Carrier detection in the Duchenne type muscular dystrophy: Preliminary observations on the place of electromyography

347

Journal of the neurological Sciences

Elsevier Publishing Company,Amsterdam- Printed in The Netherlands

Carrier Detection in the Duchenne Type Muscular Dystrophy: Preliminary Observations on the Place of Electromyography K. JACOBS* The Regional Neurological Centre, The General Hospital, Newcastle upon Tyne (Great Britain)

(Received 18 July, 1967)

INTRODUCTION Female relatives of patients with the Duchenne type of muscular dystrophy wish to know whether they are carriers of the gene responsible for this type of muscular dystrophy. In a few of them clinical examination will reveal signs suggestive of muscular dystrophy such as slight muscle weakness or pseudohypertrophy (EMERY 1963) but more conclusive findings are obtained by biochemical, histological and electrophysiological investigations. Studies of creatine and creatinine excretion in urine (LEYBURNe t al. 1961) and of serum aldolase (CHUNG et al. 1960; DREYFUSAND SCHAPmA1962) have been either negative or only occasionally abnormal in carriers. The results are more promising using the determination of serum creatine kinase; in about two-thirds of all known female carriers the activity of this enzyme in serum is consistently raised (HUGHES 1962; PEARCEet al. 1964; EMERY 1965). DREYFUSet al. (1961) using serum enzyme determinations and measurements of limb to limb circulation time detected 23 out of 26 carrier females. EMERY(1965) found that the level of creatine phosphokinase, the muscle lactate dehydrogenase isozyme pattern or the histology of muscle biopsy specimens was abnormal in 16 out of 23 carriers of the Duchenne type of muscular dystrophy. PEARCEet al. (1966) did biopsy studies on 8 known carriers and found some histological abnormalities in all their cases. They were striking in 4 cases and included atrophy, hypertrophy and abnormal variations in fibre size, focal necrosis and phagocytosis of muscle fibres, the formation of nuclear chains and abortive regenerative activity. In the other 4 cases the abnormalities observed, though definite, were slight. They suggested that diagnostic histological changes may be expected in a single biopsy specimen in about one-third of all female carriers and that less specific abnormalities are likely to be found in sections from the remainder. The first electromyographic (EMG) studies in carriers were done by VAN DEN BOSCH (1963) and BARWICK(1963). Since then, a number of studies giving rather controversial results have been published. This work has been carried out in order to * Present address: InstitutBunge,Berchem-Antwerp(Belgium). J. neurol. Sci. (1968) 6:347-356

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K. JACOBS

investigate the mean duration and amplitude of motor unit action potentials, and the proportion of polyphasic potentials, recorded electromyographically from carrier females. METHODS AND CLINICAL MATERIAL

Methods of determination of action potential duration and amplitude and of the incidence of polyphasie potentials In all of the patients, relatives and normal controls the brachial biceps muscle was examined during weak and maximal effort. The leading-off electrodes were concentric Medelec needle electrodes with an external diameter of 0.465 mm. The inner core was a platinum wire with a diameter of 0.12 m m and a leading-off area of 3 × 104/~. The impedance of the electrodes varied between 500 x 102 f2/10 ° and 100 × 104 [2/13 °, at a measuring frequency of 160 c/s. A Medelec MS3 amplifier was used. The action potentials were photographically recorded from a 10 cm cathode ray oscilloscope. The speed of the sweep was 100 cm/sec and the voltage 10/~V/mm. An isolated oscilloscope was used for visualisation and the channel was connected with a loudspeaker. The electrodes were randomly inserted several times into the brachial biceps muscle, perpendicular to the direction of the muscle fibres. Each needle was moved three times through a distance of at least 3 m m in order to assure that the potentials recorded were picked up from different motor units. In this way 10-20 different regions were examined, each at three different depths. Each potential was recorded several times and that potential was chosen for measurement which was least disturbed by activity from more distant motor units. For each individual 25-50 potentials were measured. The action potential duration was measured as the time interval between the initial deflection from the base line to the point where the terminal deflection returned to it. The mean action potential duration measured with different concentric needle electrodes of the same batch shows a variation of 20% due to some, as yet, unidentified physical variations in the characteristics of the electrodes (BuCHTHAL et al. 1954). Repeated recordings with the same electrode showed a variation of only 3%. In order to reduce this variation the same 2 electrodes were used during the whole investigation. Both electrodes had been used several times for the examination of the same muscle of the same subject and the values obtained showed a variation of less than 3% . The amplitude was measured as the difference between the greatest positive and the greatest negative deflection. The variation in the mean amplitude from subject to subject varied by 17.5% . The normal range was defined as twice this variation (35%). Polyphasic potentials were defined as potentials with more than 4 phases, and the number of phases was determined by the number of deflections across the base line. In normal individuals 3-5% of all motor unit action potentials are polyphasic (BUCHTHAL et al. 1954). The proportion of polyphasic potentials was considered to be abnormal when they were more than 10% of the total. In addition to these characteristics the pattern was recorded during maximal volitional contraction. It was classified according to its voltage (normal range 2-5 mV) and whether it was a full interference pattern, a mixed pattern or a pattern of J. neurol. Sci. (1968) 6:347-356

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349

single oscillations (BuCHTHAL 1957). Spontaneous activity at rest was recorded whenever it was observed. Clinical material The clinical material comprised 14 patients with progressive muscular dystrophy, 17 female relatives and 12 normal controls. All the patients and relatives were examined clinically and their muscles tested. Muscle power was graded according to the scale suggested by the MEDICAL RESEARCH COUNCIL (1943). The patients were classified clinically according to the criteria suggested by WALTONAND NATTRASS(1954). Patients with progressive muscular dystrophy. The 14 patients comprised 11 males and 3 females. Their age varied between 13 and 58 years. Six of them, all males, were suffering from the severe Duchenne type of progressive muscular dystrophy; 3 males and 3 females were suffering from the milder late onset Duchenne type of muscular dystrophy. The limb girdle type of muscular dystrophy was represented by 2 male patients. In most of the patients the diagnosis had been confirmed by biopsy studies. The serum creatine kinase level was determined in all but 3 cases. The results of the E M G studies are given in Table 1. Relatives of Duchenne type patients. The relatives were classified as 8 known carriers (women who have a dystrophic brother or maternal uncle and a dystrophic child, women with more than 1 dystrophic child or women with a dystrophic child who themselves had raised serum enzyme levels or abnormal muscle biopsy findings) and 9 possible carriers (women with 1 affected child and no other affected male relatives and with normal enzymes and normal biopsy) of the gene responsible for the severe Duchenne type of muscular dystrophy. Their ages varied between 18 and 64 years. Clinical examination revealed minimal signs suggestive of muscular dystrophy in 2 carriers: Case 1 presented with pseudohypertrophy of the calves and slight weakness of the biceps muscles and the quadriceps muscles; Case 7 showed pseudohypertrophy of the calves. The serum creatine kinase level was determined in all carriers (Table 2). Normal controls. Twelve normal subjects, all volunteers from the medical and technical staff of the hospital, served as normal controls. There were 4 males and 8 females. Their ages varied between 19 and 32 years. All were in good health and none of them was suffering from a neurogenic or myogenic disorder, nor had any of them a family history of myopathy. The mean action potential duration at the age of 20 years (5 subjects) was 8.46_0.18 msec; at the age of 25 years (3 subjects) 8.70 +__0.22 msec and at the age of 30 years (4 subjects) 9 +0.20 msec. Unfortunately, we did not have a sufficient number of adolescent or middle aged normal controls to be able to calculate the mean action potential duration and amplitude at every age. Using the findings of SACCO et al. (1962) who showed that the mean action potential duration in the brachial biceps muscle increased by 25% between the ages of 20 and 70 years, the mean duration was calculated at different ages by extrapolation. The mean amplitude of the motor unit action potentials was 212/zV; it does not change with age. RESULTS

Spontaneous activity at rest At rest the presence of fibrillations or positive sharp waves was noted in 1 patient J. neurol. Sci. (1968) 6:347-356

0 4

Limb girdle dystrophy 13 58 47 14 27 18

e a e f

2 4 5

F e m a ~ D u e h e n n e type 10 25 4 11 20 1 12 22 5 --+ +

++-r ----+

0

0 0

++

-r+-++ ++-+-++ ++~+÷

Atrophyb

0 0

0 F.PP 0

pp 0 0

0 0 PP,PsD 0 0 0

activity at rest e

Spontaneous

1 I

I I I

M I l

I I M M I I

3000 3000

4000 5000 6000

1500 1000 4000

750 600 400 400 800 1000

Amplitude on maximal maximal contraction contraction d (At IX)

Pattern on

G r a d e d according to the scale of the Medical R e s e a r c h Council (1943). + = minimal; ÷ + moderate; --÷÷ -- severe. F -- fibrillations; PP positive potentials: P s D = p s e u d o m y o t o n i c discharges. I = interference pattern, M - - mixed pattern (for definition see BUCHTHAL 1957). Average and s t a n d a r d error from at least 25 different m o t o r units. N o r m a l -- less t h a n 4 units.

5 5 5

Late onset Duchennetype 7 31 24 8 19 14 9 19 12

Power a

3 2 1 1 0 1

(years)

No.

Age at onset (years)

Severe Duchenne type 1 13 4 2 16 3 3 15 4 4 18 3 5 32 4 6 2O 4

Age

Case

6.40±0.39 7.40i0.39

6.56±0.36 7.70~-0.55 7.20i0.39

7.68±0.43 6.88i0.25 7.90±0.45

5.50±0.30 6.60±0.31 7i0.4t 6.30~:0.30 6~0.37 6.01~0.36

Mean AP duration (msee) e

--39 -- 11

-- 12 --9 --15

- - 15 19 --7

--31 --18 - - 13 --26 --33 --29

Decrease in mean A P duration (%)

167±18 1 8 4 ~ 19

208±25 114:=t=15 163~14

1 4 7 ± 18 1 8 6 ± 18 213±24

183±31 148±18 169±20 166±21 205±24 155±15

Mean AP amplitude in At V e

ELECTROMYOGRAPHIC FINDINGS IN BICEPS BRACHII IN PATIENTS W I T H PROGRESSIVE MUSCULAR DYSTROPHY

TABLE 1

19 23

32 33 23

41 38 27

28 32 21 46 20 36

(%)

Incidence o f polyphasic potentials

1.6 4.0

4.5 7 1.2

117 51 24

17

70

178

Serum creatine kinase r

O ©

7~

.~. .~

,-,.

Age (years)

41

8

a b e a e

0 0 0 0 0 0 0 0 0

Pseudohypertr, calves; slight weakness biceps 0 0 0 0 0 Pseudohypertr, calves 0

Clinical findings

0 0 0 0 0 0 0 0 0

0

0 0 0 0 0 0

PsD

Spontaneous activity at rest a

I I I I I I I I I

I

I I I I I I

I

maximal contraction b

Pattern on

PsD pseudomyotonic discharges. I = interference pattern. Average and standard error from at least 25 different motor units. Normal = less than 4 units. N = normal

Possible carriers 9 30 10 19 I1 39 12 63 13 40 14 18 15 20 16 44 17 35

64 56 42 37 32 38

2 3 4 5 6 7

Known carriers 1 39

Case No.

ELECTROMYOGRAPHIC

TABLE 2

4000 3500 5000 4000 4000 5000 5000 4000 4000

4000

4000 1000 4000 5000 4000 4000

1500

Amplitude on maximal contraction O, V)

8.834-0.35 8.214-0.31 9.404-0.36 9.904-0.53 9.204-0.46 8.104-0.36 8.404-0.33 8.904-0.44 9.104-0.44

8.844-0.33

7.654-0.19 7.894-0.37 9.604-0.37 8.404-0.41 8.706-0.39 9.284-0.76

6.124-0.22

duration ( msec) e

Mean A P

N N N N N N N N N

N

--27 --21 Ne N N N

--35

Decrease in mean duration (%)

FINDINGS IN BICEPS BRACHII IN CARRIERS

1784-13 226± 16 2184-18 289-k35 200_]_ 17 1704-13 1894-19 211+25 2574-22

2294-24

208-4-10 1864-19 207+23 199-4-17 1844-13 1834-16

1834-36

(vv)~

Mean A P amplitude

0 8 0 5 3 7 5 0 6

7

5 12 7 6 3 6

24

(%)

Incidence o f polypha~ic potentials

1.8 1.3 2.1 2.3 2.1 2 2.8 1.5 3.8

1.6 5.4 6 1.7 3.7 30

25

Serum creatine kinase a

L~

-]

r'n

Z Z

352

K. JACOBS

with the severe Duchenne type of muscular dystrophy, in 1 patient suffering from the late onset Duchenne type and in 1 female patient with early onset dystrophy. The same patient suffering from the severe Duchenne type showed also pseudomyotonic discharges at rest. In none of the known or possible carriers was there any spontaneous activity, except in Case 1 where pseudomyotonic discharges were present.

Maximal contraction Two patients with the severe Duchenne type and 1 with the late onset Duchenne type showed a mixed pattern on maximal volitional contraction. All the others presented with a full interference pattern. All showed at minimal contraction a pattern richer than the power of contraction would suggest. All the carriers showed a normal interference pattern. The amplitude at maximal contraction was reduced in all the patients suffering from the severe Duchenne type of muscular dystrophy and in 2 of the 3 patients with the late onset Duchenne type. It was normal in the patients of the female Duchenne type and the patients with limb girdle dystrophy, as it was also in all the carriers. Mean action potential duration and amplitude The mean action potential duration was significantly diminished in the patients with the severe Duchenne type of muscular dystrophy and in 6 of the 8 patients suffering from the milder forms of muscular dystrophy. Three of 8 known carriers showed a significant decrease in mean action potential duration; in the others the mean duration was decreased but within the normal range. The 9 possible carriers all had mean durations within the normal range, although in 6 of them the mean duration was below the mean. The mean amplitude of the motor units was decreased in all the dystrophic patients but still within the normal range except in 1 (Case 11) in whom there was a decrease of 46~o, well outside the normal range. The mean amplitude of the motor units of all the known and possible carriers was reduced but within the limits of normal variation. Incidence of polyphasic potentials The proportion of polyphasic potentials was significantly increased in all the patients; the increase varied from 19 to 46%. Of the known carriers 2 (Cases 1 and 3) showed an increase in the number of polyphasic potentials above the normal range; the occurrence of polyphasic potentials in all possible carriers was within normal limits. DISCUSSION The first reports of electrophysiological abnormalities in female carriers of the gene responsible for the severe Duchenne type muscular dystrophy were by VAN DEN BOSCH (1963) and BARWlCK (1963). BARWICK described a "myopathic pattern" in the electromyogram (EMG) of some carriers; he concluded that a very abnormal E M G obtained from several muscles was suggestive of the carrier state and that electromyography appeared to have an uncertain place in the identification of carriers. VA~ DEN BOSCH counted the number of phases/potential and divided this number into the

J. neurol. Sci. (1968) 6:347-356

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353

duration of the potential, thus giving the average duration of each phase. He then expressed his results in terms of the number of phases/sec which he referred to as ~ or the mean number of c/s. He concluded that carriers could probably be identified by his method. Using the same method DAVEY AND WOOLF (1965) concluded that the measurement of the ~ value of polyphasic potentials will only occasionally permit suspicion of the carrier state and even this occasional suspicion cannot be accepted without reserve. The absolute refractory period of muscle i.e. the shortest time interval at which both a conditioning and a testing stimulus evoked action potentials, was shorter than normal, as short as in patients, in 11 of 14 female relatives of 12 patients with the clinically severe type of Duchenne dystrophy (CARUSO AND BUCHTHAL 1965). The decrease occurred in all definite and probable carriers and in half of the possible carriers. The authors concluded that these differences indicate that the procedure is useful in identifying carriers. The mean duration and amplitude of the action potentials and the percentage of polyphasic potentials have been investigated by CARUSO AND BUCHTHAL(1965) and by DAVEY AND WOOLF (1965) who found differing results. CARUSOAND BUCHTHAL(1965) found that the average duration and amplitude of motor unit potentials was diminished and the incidence of polyphasic potentials was increased in these female relatives. Nevertheless, they could not be used to identify a given carrier because the values obtained, although showing a statistically significant deviation, were slight and fell within the limits of normal variation. DAVEY AND WOOLF (1965) found that the potentials recorded from the carriers were of slightly larger average duration than those obtained from controls. However, the mean amplitude was reduced and the percentage of polyphasic potentials was increased but the results were not statistically significant. The ages of their patients and of the normal controls were not given in the study so that it is difficult to discuss their results. In the present study no spontaneous activity at rest could be detected in any of the carriers except in 1 case where pseudomyotonic discharges were present. Although there was a decrease in the amplitude of the pattern obtained from 2 of the known carriers, it is nevertheless unlikely that the pattern at maximal contraction will be of any value in the detection of carriers; indeed, all the definite and possible carriers showed a normal interference pattern and on minimal contraction the pattern was not more profuse than the degree of contraction would suggest, as is the case in patients with dystrophy. The mean action potential duration was helpful in identifying 3 of the 8 known carriers; although in the other known and in the possible carriers the mean duration was usually slightly decreased below normal, this parameter could not be used to identify a given carrier with certainty, because in most of those tested the slight deviations from normal were still within the normal range. The average mean amplitude of the motor units recorded from the carriers was also decreased, but again this diminution of the mean amplitude was still within the normal range, although at the lower limit. The incidence of polyphasic potentials was outside the normal range in 2 of the 8 known and in none of the possible carriers, and was thus helpful in the identification of 2 known carriers. .L neurol. Sci. (1968) 6:347-356

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K. JACOBS

All the deviations from normal that could be detected in carriers occurred in 3 known carriers (Cases 1, 2 and 3) and they will now be considered in more detail. Case 1 was a female, aged 30 years, who showed on clinical examination pseudohypertrophy of the calves and some weakness of the brachial biceps muscles. In the E M G there were pseudomyotonic discharges at rest, a slight decrease in amplitude on maximal contraction, a significant decrease in mean duration of the motor units and a significant increase in the number of polyphasic potentials. EMERY (1963) found a reduction in mean duration and/of amplitude of action potentials in 2 female carriers, each of whom showed on examination clinical signs suggestive of muscular dystrophy. CARUSO AND BUCHTHAL(1965) could not find a significant decrease in mean duration and amplitude in any of the 43 relatives of patients with muscular dystrophy they examined. None of their cases presented with clinical signs suggestive of muscular dystrophy. Our Case 1 (Table 2) may well be a manifesting heterozygote. Following the LyON hypothesis (LYON 1961) concerning gene action on the X-chromosome, a carrier might be expected to have 2 populations of muscle fibres, one group normal and one dystrophic, and the presence of clinical manifestations of dystrophy would depend upon the proportion of dystrophic fibres. In this case it seems likely that the proportion of dystrophic fibres was greater than in the majority of carriers. The abnormalities found on E M G were probably explained by the fact that there was a large proportion of dystrophic fibres present in the muscle examined. Clearly, however, there are some other carriers in whom the proportion of affected muscle fibres is much less, and the E M G parameters we hayed used are much less helpful in their detection. Case 2, aged 64 years, showed no clinical abnormalities. In the E M G there was a significant decrease in mean action potential duration. Case 3, aged 56 years, presented without clinical abnormalities. The E M G showed a decrease in mean amplitude on maximal contraction, a significant decrease in mean action potential duration and a slight increase in the number of polyphasic potentials. The latter 2 carriers were both much older than the others we tested. DAVEY AND WOOLF (1965) found a marked increase in polyphasic potentials in 1 carrier, who was a middle aged woman. They suggested that in young carriers active degeneration of muscle fibres may still be proceeding slowly whereas at a later age most of the muscle fibres that are going to degenerate will have done so. This might explain why in younger carriers the E M G results are normal or show only a minor deviation from normal towards the "myopathic pattern", whereas in older patients, in whom the degeneration of the dystrophic fibres is far advanced or complete, a decrease in mean action potential duration is found. The importance of the decrease will depend upon the proportion of dystrophic fibres and consequently upon the proportion of fibres that have disappeared from the motor units. On the other hand, this finding could mean, if confirmed, that these E M G methods are likely to be of little value at the age when carrier detection is most important, that is in a young woman who wishes to know whether she should have children. CONCLUSIONS

Electromyographic measurements for the detection of carriers of muscular dystrophy ~ n e u r o l . Sci.

(1968) 6:347-356

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seem to be somewhat disappointing. The amplitude and characteristics of the interference pattern at maximal contraction are not different from results obtained in normal controls. The mean action potential duration was significantly diminished in 3 known carriers, of whom 1 showed some clinical evidence of muscular dystrophy (a manifesting carrier) and the 2 others were much older than most of the carriers we tested; in the 5 other definite carriers the mean action potential duration, although diminished, was still within normal limits. The average findings for mean amplitude were within the range of normal variation although there was a shift towards the lower limit of normal. Only in 2 of the definite carriers was there an increase in the percentage of polyphasic potentials. The abnormalities that have been found occurred in 3 of the 8 definite carriers and in none of the possible carriers. The 3 carriers presented either with minor clinical abnormalities (Case 1) suggesting that areasonably high proportion of the muscle fibres was dystrophic, or were of an older age group (Cases 2 and 3). Hence these E M G methods may not help in the detection of the carrier state in a given case, except in older individuals or in cases in which clinical examination will already suggest the carrier state. It is in just such individuals that serum creatine kinase activity is usually at its highest so that there is little likelihood that the E M G methods we have used will be successful in identifying the 30% or more of carriers in whom serum creatine kinase activity is normal. Serum enzyme studies still seem to be the most reliable method of identifying carriers. ACKNOWLEDGEMENTS This work was carried out during the tenure of a travelling fellowship in the Regional Neurological Centre, Newcastle upon Tyne, in 1965-1966. I am grateful to Dr. J. N. WALTONfor suggesting the investigation and for providing facilities. The work was aided by grants from the Muscular Dystrophy Association of America, Inc., and the Muscular Dystrophy Group of Great Britain. SUMMARY Electromyographic studies were done in 14 patients with progressive muscular dystrophy, and in 8 definite and 9 possible carriers of the gene responsible for the severe Duchenne type of muscular dystrophy. The electromyographic parameters which were examined, namely, mean duration and amplitude of motor unit action potentials and the percentage of polyphasic potentials, were outside the normal range in 3 of the 8 known carriers: viz. in 1 carrier with clinical findings suggestive of muscular dystrophy and in 2 older women. In the other known carriers and in the possible carriers the mean duration and amplitude, although diminished, were within normal limits as was also the percentage of polyphasic potentials. REFERENCES BARWICK,D. D. (1963) In: The Members of the Research Committee of the Muscular Dystrophy Group (Eds.), Research in Muscular Dystrophy, Pitman, London, pp. 10-19. BUCHTHAL,F. (1957)An Introduction to Electromyography, Munksgaard, Copenhagen. J. neurol. Sci.

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BUCHTHAL, F., C. GULD AND P. ROSENFALCK (I954) Action potential parameters in normal human muscle and their dependence on physical variables, Acta physiol, scand., 32 : 200-218. CARUSO, G. AND F. BUCHTHAL(1965) Refractory period of muscle and electromyographic findings in relatives of patients with muscular dystrophy, Brain, 88 : 29-50. CHUNG, C. S., N. E. MORTON AND n . A. PETERS (1960) Serum enzymes and genetic carriers in muscular dystrophy, Amer. J. hum. Genet., 12: 52-66. DAVEY, M. AND L. WOOLF (1965) An electromyographic study of carriers of muscular dystrophy, 6th int. Congr. LEG clin. Neurophysiol., Vienna, pp. 653-658. DREYFUS, J. C. AND G. SCHAPIRA (1962) Biochemistry of Hereditary Myopathies, Thomas, Springfield, Ill. DREYFUS, J. C., G. SCHAPIRA, F. SCHAPIRA AND J. DEMOS (1961) The detection of heterozygotes of myopathy, 2nd int. Conf. hum. Genet., Rome. DUaOWlTZ, V. (1963) Myopathic changes in a muscular dystrophy carrier, J. Neurol. Neurosurg. Psychiat., 26: 322-325. EMERY, A. E. H. (1963) Clinical manifestations in two carriers of Duchenne muscular dystrophy, Lancet, 1 : 1126-1128. EMERY, A. E. H. (1965) In: The Members of the Research Committee of the Muscular Dystrophy Group (Eds.), Research in Muscular Dystrophy, Second Series, Pitman, London, pp. 90-97. HUGHES, B. P. (1962) Serum enzymes in carriers of muscular dystrophy, Brit. med. J., 2 : 963-964. LEYBURN, P., W. H. S. THOMSONAND J. N. WALTON (1961) An investigation of the carrier state in the Duchenne type muscular dystrophy, Ann. hum. Genet., 25: 41-49. LYON, M. F. (1961) Gene action in the X-chromosome of the mouse, Nature (Lond.), 190: 372-373. MEDICAL RESEARCH COUNCIL (1943) Nerve Injuries Committee, M. R. C. War Memorandum. No. 7. PEARCE, G. W., J. M. S. PEARCE AND J. N. WALTON (1966) The Duchenne type muscular dystrophy: histopathological studies of the carrier state, Brain, 89 : 109-120. PEARCE, G. W., R. J. PENNINGTON AND J. N. WALTON (1964) Serum enzyme studies in muscle disease, Part 3 (Serum creatine kinase activity in relatives of patients with the Duchenne type of muscular dystrophy), J. Neurol. Neurosurg. Psychiat., 27: 181-185. SACCO, G., F. BUCHTHAL AND P. ROSENFALCK (1962) Motor unit potentials at different ages, Arch. Neurol. (Chic.), 6: 366-373. VAN DEN BOSCH, J. (1963) In: The Members of the Research Committee of the Muscular Dystrophy Group (Eds.), Research in Muscular Dystrophy, Pitman, London, pp. 23-30. WALTON, J. N. AND F. J. NATTRASS(1954) On the classification, natural history and treatment of the myopathies, Brain, 77:169 231.

J. neurol. Sci. (1968) 6:347-356