. Nerve terminals were reduced 3- to 4-fold in size. The postsynaptic membrane was reduced in size. The postsynaptic folds revealed focal degeneration and many were distended by labyrinthine membranes. The AChR was normal, but AChE was absent from the end-plate. A congenital defect in the molecular assembly of AChE or in its attachment to the postsynaptic membrane was suggested as the primary defect.
Reduced Number of AChRs Two unrelated boys were reported several times by a single group of investigators; the primary defect was a congenital paucity of secondary synaptic clefts [14-16]. Decreased fetal movements were observed during both pregnancies and arthrogryposis was present at birth in both. Both newborns had respiratory distress, feeding difficulties, facial weakness, hypotonia, and mild ptosis without ophthalmoplegia. Early mental development was normal, but motor development was delayed and weakness was exacerbated by intercurrent illness. Edrophonium tests were positive. Single nerve stimulation was normal, but repetitive stimulation at 3-5/sec produced an abnormal decremental response. Oral pyridostigmine improved function, decreased limb deformities, and improved motor development. MEPP amplitudes were reduced in size. There was a predominance of type I fibers and some type grouping. AChE activity was reduced and preterminal axons had increased branching. The postsynaptic membrane had few, if any, folds. There was no sign, however, of degeneration. AChRs were deficient in number and abnormal in distribution. Extrajunctional AChRs were present in some muscle fibers.
Impaired Function of AChRs Five patients with impaired function of AChRs have been reported by 2 different groups [6,11]. The defects were not the same. The first patient, a male, presented at birth with ptosis, feeding difficulty, and a weak c r y Vincent et al., Patient 3 [6]. At age 3 years, he was easily fatigued; edrophonium was administered to increase strength. Neostigmine treatment was initiated but the results of treatment were not reported. By 13 years of age, he had ptosis, almost complete ophthalmoplegia, and slight weakness and fatiguability of the face and limb muscles. Physical examination was not substantially different when studies of neuromuscular function were conducted at 16 years of age. Repetitive stimulation at 3 Hz produced an abnormal decremental response. The amplitude of MEPPs was reduced, ~-bungarotoxin binding was slowly reversible, and some muscle fibers had multiple end-plate regions. Ultrastructural studies of end-plate morphology were not performed. ACh content of the muscle was normal. These
investigations suggested an abnormality of the AChR macromolecule, the most prominent finding being the slow reversibility of c~-bungarotoxin binding. The second patient, an 18-year-old woman whose parents were cousins, had appeared normal at birth but developed bilateral ptosis at 7 months of age - - Lecky et al., Patient 2 [11]. The parents, related by a common maternal grandfather, both had congenital bilateral ptosis. Subsequent motor development was normal. At 16 years of age, she developed proximal limb weakness and abnormal fatiguability. Examination revealed ptosis, ophthalmoplegia, and limb weakness; all of which were increased by exercise. Tendon reflexes were normal. Edrophonium reversed the ocular and limb weakness; pyridostigmine therapy provided moderate improvement. Repetitive nerve stimulation at I, 3, and 5 Hz demonstrated neither an abnormal decremental nor incremental response and an abnormal decrement could not be provoked at 3 Hz after maximal voluntary contraction. Type 2 muscle fiber atrophy was observed in both the intercostal and triceps muscles. AChE activity was abundant in most intercostal muscle fibers, but the end-plates were elongated and multiple. Ultrastructure examination revealed a normal number of synaptic vesicles and a normal configuration of postsynaptic folds. Postsynaptic c~bungarotoxin binding was markedly reduced but without extrajunctional binding, suggesting that binding was very slow or rapidly reversible. This finding suggested a defect in the AChR macromolecule, although different from that reported by Vincent et al. (Patient 3) [61. Three additional patients reported by Vincent et al. (Patients 2,4,5) [6] revealed abnormalities of the AChR similar to those found in the patient reported by Lecky et al. [ l 1]. Patient 2 was the son of second cousins. He had ptosis and hypotonia at birth and developed generalized weakness by 4 years of age. There was some limitation of extraocular movements. Treatment with prednisone and plasma exchange had no beneficial effect. Although facial weakness was present, there were no respiratory or feeding difficulties. Repetitive stimulation at 3 Hz did not provoke a decremental response. Patient 4 was normal at birth but had slow gross motor development and ptosis which was evident by 18 months of age. Limitation of extraocular movements developed later. Repetitive stimulation at 3 Hz produced a decremental response. Patient 5 was the brother of Patient 4. Feeding difficulty, ptosis, and impaired extraocular motility were prominent shortly after birth. Generalized weakness and fatiguability were observed by 1 year of age. Repetitive stimulation at 3 Hz did not produce a decremental response. These 3 patients demonstrated reduced MEPP amplitude and reduced binding of c~-bungarotoxin. Similar abnormalities are found in patients with autoimmune myasthenia gravis. Although the findings in these patients could be explained by a reduction in the number of AChRs, the most likely explanation is an alteration in the receptor, whereby the affinity for ~-bungarotoxin is reduced or the binding is readily revers-
Misulis and Fenichel: MyastheniaGravis 207
ible. A definitive determination cannot be made on the basis of available data.
Slow Channel Syndrome The slow channel syndrome has been described in 2 male siblings from 1 family, 7 individuals from 3 generations of a second family, 3 individuals from 2 generations of a third family, and 2 sporadic patients [17,18[. The defect is transmitted by autosomal-dominant inheritance with incomplete penetrance; some individuals have electromyographic (EMG) evidence of disease without clinical symptoms. Onset occurred during infancy in 4 patients, at 12 years of age in 1, and during the third decade of life in 3. None was symptomatic at birth. Weakness of the cervical and scapular muscles often is the initial feature. Other features present in most patients include exercise intolerance, ophthalmoparesis, and muscle atrophy. Ptosis, bulbar dysfunction, and leg weakness are less common. The rate of progression is measured in decades and patients were not evaluated until 12-34 years after onset. None of the patients responded to injection or oral administration of anticholinesterase medication. Two appeared to be hypersensitive to the injection and responded with muscarinic side effects. Repetitive nerve stimulation at 3/sec produced an abnormal decremental response and single nerve stimuli produced a repetitive muscle potential in all patients tested. Light microscopy demonstrated type 1 fiber predominance. Some specimens also demonstrated group atrophy, tubular aggregates, and an abnormal endplate configuration. AChE activity at the end-plate was abundant. The EPP was markedly prolonged and the amplitude of the MEPPs was reduced. Fibers with the lowest MEPP amplitude demonstrated focal degeneration of junctional folds with loss of AChR. The basic abnormality is believed to be a prolonged open time of the ion channel associated with AChR.
Discussion Genetic forms of myasthenia gravis are generally distinguished from acquired forms by early onset and absence of circulating antibodies directed against AChR; however, these criteria are not absolute because both acquired and genetic forms of myasthenia gravis may manifest during the neonatal period, infancy, or early childhood, and acquired myasthenia also may be antibody-negative. Studies of human leukocyte antigen (HLA) suggest that a genetically determined breakdown in immune tolerance may be responsible for acquired myasthenia gravis in young females, as well as other autoimmune disorders [19-21]. Generalized myasthenia gravis in young females is associated with an increased frequency of HLA-B8 [1921], B8, and/or DRw3 [21]. Ocular myasthenia and myasthenia gravis associated with thymoma are not associated with a specific HLA type. Seybold and Lindstrom described 3 siblings, 1 of whom had clinical myasthenia gravis and elevated concentrations
208 PEDIATRICNEUROLOGY Vo|. 5 No. 4
of antibody to the acetylcholine receptor protei~ tAChRP): 2 were asymptomatic but had detectable AChRP antibodies at lower concentrations 1221. All 3 carried the tt I.,AB8 allele, as well as 4 other family members. The most perplexing patients are those presumed to have acquired myasthenia who are antibody-negative. In most patients, onset occurs during childhood, usually alter age 5 years, and the only symptoms are ptosis and diplopia. The response to AChE inhibitors and corticosteroids is variable and often difficult to assess because the natural history is characterized by spontaneous remissions and exacerbations [23]. The pathophysiology of such disorders is unknown and although they have been designated as acquired in Table 1, a genetic basis cannot be excluded. The nosology offered in Table 1 is tentative and reflects an incomplete state of knowledge concerning bx~lh acquired and genetic forms of myasthenia gravis. Its main purpose is to direct attention away from pure clinical classifications which have served a useful purpose in the past but now are misleading.
Approach to Patients with Possible Genetic Myasthenia The major difficulty in classifying the genetic myasthenic syndromes by their pathophysiologic defect is lack of information. Clinical descriptions are meager in some reports and laboratory evaluations are not equivalent. It is probably not possible to perform all tests needed for accurate characterization of the neuromuscular defect on 1 infant, especially when the infant is ill; however, a pathophysiologic determination would be helpful not only for evaluation of other potentially treatable diseases, but would also add to our understanding of congenital myasthenia. An ideal evaluation would include all of the following elements: The following features of the history and physical examination should be documented." (I) The onset and severity of feeding difficulty, respiratory dysfunction, ophthalmoparesis, ptosis, hypotonia, and limb fatiguability; (2) The absence of signs in (1) as important negative features: (3) All developmental milestones and progression or regression of symptoms during infancy and childhood; (4) The response to parenteral and oral anticholinesterase medications and to guanidine and corticosteroids; and, (5) A complete family pedigree that indicates which members were examined or studied electrophysiologically, those with any neuromuscular disease, and fetal or neonatal deaths. A complete laboratory evaluation shmdd include: (1) Needle EMG of weak muscles, motor and sensory nerve conduction velocities, and nerve repetitive stimulation at 3 and 10 Hz before and after exercise; (2) Serum concentrations of antibody directed against AChR and skeletal muscle;
Neonatal Respiratory I or Feeding Difficulty I Yes I
No
V
,
I
IOphthalmoparesisl Yes I No AChR
I Ophthalmoparesisl Yes I No
|
toxin Binding
Yes
I
Reduced AChR Number
No
to Single Stimulus
ACh Release
Yesl
No
Presynaptic [ACHE Deficiency I Defect Yes I No AChE Deficiency
Defective AChR
Slow Channel Syndrome
Figure 1. Flow chart for the diagnosis of the genetic myasthenias, developed from data presented in this review. The flow indicates the minimum information required to establish a provisional diagnosis. Confirmation of the defect (e.g., defective AChR) requires additional laboratory testing.
(3) Skeletal muscle biopsy for light microscopy and intercostal muscle biopsy for electron microscopy, immunohistochemistry, receptor binding studies, ACh content, and AChE assay. This latter portion often is performed in conjunction with in vitro electrophysiology. (4) In vitro electrophysiology for determination of MEPP amplitude, frequency, and quantal content which is typically performed on intercostal muscle. Briefly, intracellular recordings are obtained using a glass microelectrode with stimulation of nerve fibers facilitated by suction
electrodes. Potentials measured are the spontaneously occurring MEPPs and EPPs evoked by stimulation of a nerve branch. Quantal content is calculated from knowledge of the MEPP and EPP characteristics. The most important of the measurements is MEPP amplitude. Figure 1 is a flow chart for diagnosis of genetic myasthenias which was constructed from patient data presented above. Table 2 summarizes these data. As additional patients with these syndromes are reported, Table 2 and Figure 1 will undoubtedly be modified.
Table 2. Summary of distinguishing features in genetic myasthenia Presynaptic Defects Abnormal Abnormal ACh ACh RM Release
End-plate AChE
Deficiency
Postsynaptic Defects Reduced Impaired Number
AChR
of AChRs
Function
Slow Channel Syndrome
History Neonatal respiratory difficulty Neonatal feeding difficulty etosis Ophthalmoparesis Easy fatigability
+ + + + + + + + .9
+
+ + +
+ + + -+
+ ? ?
+ + - + +
+ + -
_ _ ? - - ?
_ _ - -
_ +
+ + +
+ + + -+
+ +
+ + +
+ + + +
? ? + + +
++++ ++++ ++++
+ + - + +
+ - - - ? ? ? - + + +
++++ ++++ +++--
+ + _+ ?
+ _ ? .9 - ? ? ?
+
+ + +
+ ? + + +
Electrophysiology Decrement Repetitive Decreased
at 3/sec discharge MEPP amplitude
+ + +
Biochemistry Decreased AChR AChE deficiency
number
* Data for this table are from the references therefore
not clearly
a genetic
cited in this review.
Not all patients
from all papers
were tabulated
because
some
had late onset and
etiology.
Abbreviations: +
= Finding
reported
-
was absent
Finding
in 1 patient
_+
= Finding
was equivocal
?
= Presence
not reported
RM
= Resynthesis
or mobilization
Misulis
and Fenichel:
Myasthenia
Gravis
209
References [1] Fenichel GM. Clinical syndromes of myasthenia m infancy and childhood. Arch Neurol 1978;35:97-103. [21 Lindstrom JM, Seybold ME, Lennon VA, Whittingham S, Duane DD. Antibody to acetylcholine receptor in myasthenia gravis. Prevalence, clinical correlates, and diagnostic value. Neurology 1976: 26:1054-9. [3] Lefvert AK, Osterman PO. Newborn infants to myasthenic mothers: A clinical study and an investigation of acetylcholine-receptor antibodies in 17 children. Neurology 1983;33:133-8. [4] Morel E, Eymard B, Vernet-der Garabedian B, Pannier C, Dulac O, Bach JF. Neonatal myasthenia gravis: A new clinical and immunological appraisal on 30 cases. Neurology 1988;38:138-42. [5] Gordon N. Congenital myasthenia. Dev Med Child Neurol 1986;28:810-3. [6] Vincent A, Cull-Candy SG, Newsom-Davis J, Trautman A, Molenaar PC, Polak RL. Congenital myasthenia: End-plate acetylcholine receptors and electrophysiology in five cases. Mumle Nerve 1981;4:306-18. [71 AIbers JW, Faulkner JA, Dorovini-Zis K, Barald KF, Must RE, Ball RD. Abnormal neuromuscular transmission in an infantile myasthenic syndrome. Ann Neurol 1984; 16:28-34. [8] Engel AG. Morphologic and immunopathologic findings in myasthenia gravis and in congenital myasthenic syndromes. 1 Neurol Neurosurg Psychiatry 1980;43:577-89. [9] Hart ZH, Sahsashi K, Lambert EH, Engel AG, Lindstrom JM. A congenital familial myasthenic syndrome caused by a presynaptic defect of transmitter resynthesis or mobilization. Neurology 1979;29: 556-7. [10] Mora M, Lambert EH, Engel A. Synaptic vesicle abnormality in familial infantile myasthenia. Neurology 1987;37:206-14. [11] Lecky BRF, Morgan-Hughes JA, Murray NMF, Landon DN, Wray D, Prior C. Congenital myasthenia: Further evidence of disease heterogeneity. Muscle Nerve 1986;9:233-42. [12] Vincent A. Disorders affecting the acetylcholine receptor: Myasthenia gravis and congenital myasthenia. J Recept Res 1987; 7:599-616.
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[13] Engel AG, Lambert EH, Gomez MR. A mv~ m?a~theni~ syndrome with end-plate acetylcholinestera~ deficiency, small nerve terminals, and reduced acetylcholine release. Ann Nctm~! 1977: 1:315-30. [14] Hageman G, Smit LME, Ftoogland R..Icnm:kens [~'(il, Willemse J. Muscle weakness and congenital contractures m :~ case of congenital myasthenia. J Pediatr Orthop 1986:6:227-3I. [15] Smit LME, Hageman G, Veldman H, Molenaar P(', {)eh BS. Jennekens FGI. A myasthenic syndrome with congenita! paucity of secondary synaptic clefts: CPSC syndrome. Muscle Nerve 1988"11: 377-48. [161 Smit LME, .lennekens FGI, Veldman H, Barth PG. Paucity of secondary synaptic clefts in a case of congenital myasthenia with mul-. tiple contractures: Ultrastructural morphology of a developmental disorder. J Neurol Neurosurg Psychiatry 1984;47:1091-7. [17] Engel AG, Lambert EH, Mulder DM, et al. A newly recognized congenital myasthenic syndrome attributed to a prohmged open time of the acetylcholine-induced ion channel. Ann Neurol 1982; 11:553-69. [18] Oosterhuis HJGH, Newsom-Davis J, Wokke JHJ, el al. The slow channel syndrome. Two new cases. Brain 1987;1 Ill: 1061-79. [19] Fritze D, Hermann C Jr, Naeim F, Smith GS, Zeller E, Walford RL. The biologic significance of HL-A antigen markers in myasthenia gravis. Ann NY Acad Sci 1976;274:440-50. [20] Pirskanen R. Genetic aspects in mya~sthenia gravis. A family study of 264 Finnish patients. Acta Neurol Scand 1977;56:365-88. [21] Compston DAS, Vincent A, Newsom-Davis J, Batchelor JR. Clinical, pathological, HLA antigen and immunological evidence for disease heterogeneity in myasthenia gravis. Brain 1980; 103:579-601. [22] Seybold ME, Lindstrom JM. Antiacetylcholine receptor antibody and its relationship to HLA type in asymptomatic siblings of a patient with myasthenia gravis. Neurology 1981:31:778-80. [23] Rollinson RD, Fenichel GM. Relapsing ocular myasthenia. Neurology 1981;31:325-6.