Immaturity of muscle fibers in the congenital form of myotonic dystrophy: Its consequences and its origin

Immaturity of muscle fibers in the congenital form of myotonic dystrophy: Its consequences and its origin

Journal of the Neurological S cwnces, 1988, 83 145-159 Elsevaer 145 JNS 02937 Immaturity of muscle fibers in the congenital form of myotonic dystro...

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Journal of the Neurological S cwnces, 1988, 83 145-159 Elsevaer

145

JNS 02937

Immaturity of muscle fibers in the congenital form of myotonic dystrophy: its consequences and its origin E. F a r k a s - B a r g e t o n , J . P . Barbet, S. D a n c e a , R. Wehrle, A. C h e c o u n and O. D u l a c Laboratozre de Neuropathologle de l'H6pital Saint-Vincent-de-Paul, Pans (France) (Recewed 23 March, 1987) (Revised, recewed 4 September, 1987) (Accepted 4 September, 1987)

SUMMARY

Skeletal muscle maturation is unpaired in children with congenital myotonic dystrophy. This immaturity is characterized at the light microscopy level by an abnormal presence of myotubes, small fascicles of muscle fibers, tlun myofibers, and delayed muscle fiber type differenUation with a peripheral halo laclong mitochondnal oxidative enzyme activity. At an ultrastructural level, the characteristics are a paucity ofmyofibnls with a peripheral nm devoid of mitochondria and myofibrils in the fibers. In ttme the muscle is able to gain a certain degree of maturity as shown in one of our cases who had two successwe muscle biopsies. The muscle, however, never becomes normal but retaans discrepancies in fiber size and fiber type distribution and shows some fiber necrosis. Maturation of the motoneurons is normal, which may explain necrosis of immature muscle fibers. In an experimental study carried out to look for evidence of a ctrculatory factor m mothers of children with congenital myotonlc dystrophy, it was found that sera from these mothers administered intra-peritoneaUy to newborn rats does in fact impair muscle maturation, whereas rats injected similarly with sera from control women showed normal muscle maturation.

Key words: Immaturity of muscle fibers; Neuronal cell bodies of VII nucleus; Myotonic dystrophy

Correspondence to J P Barbet, Laboratolre d'Anatom~e Pathologlque, H6pltal Saint Vincent-dePaul, 74 ave Denfert-Rochereau, F-75014 Pans, France 0022-510X/88/$03 50 © 1988 Elsevier Science Pubhshers B V (Biomedical Division)

146 INTRODUCTION In a p r e v i o u s study ( F a r k a s - B a r g e t o n et al. 1974), we d e s c r i b e d t h e particular a s p e c t o f m u s c l e fibers in n e o n a t e s w i t h c o n g e n i t a l m y o t o m c d y s t r o p h y ( C M y D ) . A p e r i p h e r a l h a l o d e v o i d o f m i t o c h o n d r i a l o r a d a t i v e e n z y m e activities, c o r r e s p o n d e d in E M to an a r e a d e v o i d o f m i t o c h o n d r i a a n d myofibrils. T h e h y p o t h e s i s t h a t this w a s a sign o f t m m a t u r i t y w a s later c o n f w m e d w h e n it w a s o b s e r v e d in n o r m a l h u m a n fetal fibers ( F a r k a s - B a r g e t o n et al. 1977). In 4 n e w c a s e s o f C M y D , w e h a v e f o u n d a d d i t i o n a l signs o f m u s c l e immaturity, t o g e t h e r with n e c r o t i c fibers as in o u r p r e v i o u s c a s e s ( F a r k a s - B a r g e t o n et al. 1974). A c c o r d i n g to L o w n e et al. (1982), fiber n e c r o s i s m a y o c c u r w h e n m a t u r e m o t o n e u r o n s i n n e r v a t e i m m a t u r e muscle. T o test such a possibility in C M y D , m o t o n e u r o n s o f the factal nucleus w e r e m e a s u r e d a n d c o m p a r e d w i t h n o r m a l controls. S i n c e a role o f a m a t e r n a l e n v t r o n m e n t a l f a c t o r during i n t r a u t e r i n e life h a d b e e n p o s t u l a t e d ( H a r p e r and D y k e n 1972), w e c h e c k e d the effect o f s e r u m f r o m 2 C M y D m o t h e r s o n d e v e l o p i n g rat muscle. N e w b o r n rats w e r e c h o s e n r a t h e r t h a n p r e g n a n t females, s m c e m o s t a n o m a l i e s o b s e r v e d in n e o n a t a l C M y D d e a l w i t h a p e r i o d o f m u s c l e d e v e l o p m e n t a n d m a t u r a t t o n o c c u r r i n g after birth in rats

MATERIAL AND METHODS

Case reports Case 1. A preterm girl of 30 weeks. Her mother, maternal grandparents, an uncle and a great-uncle all had MyD The mother had had stx pregnancies: 2 miseamages, 3 preterm infants (32, 33, and 35 weeks) who died m the neonatal period and the present pregnancy This was comphcated by polyhydrammos and a girl of 1700 g with generalized hypotoma and poor resptratory control was delivered by cesarean seetaon at 30 weeks_ She presented a tnangnlar mouth, poor facial expression, difficulty in swallowing, anhrogryposls with flexion of the hips and 30 ° tahpes. Chest X-rays showed thin ribs and elevation of the nght side of the diaphragm CMyD was suspected and a muscle biopsy performed on day 2_ The child died on day 3 from persistent severe respiratory distress Case 2. A preterm girl of 33 weeks. The mother had slow relaxation of the fingers and a tustory of previous miscarriages at 4t/2 months and 8 weeks The present pregnancy was complicated by polyhydramnios and a severely asphyxmted girl of 2300 g was delivered by cesarean section at 32 weeks After neonatal resusc]taUon, gencrahzed hypotonia was noted as well as difficulty in swallowing, an open tnangular mouth and btlatcral nearly fixed tahpes A chest X-ray disclosed thin ribs and elevation of the nght side of the dmphragm. The clinical diagnosis of CMyD was based on this data and on the mother's case history_ The infant suffered persistent respiratory chstress and a muscle biopsy was performed on day 2, just before death_ Case 3. A preterm girl of 37 weeks_ The 24-year-old mother, with percuss]on myotoma and a cataract, had had 2 previous pregnancies, one child was still ahve and doing well, while the second had died alter 6 days The first pregnancy with the present procreator was marked by polyhydrammos and early neonatal death In the present pregnancy there was a small excess of ammotic fired, the 2000-g girl was resuscitated at birth following an absence of respiratory movements Chest X-rays showed diaphrasmat]c immobility on the right side and elevauon on the left side. The diagnosis of CMyD was made and a muscle biopsy performed No respiratory control was gained and the child died aftcr 12 days. Case 4 A 3-year-old boy The mother, maternal grandmother and uncle all had MyD Thts boy was delwered by cesarean secuon at 32 weeks because of a narrow pelvis, breech presentation and fetal distress_ the child weighed 1950 g, was severely asphyxiated and reqmred resuscitation. Severe generalized hypotoma was noted together with swallovnng dfl~icult~es necessitating tube feeding The child left the hospital after

147 2 months_ At the age of 3 months (55 cm, 3320 g), he presented poor facial expression, a tent-shaped mouth, generalized hypotonia and gastroesophageal reflux The diagnosis of CMyD was made and a muscle biopsy performed at 6 months The cluld did faarly well during the following months, sat at 10 months and walked at 2 years, 8 months At 3 years 3 months, he showed no muscle weakness but did not speak well and stdl had a poor facml expressmn A second muscle bmpsy was performed m the same regmn as the first one.

Me~o~ Muscle biopsies All muscle biopsies were performed on the vastus lateralis. Each sample was divided into 3 parts: the ftrst was processed in paralTm, the second frozen in isopentane cooled in liquid N 2 for histochemistry and the third embedded in Spurr's resin for electron microscopy (EM). The routine histoenzymology included ATPase at pH 9.4 and after acid preincubations at pH 4.63 and 4.35 (Brooke and Kaaser 1970), NADHtetrazolium reductase (NADH-TR), succinate dehydrogenase (SDH) and g-glycerophosphate dehydrogenase (GPDH).

Neurons of the facial nucleus The brains of cases 1 and 2 and of 2 controls of 30 and 33 weeks without any neuromuscular involvement were fixed in 10~o formalin for several months. Then a block containing the upper part of the medulla oblongata and the lower part of the pons was dissected, embedded in paraffin and serial sections of 7 #m were cut and stained with cresyl violet. Ten sections of the facial nucleus separated one from another by 70 #m were selected m each case. Neuronal cell bodies containing a nucleus and a nucleolus were measured in the facial nucleus using a Leitz semiautomatic image analyser (a previous study showed that histograms of quantttication of neuronal cell bodies are identical whatever the level chosen in the facial nucleus, indicating its homogeneous neuronal structure).

Experimental study In 6 pregnant Sprague-Dawley rats, each litter was restricted to 8 animals soon after birth. In each group, 4 animals were treated with serum obtained from the mothers of cases 2 and 4 (0.2 ml per day i.p.) and 4 control animals were either untreated or injected with serum from healthy women depending on the group. Animals were killed on days 5, 10 and 15 and samples collected from the gastrocnemius (rectus femoris)for histoenzymology and EM. The only difference from the techniques used on human muscle was that ATPase acid preincubations were carried out at pH 4.5 and 4.3 (Brooke et al. 1971).

148 RESULTS

Muscle biopsies Case 1. M a n y myotubes with central nuclei were observed and the fibers and fascicles were very small for the age of the child (Table 1) P r e p a r a t i o n s for m i t o c h o n d n a l enzymes showed the activity to be grouped in the center of the fibers or a r o u n d the central nuclei, leavmg a peripheral halo devoid o f reactivity. Analysis by s t a n d a r d A T P a s e reaction revealed only occasional type I fibers a n d these h a d a weaker activity than the others. M o s t fibers h a d a strong A T P a s e activity although they were devoid o f myofibrillar reactivity either m their center or at their periphery (Fig. l a ) Following acid p r e m c u b a t i o n m o s t fibers maintained an intense staining reactivity, indicative of immature type I I C fibers. E M : m o s t fibers were m fact myotubes with a central nucleus or a central area devoid o f myofibnls and contmnmg m l t o c h o n d n a and reticulum. Myofilaments were poorly organized, often in c o m p a c t masses with irregular interruptions. Myofibers at different degrees o f maturation could be seen within a single b a s a l m e m b r a n e (Fig. 2) M a n y fibers h a d peripheral rims devoid o f myofibrlls and m l t o c h o n d n a , but containing glycogen and sometimes myofdaments, retlculum or ribosomes. Necrotic fibers were also seen. Case 2 All fibers were very small (Table 1) and formed abnormally small fascicles. M a n y myotubes had voluminous central nuclei. On p r e p a r a t i o n s for mltochondrial enzymes, actwlty was centrally located with a clear inactwe peripheral halo. On s t a n d a r d

TABLE 1 QUANTITATIVE DATA FROM MUSCLE BIOPSIES IN CASES OF CMyD AND CONTROL INFANTS Cases

Age

Control Control Case 1 Case2 Case 3 Case4

27 ~k 30 wk 30 wk 33wk 37 wk 6mo

Control Case 4

6 mo 3 yr

Control

3 yr

Mean fiber diameter in cases of CMyD (/zm)

Mean fiber dmmeter in control cases (#m)

Proportion of rnyotubes ( ~ )

11 6 _+2 4 9 8 + 18

0 0 81 24 25

4 3 + 12 54+ 1 1 7.5 _+0 9 1 9_8+74(r¢=7482) II 1 4 2 + 3 6 ( ~ = 2 5 0 6 ) 14"

1 0

25*

0 0

I 17_7 + 6.5 (~ = 368 5) II 257+ 57(~=2224)

* After Brooke and Engel (1969) = Vanablhty coefficient

149

Fig I. Muscle bLopstes ( x 544). (a) Case I, ATPase p H 9 4 myotubes wlth mactwe central area, some fibers with mactlvc peripheral areas. (b) Case 2, ATPase p H 9.4: simdar aspect to that of case I_ (c) Case 3, N A D H - T R : peripheral halo devold ofactiwty in most fibers (d) Case 3, ATPasc p H 4 35 3 fibertypes, dark type I, clear type IIB and intermcchate type IIC

Fig 2 C a s e 1, E M , x 8400_ A m y o b l a s t , a myofiber a n d a necrotic fiber in same b a s a l m e m b r a n e

150 ATPase, most fibers reacted strongly but a few large or small fibers had a low activity (Fig. lb). These type I fibers were darker on the ATPase after acid preincubatlons. Most type II fibers were of subtype IIC but some were lighter than the others at pH 4.35, indicating that there were some fibers of subtype IIB. Fiber typing was, however, impatred and often imprecise because of the paucity of myofibrlls. EM' many myotubes were present, as well as some myoblasts with a basal membrane and a scanty cytoplasm rich in glycogen but devoid of myofflaments. Most fibers had a peripheral rim devoid of mitochondria and myofibrils, and some had few myofibrils in their center. Several fibers at different degrees of maturation could be seen inside a single basal membrane. Necrotic fibers were also encountered exceptionally. Case 3. Myofibers were very small (Table 1) and there were some myotubes. On rmtochondrial enzyme preparations, most fibers had an important peripheral halo devoid of activity (Fig. lc) and some only reacted in their center or around the central nuclei. On standard ATPase, a few large or small fibers had a lower activity than the others. Following acid preincubation there seemed to be 3 types of fibers: I, IIB and IIC (Fig. ld). Type I and liB fibers were more numerous than in cases 1 and 2 but here again fiber typing was impaired by paucity of myofibrlls, the peripheral rim being devoid of myofibrlls and necrotic changes. EM: myotubes with central nuclei were seen, as well as some Isolated myoblasts. Myofilaments were often accumulated without any clear organization in myofibrils (Fig. 3). When segregated, myofibrils were irregular, varied m form and sometimes even lacked striations. Many fibers were either grossly disorganized or necrotic with completely hyalinlzed structures and autophagic vacuoles (Fig. 4). Case 4. The ftrst biopsy at 6 months showed a marked variability in muscle fiber size (Table 1). On preparations for mitochondrial dehydrogenases most fibers of both types had a peripherally inactive halo (Fig. 5a). Standard ATPase clearly showed the presence of two fiber types, and fibers of type I were smaller than type II fibers and showed greater variability in size. Acid preincubation showed that the muscle was only composed of three fiber types: I, liB and IIC. The second biopsy performed at 3 years 3 months showed a clear increase in fiber size (Table 1) and a disappearance of the peripherally inactive halo (Fig. 5b). IIA fibers had appeared but were less numerous (13 ~o) than IIB fibers (27~o), and the IIC fibers had disappeared. Type I fibers were still smaller than type II fibers but were now the major fiber type present (60~o). EM' in the first biopsy, many fibers contained few myofibrils and some had a more or less complete peripheral rim devoid of myofibrils and mitochondria (Fig. 6a). Myotubes were only encountered exceptionally (Fig. 6b). The second biopsy showed normal mature fibers with marked differences in size and many empty basal membranes (Fig. 7) indicating complete degeneration of the structures they contained.

Neurons of the facial nucleus The histological aspect of both small and large motoneurons of facial nuclei in cases 1 and 2 was similar to controls, as well as thetr density (Fig. 8a and b). Mean

151

Fig. 3. Case 3, EM, x 3360 Myotubes and myofibers showing size discrepancy Dlsorgantzed myofdaments m a myotube (arrow). Peripheral runs contmmng glycogen

F~g 4 Case 3, EM, x 7000 Dlsorgamzed myofibcrs w t h swollen reticulum, and one necrotic fiber w t h autopbaglc vacuoles

152

Fig 5 Muscle biopsies in case 4, NADH-TR, x 214 (a) First biopsy at 6 months: peripheral rtm devoid of actwity m most fibers (b) Second biopsy at 3 years: increase m mean fiber size and disappearance of the peripheral mactwe halo

neuronal surface area was 358 + 139 #m 2 in case 1 and 500 + 152 #m z in case 2 (not different from the control fetuses, whose surface areas were 359 + 146 #m z and 500 + 150 #m 2, respectively). Histograms of neuronal cell body surface areas were similar in both diseased and control fetuses.

Experimental study Hlstoenzymology 5-Day-old rats' m all rats, most fibers had a peripheral halo devoid of activity on the preparations for N A D H - T R and SDH. Size variability was more important in the treated animals than m the two control groups (Table 2). Furthermore, there were only 2 fiber types present in MyD-treated animals on the ATPase following preincubation at pH 4.3 (type I and IIC), whereas there were 3 fiber types in both control groups (type I, l i b and IIC) (Fig. 9a and b). 10-Day-old rats: m the 3 groups of rats, the peripheral halo on prepaxaUons for N A D H - T R and S D H disappeared and type I, IIB and IIC fibers were present. In the control groups, type I fibers were no longer distributed over the entire surface of the muscle but occupied only two-thirds of it. In the rats injected with MyD sera, fibers were smaller (Table 2) and type I fibers were distributed throughout the entire cross-sectional area of the muscle. 15-Day-old rats: type I fibers occupied only half the surface area of the muscle in the two control groups and a greater surface in the MyD treated animals (Fig. 10a and b). Type II fibers were smaller in the treated group (Table 2).

EM In 5-day-old control rats, myofibers with single peripheral nuclei were fiUed with myofibnls and very few satellite cells could be seen (Fig. 1 lb). However, rats rejected with MyD sera had numerous satellite cells (Fig. 1 la); the fibers frequently had multiple or folded nuclei and several fibers at different degrees of maturation were encountered within a single basal membrane.

153

Fig_ 6 Case 4, first biopsy (a) EM, x 4200: many fibers with a peripheral nm devoid of myofibrds and contmning glycogen (b) EM, x 9800 longltuchnal section of a myotube with centrally located rmtochondria.

154

Fig. 7 Case 4, EM, x 7000. Presence of completely empty basal membrane between normal myofibers.

TABLE 2 MEAN DIAMETER (~m), STANDARD DEVIATION A N D VARIABILITY C O E F F I C I E N T OF MUSCLE FIBERS OF TYPES I A N D II IN THE D I F F E R E N T GROUPS OF RATS S T U D I E D (3 RATS WERE M E A S U R E D IN EACH AGE GROUP) Age group

Control rats

Animals treated wRh normal sera

Antmals treated with sera from affected mothers

5 days

I 930+406 436 23 II 16.59 + 270 162.82

11005+423 ~ 330 89 II 13.23 __+308 ~ 232 80

I ~ II ~

979+493 504 12 13 51 __+3 60 1002.17

10 days

I 1461 + 342 234.11 II 15 77 + 5.02 318 80

I ~ II ~

I ~ II ~

12.15 + 15 315.20 12 82 + 261 204.19

15 days

I 21.45 + 395 Vr 184 16 II 19_73 + 495 251 08

I 21 10 + 236 ~ 109_01 II 18 62 + 3.31 ~ 177_76

= Vanabdlty coefficlent

14.40+ 323 224.85 15.56__+ 405 262.05

I 20 42 + 4 54 ~ 222 59 II 15 67 + 464 ~ 296 73

155

Fig. 8 Nucleus of the VII, cresyl wolet, × 48. In case 2, normal mature cell bodies with well developed Nissl bodies (a) were similar to those m the control (b).

Fig 9 Experimental 5-day-old rats, ATPase, pH 4.3, x 240. (a) Rat rejected with MyD sera: fiber types I (dark) and IIC (intermediate) (b) Rat injected with normal sera: fiber types I (dark), IIB (clear) and IIC (intermediate)

8 w

Fig 10_ Experimental 15-day-old rats, ATPase, pH 43, x 16. Type I fibers occupied a greater surface area in the rat rejected with MyD sera (a) than m the rat injected vath normal sera (b)

156

Fig_ 11 Expenmental 5-day-old rats, EM, x 2000 (a) Rat injected with MyD sera: 2 fibers with satellite cells surrounding 4 fibers at different degrees ofmaturaUon within a single basal membrane (b) Rat injected vath normal sera: absence of satelhte cells and fibers vath slmdar degrees of maturatmn

157 DISCUSSION Many cases of CMyD have been reported since the first description by Vamer in 1960 (for detailed references, see Harper 1986). The clinical picture is quite different from forms with later onset and myotonia cannot be detected before the second year of life. The main clinical features are hypotonla, facial diparesis and dysphagia with frequent respiratory distress, and talipes. Karpati et al. (1967) first examined muscle biopsies m older children with CMyD and observed histological signs of the adult form. In our first early biopsies (Farkas-Bargeton et al. 1974) the changes were different from those seen in older children or adults, and immaturity was evadent though not discussed. The abnormal presence of myotubes, presence of fibers at different degrees of maturation within a single basal membrane, small diameter of both type of fibers as well as the scarcity of myofibrils inside them and the excess in the number of type II fibers were all signs of immaturity which became evident after the study of human fetal muscle maturatmn (Farkas-Bargeton et al. 1977). Sarnat et al. (1963) were the first to evoke immaturity of muscle fibers in CMyD, although they did not perform any histochermstry. The small size and large variability coefficient of the fibers noted both m our 2 cases and in the studies by Sahgal et al. (1983) and Silver et al. (1984), is probably the result of a delay in muscle fiber maturation. Fiber type differentmtion was delayed in all our cases, and was only observed in the 37 week gestation premature infant of the series of Sahgal et al. (1983). However, fiber typing was very difficult due to the varying amount and repartitlon ofmyofibrils in the fibers, the peripheral cytoplasmic rim devoid of myofibrtls and secondary degenerative changes. Abnormal subsarcolemmal areas were found by Sarnat et al. (1975) and were called "sarcoplasmlc masses" smular to those observed m adults. However, both our prevmus (Farkas-Bargeton et al. 1974) and our present studies show that the peripheral rim is devoid of rmtochondria and myofibrils and is different from the sarcoplasmic masses of adult MyD which are rich in mltochondria and vesicular structures (Lapresle and Fardeau 1965; Casanova and Jerusalem 1979). This peripheral rtm is a sign of immaturity, since it ~s normally only observed in human fetal fibers (Farkas-Bargeton et al. 1977). Furthermore, as shown in case 4, ~t disappears with time in children with CMyD. Muscle maturation, although delayed, still takes place in CMyD as shown by: (a) an increase m mean diameter of the fibers between the 2 biopsies in our case 4 from 6 months to 3 years (although type I fibers remmned small with a large variability coefficient), (b) increase in mean fiber diameter with age in the different cases, (c) decreasing percentage of satellite cells, disappearance of myotubes and progressive hlstochemical differentaation with age (Sahgal et al. 1983). It can thus be concluded, in accordance with Samat et al. (1976), Sahgal et al. (1983) and Iannaccone et al. (1986), that the abnormal features of muscle fibers encountered m CMyD are signs of immaturity and will improve progressively with time, which can explam the chnical improvement in surviving children However, fiber necrosis cannot be explained by simple immaturity, and was maxmaal m the 2 oldest children of our series. Abnormally immature muscle fibers can degenerate as their motoneurons mature normally (Lowne et al. 1982, Vrbova 1983). When skeletal muscles of immature animals are denervated and then reinnervated by

158 the nerve that originally supphed them, slow muscles recover (Lowrie et al. 1982) but fast muscles remain weak, due to selective degeneration and disappearance of fast glycolytic fibers. Since our examination showed normal motoneurons in the facial nucleus of cases 1 and 2 with faoal diplegia, it can be postulated that the development of the facial motoneurons was normal, and probably that of all motoneurons. The decrease in cell body area of limb motoneurons observed by Walton et al. (1977) in adult MyD is probably secondary to longstanding low activity, which does not occur m early cases of CMyD. Necrosis of type II fibers could then result in CMyD from normal innervatlon of immature muscle fibers, which could explain the predominance of type I fibers m older children (Argov et al. 1980), the presence of empty basal membranes and enhanced fiber atrophy. Paucity or absence of type IIA fibers as m our case 4 is in contrast to the absence of type liB fibers observed by Argov et al. (1980) and could be explained either by delayed maturation or by degeneration. The role of a maternal environmental factor in the pathogenesls of CMyD has been postulated since the neonatal form only occurs in offspring of MyD mothers and not in the occasional cases of MyD in whom the gene was inherited from the father (Harper and Dyken 1972). Moreover, the number of affected children in the offspring of MyD mothers can be much higher than expected for genetic transmission (Dyken and Harper 1973). Our experimental results strongly support the role of a maternal serum factor, since they show a delay of muscle maturation in the rats injected with MyD serum when compared to the controls. This was charactensed by an increase in the variability coefficient of both fiber types and a greater surface area occupied by type I fibers in the 15-day-old treated animals than in the controls (the number and the extension of this type of fibers normally decreases after birth in the rectus femoris of rats). Numerous myotubes, satellite cells and fibers at different degrees of maturalaon within a single basal membrane were also seen by EM in treated rats. The maternal factor revolved in CMyD is probably related to the metabolic &sturbances described in MyD: hypercatabolism of IgG, low serum levels of D H A and DHAS by decreased adrenal production (Carter and Steinbeck 1986) and hyperinsulinemia (Huffet al. 1967) probably secondary to peripheral insulin resistance (Kobayashi et al. 1977). Although not proven, it could be postulated that CMyD mothers have high insulin levels and are relatively unable to satisfy the fetal needs for glucose and amino acids, which m turn could lead to impaired fetal muscle development and maturation. Silver et al. (1985) suggested that the children with CMyD may not have the MyD gene while the unaffected fetuses may have it: a relative insensittvity of the muscles to insulin in genetically affected fetuses could increase the fetal pancreatic secretion of insulin (main growth factor during the third trimester of pregnancy) and thus protect them from the congenital form of myotonic dystrophy.

REFERENCES Argov, Z, D Gardner-Medwm,M A Johnson and F L Mastagha (1980)Congemtalmyotomcdystrophy, A r c h N e u r o l , 37 693-696

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