Download Report

Downloaded from jnnp.bmj.com on October 6, 2014 - Published by group.bmj.com
Letters to Editor
should be mentioned at the outset that the accuracy
of the applied method is limited' (Panayiotopoulos and
Scarpalezos, 1975).
Problem 2 The factors of error involved in the
motor axon counting technique would affect disproportionately the results in health, denervation, and
muscular dystrophy, thus precluding comparative
studies between them.
1. In denervation, although the 'alternating' phenomenon (McComas et al., 1971) is less troublesome
(Panayiotopoulos and Scarpalezos, 1975), the additive
summation of newly recruited MUP is less satisfactory than in control subjects (McComas et al.,
1971b; Panayiotopoulos and Scarpalezos, 1975). Moreover, ephaptic activation of degenerated motor axons
cannot be totally excluded.
2. In Duchenne muscular dystrophy the factors of
error contributing to an underestimation of the number of motor axons are much greater than in healthy
subjects and denervation (a detailed discussion can
be found in Panayiotopoulos (1976) as well as in
previous reports: Scarpalezos and Panayiotopoulos,
1973; Panayiotopoulos et al., 1974). It would be also
impossible with the 'new methods' to determine 'the
firing levels' of 'small MU (which) could make an
undetectable contribution to the surface EMG'
(Brown et al., 1976). One might ask why we applied
the method in neuromuscular diseases despite our
awareness of the above several problems. Our purposes were (a) to point out the fundamental difficulties
of the original technique, results from which were
considered the strongest evidence to support the
neurogenic hypothesis, and (b) to show that by
approximately equalising the factors of error between health and muscular dystrophy there is in
fact no loss of motor axons in muscular dystrophy
and that 'the neurogenic hypothesis is based on poor
and vulnerable evidence' (Panayiotopoulos and
Scarpalezos, 1975; Panayiotopoulos et al., 1976).
McComas has recently participated in an experimental 'study which provides direct evidence against
a neuropathogenesis for murine dystrophy' (Law
et al., 1976).
In the results of the 'new methods' (Milner-Brown
and Brown, 1976) two out of four cases of muscular
dvstrophy have numbers of motor units below the
lowest normal value. In view of the recent work (Law
et al., 1976) the above findings may be an indirect
evidence that the 'new methods' do not equalise
the factors of error between health and muscular
dystrophy.
In conclusion, we believe that the above main and
other problems of the motor axon counting technology should be vigorously discussed and tested if
'new methods' and 'new results' are to be presented
in the future.
C. P. PANAYIOTOPOULOS
Eginition Hospital,
74, Vasilissis Sophioy A v.,
A thens, Greece.
929
SIR.-Our intention in the papers (Brown and MilnerBrown, 1976; Milner-Brown and Brown, 1976) has
been to highlight errors in the method originally
proposed to estimate motor unit numbers (McComas
et al., 1971a), and report modifications that can correct for one of the most important errors: the
fluctuation in the electrical excitability and overlap
in the firing levels of motor axons. The modifications
are part of a continuing programme to develop a
method, acceptable to patients, and capable of providing accurate quantitative estimates of the number
of motor units (MU) in muscles.
Differences in the latencies from the stimulus
shock to maximum voltage(s) in MU can partially,
indeed completely, hide the voltage contribution of
MU to the incremental sum, particularly if the range
in latencies of the single MU is large. Precisely for
that reason we have stated that MU isolated using
isometric contraction, F discharge, or multiple point
stimulation cannot be used directly to calculate a
mean MU potential voltage (Feasby and Brown, 1974;
Milner-Brown and Brown, 1976). These methods
were, however, used to isolate single MU in order to
indicate the presence of large MU in the muscle and
to emphasize how unrepresentative the sample of MU
used in calculating the mean motor unit potential
(MMUP) are.
In 'problem 1', Dr Panayiotopoulos states that 'The
most essential requirement is that the MUP be
summated in a precisely additive manner'. The lack
of temporal coincidence of the potential maxima of
individual MU is evident, and has been quantitatively
illustrated by comparing the mean amplitudes of
MUP obtained directly from incremental responses
with MUP isolated using graphical subtraction
(Panayiotopoulos et al., 1974), or computer methods
(Ballantyne and Hansen, 1974; Brown et al., unpublished). However, in the original method
(McComas et al., 1971a) and our modified methods
(Milner-Brown and Brown, 1976), the probable
'error', due to the lack of temporal coincidence of
potential maxima of individual MU, is inherent in
both the summation of the compound potential (CP)
of the first 10 MU used in calculating the MMUP
as well as the maximum CP. As there is no obvious
reason to suggest that the algebraic summations are
not the same in both CPs, the probable 'errors' should
cancel out when the maximum CP is divided by the
MMUP. Therefore, the relative importance of the
pattern of summation in the overall MUP seems to
have been overemphasised by Dr. Panayiotopoulos.
Panayiotopoulos and colleagues seemed to imply
that their modified method was an improvement on
the original method, because in their method 'an
effort was made to minimise the possibility of missing
small amplitude MUAP' (Panayiotopoulos et al.,
1974; Panayiotopoulos and Scarpalezos, 1975). In a
recent study we showed that motor units are excited
in order from small to large by graded increases in
the stimulus intensity (Kadrie et al., 1976). Therefore,
the case for exclusion of the very small MU by the
Downloaded from jnnp.bmj.com on October 6, 2014 - Published by group.bmj.com
Letters to Editor
930
original method has been overstated, the methods
and observations on which it has been based being
incorrect. The recognition cf the voltage contribution of single MU to the surface record depends on
the potential steps occurring in an all-or-nothing
manner, the 'all' responses being clearly reproduceable in configuration and beyond the possible errors
introduced by noise, stimulus artefact, and the inclusion of out of order summation 'alternation'
potentials. In our opinion, none of the above conditions is met by the methods reported by
Panayiotopoulos et al. (1974).
In conclusion, we would like to add that, even
though no method for estimating MU numbers has
been found to date that is entirely acceptable, the
two modified methods incorporating an important
correction should help to make the method of estimating MU numbers more accurate. However, it is
premature in our opinion to hypothesise on the
aetiology of neuromuscular disorders based on data
obtained frcm motor unit estimates alone.
WILLIAM F. BROWN
H. S. MILNER-BROWN
Department of Clinical Neurological Sciences,
Division of Neurology,
University Hospital,
London, Ontario.
REFERENCES
Ballantyne, J. P., and Hansen, S. (1974). New method
for the estimation of the number of motor units in
a muscle. 1. Control subjects and patients with
myasthenia gravis. Journal of Neurology, Neurosurgery, and Psychiatry, 37, 907-915.
Ballantyne, J. P., and Hansen, S. (1975). Computer
method for the analvsis of evoked motor unit
potentials. 2. Duchenne, limb-girdle, facioscapulohumeral and myotonic muscular dystrophies. Journal of Neurology. Neurosurgery, and Psychiatry. 38,
417-428.
Brown, W. F., and Milner-Brown, H. S. (1976). Some
electrical properties of motor units and their effects
on the methods of estimating motor unit numbers.
Journal of Neurology, Neurosurgery, and Psychiatry, 39, 249-257.
Brown, W. F., Milner-Brown, H. S.. and Drake, J.
(1976). Sources of error in methods for estimating
motor unit numbers. In Recent Advances in Myology (Proceedings of the IlIrd International Congress on Muscle Diseases, Newcastle upon Tyne,
September, 1974), pp. 108-115. Edited by W. G.
Bradley, D. Gardner-Medwin, J. N. Walton. Excerpta Medica American Elsevier.
Feasby, T. E.. and Brown, W. F. (1974). Variation of
motor unit size in the human extensor digitorum
brevis and thenar muscles. Journal of Neurology,
Neurosurgery, and Psychiatry, 37, 916-926.
Kadrie, H. A., Yates. S. K., Milner-Brown, H. S., and
Brown, W. F. (1976). Multiple point electrical
stimulation of ulnar and median nerves. 1. Rank
order of recruitment of motor units. 2. Axonal
branching. Journal of Neurology, Neurosurgery,
and Psychiatry. (In press).
Law, P. K., Cosmos. E. Butler, J., and McComas.
A. J. (1976). Absence of dystrophic characteristics
in normal muscles successfully cross-reinervated by
nerves of dystrophic genotype: physiological and
cytochemical study of crossed solei of normal and
dystrophic parabiotic mice. Experimental Neurology, 51, 1-21.
McComas, A. J., Fawcett, R. R. W.. Campbell, M. J.,
and Sica, R. E. P. (1971a). Electrophysiological
estimation of the number of motor units within a
human muscle. Journal of Neurology, Neurosurgery, and Psychiatry, 34, 121-131.
McComas, A. J., Sica, R. E. P., and Campbell, M. J.
(1971b). 'Sick' motoneurones. A unifying concept
of muscle disease. Lancet, 1, 321-325.
Milner-Brown, H. S., and Brown, W. F. (1976). New
methods of estimating the number of motor units
in a muscle. Journal of Neurology. Neurosurgery,
and Psych.atry, 39, 258-265.
Panayiotopoulos. C. P. (1976). Workshop on the
aetiology of Duchenne Muscular Dystrophy. In
Recent A dvances in Myology (Proceedings of the
IlIrd Interniational Congress on Muscle Diseases,
Newcastle upon Tyne, Scptember, 1974), pp. 159162. Edited by W. G. Bradley, D. Gardner-Medwin,
J. N. Walton. Excerpta Medica American Elsevier.
Panayiotopoulos, C. P., and Scarpalezos, S. (1975).
Electrophysiological estimation of motor units in
limb-girdle muscular dystrophy and chronic spinal
muscular atrophy. Journal of Neurological Sciences,
24, 95-107.
Panayiotopoulos, C. P., Scarpalezos, S. and Papapetropoulos. Th. (1974). Electrophysiological estimation of motor units in Duchenne muscular
dystrophy. Journal of N'eurological Sciences. 23,
89-98.
Panayiotopoulos, C. P., Scarpalezos, S., and Papapetropoulos, Th. (1976). Electrophysiological estimation of motor units in neuromuscular diseases.
In Recent Advances in Myology (Proceedings of the
IlIrd International Congress on Muscle Diseases.
Newcastle upon Tyne, September, 1974), pp. 91-98.
Edited by W. G. Bradley, D. Gardner-Medwin,
J. N. Walton. Excerpta Medica: Amsterdam.
Scarpalezos, S., and Panayiotopoulos. C. P. (1973).
Duchenne muscular dystrophy: reservations to the
neurogenic hypothesis, Lancet. 2, 458.
Downloaded from jnnp.bmj.com on October 6, 2014 - Published by group.bmj.com
Letters
William F. Brown and H. S. Milner-Brown
J Neurol Neurosurg Psychiatry 1976 39: 929-930
doi: 10.1136/jnnp.39.9.929
Updated information and services can be found at:
http://jnnp.bmj.com/content/39/9/929.citation
These include:
Email alerting
service
Receive free email alerts when new articles cite this
article. Sign up in the box at the top right corner of
the online article.
Notes
To request permissions go to:
http://group.bmj.com/group/rights-licensing/permissions
To order reprints go to:
http://journals.bmj.com/cgi/reprintform
To subscribe to BMJ go to:
http://group.bmj.com/subscribe/