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Electrophysiological and clinical correlation in myasthenia gravis.

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Electrophysiological and Clinical
Correlation in Myasthenia Gravis
Shin J. Oh, MD,' Nasrollah Eslami, M D , t Takeo Nishihira, M D , t
P. K. Sarala, MD,I Tadashi Kuba, MD,? Robert S. Elmore, MD,?
I1 Nam Sunwoo, M D , t and Young I1 Ro, M D t
Repetitive nerve stimulation testing of the ulnar nerve was systematically performed on 21 normal controls and 120
patients with myasthenia gravis (MG). Diagnostic sensitivity increased from 0% in MG in remission and 17.2% in
ocular MG to 100% in severe generalized MG. Six types of responses were observed in MG and could be classified
into two distinct patterns based upon disease severity: ( 1 ) in mild MG, an abnormal decremental response at low
rate of stimulation, normal response at high rate of stimulation, and prominent posttetanic facilitation and exhaustion phenomena; and (2) in severe MG, abnormal decremental responses at low as well as high stimulation rates and
less common posttetanic facilitation and rare posttetanic exhaustion phenomena. This difference is most likely due
to the severity of the neuromuscular block in MG.
O h SJ, Eslami N , Nishihira T, Sarala PK, Kuba T, Elmore RS, Sunwoo I N , Ro YI: Electrophpsiological and
clinical correlation in myasthenia gravis. Ann Neurol 12:348-354, 1082
In 1941 Harvey and Masland [ 1I ] described a repetitive nerve stimulation test of the ulnar nerve to
study the response in cases of myasthenia gravis
(MG). Since then, this test has been widely used as a
diagnostic tool in MG and other related diseases.
However, few reports have studied the relationship
between the repetitive nerve stimulation test and
severity of MG [14, 281. We report such a study
here.
Methods and Materials
For study of neuromuscular transmission we used the
Harvey-Masland method with the surface-recording electrode o n the abductor digiti quinti muscle and the surfacestimulating electrode o n the transsulcal segment of the
ulnar nerve [ 111. For the recording electrode, the active
electrode was placed over the belly of the muscle and the
reference electrode over the tendon. For stimulation of the
nerve, we used the supramaximal 0.2 msec stimulus duration.
Each subject lay on a bed with the forearm and hand
fixed o n a stand with a heavy base (281. The nerve was
stimulated at 3isec for 2 seconds, at 5isec for 1 sec-ond,and
at 50/sec for I second, using DISA 14 and 1500 EMG
machines. There was at least a I-minute interval between
each test. Immediately and 4 minutes after tetanic stimulation at 50isec, the nerve was stimulated at S/sec for 1 second each time.
From the tDepartment of Neurology, T h e Medical Center o f the
University of Alabama in Birmingham and 'Veterans Administration Hospiral, Birmingham, AL.
T h e peak-to-peak amplitude of each muscle potential
was measured. T h e percentage of decrement or increment
was calculated by comparing the first response with the
lowest or highest among the first five responses at the low
rate of stimulation and during t h e first second at the high
rate of stimulation. When the results differed by 2
standard deviations from the mean in controls, they were
considered abnormal.
Posttetanic facilitation and exhaustion phenomena were
defined as having occurred when the decremental response
at Sisec immediately after and 4 minutes after tetanic
stimulation showed improvement or aggravation, respectively, compared with the response at 5isec prior to tetanic
stimulation.
T h e present analysis is based on 2 1 normal controls a d
120 patients with MG. M G was diagnosed by a combination of clinical examination and consistent reversal of signs
and symptoms upon parenteral administration of edrophonium or neostigmine. Among 103 patients with
symptomatic MG, the test was performed in 79 before any
medication was started (no-anticholinesterase group), in
18 after anticholinesterases had been discontinued for at
least 12 hours (anticholinesterase-off group), and in 6 patients with myasthenic crisis within 6 hours after administration of anticholinesterase medication.
T h e severity of M G was graded according to the
classification recommended by the Medical Advisory
Board of the Myasthenia Gravis Foundation 1201: I, ocular
MG; IIA, mild generalized MG; IIB, moderate generalized
Received Aug 13, 1981, and in revised form Feb 10, 1982. Accepted for publication Feb 13, 1982.
Address reprint requests to D r Oh, Department of Neurology,
University of Alabama in Birmingham, Univrrsity Station, Birmingham, AL 35294.
348 0364 5 114/82/100348-07$01.25 @ 1982 by the American Neurological Association
Table 1 . Data from the Repetitive Nerve Stimuhtion Test on the Abductor Digiti Quinti in 21 Normal Controls
Determination
Normal Limit
Amplitude of evoked muscle potential
Amplitude of evoked muscle potential
after 30 seconds of exercise
Response at the low rate of stimulation
2lsec
3Isec
5lsec
Response at the high rate of stimulation
( 5 O/sec)
Posttetanic stimulation
5isec immediately after tetanic
stimulation
5isec 4 minutes after tetanic
stimulation
4,821 pV
+36.80%,”
“Mean + 2 SD.
”Mean -+ 2 SD. In all others, normal limit
=
-6.8096
-6.75%
-4.93%
-17.6 to +42.43(,?6”
- 10.04%
-7.71%
mean
-
2 SD.
+ = increase or incremental response; - = decremental response.
Table 2. Diagnostic Sensitivity Acrording t o the Severity of Myasthenia Gravis
Generalized MG
Ocular M G
(I)
Classification
IIA
IIB
IIC
Subtotal
Normal response
Abnormal response
9
23 (71.9%)
2
23 (92.0%)
0
17 (100%i)
11
63 (85.1%)
24
32
25
17
74
29
Total remonses
MG; and IIC, severe generalized MG. When patients were
symptom free without any medication, M G was termed “in
remission.” When patients were symptom free with medications, M G was termed “asymptomatic.”
Results
Table 1 summarizes the findings among norm‘‘11 controls. We encountered no technical difficulty with
our methods in the normal subjects. They tolerated
the test well, including the most painful 5O/sec
stimulation test for 1 second.
Diagnostic sensitivity increased with the severity
of MG (Table 2). All 7 patients with M G in remission
had a normal response. An abnormal decremental response was noted in 17.2% ( 5 patients) of those with
ocular M G and in 85.1% (63 patients) of those with
generalized MG. Among 10 asymptomatic M G patients, l showed the posttetanic exhaustion phenomenon.
Six types of responses were noted in M G (Table
3). The most frequent was characterized solely by an
abnormal decremental response at a low rate of
stimulation. This response, termed type 1, was observed in 48.5% of the abnormal responses. Type 2,
5 (17.2%)
Total
35
68 (66.0%)
103
abnormal decremental responses at both low and
high stimulation rates, were observed in 23.5% of
the abnormal responses, mostly in IIB and IIC MG.
In 5 patients an abnormal decremental response at
high stimulation rate was the sole abnormality (type
3). In 6 patients an abnormal decremental response at
a low rate of stimulation was observed 4 minutes
after tetanic stimulation (type 4). A type 5 response,
low amplitude and abnormal decremental responses
at low and high stimulation rates, was observed in 5
patients, all with IIB or IIC MG. A type 6 response,
which is characteristic of the Eaton-Lambert syndrome, was observed in 3 patients. Persistent type 6
response was observed in 1 patient with ocular M G
[21], and in 2 patients with IIB M G a type 6 response
was observed transiently in the late afternoon. Thus,
various combinations of responses were noted in
MG, the decremental responses at a low rate of
stimulation (type 1) being most common in mild generalized M G (IIA), while decremental responses at
both low and high stimulation rates (types 2 and 5)
were most common in severe generalized MG (IIC).
Posttetanic facilitation was observed with 87.0%
of the abnormal responses and posttetanic exhaus-
Oh et al: EMG and Clinical Findings in MG
349
Table 3 . Frequency of the Various ResponseJ According t o Severity of Myasthenia Gra6'is
Amplitude
of Muscle
Potential
Classification
Posttetanic
Exhaustion
Generalized M G
Low Rate
Stimulation
High Rate
Stimulation
IIA
IIB
IIC
1
N
15
6
1
1
1
1
9
10
7
1
1
2
2"
2
32
25
Subtotal
Ocular
MG
(1)
Total
~
N
N
N
N
L
L
N
N
N
N
4
1
1
1
t
N
N
1
3
0
0
0
0
31
16
5
4
5
2
11
17
74
8
0
0
3
2
0
0
33
16
5
2
6
0
1
24
5
3
35
29
103
"One patient did not have a decremental response at low rate of stimulation.
N
=
normal; L
=
low amplitude;
1 = decremental response; t
= incremental response
Table 4 . Frequency of Posttetanic Facilitation and Exhamtion Phenomena Among Patients
with Abnormal Decremental Responses at 5 lsec Stinidation
Generalized MG
Response
IIA
IIB
IIC
Ocular MG (I)
Posttetanic facilitation
93.3%
56 3%
16
93.3%
66.7%
16.6%
100%
87.0%
25.0%
100%
37.0%
16
12
Posttetanic exhaustion
N o . of abnormal responses
18
- 40
~
I7 -
- 30
I6 -
15-
- 20
I4 -
13-
3.-
c
12 -
AS
II-
R
N
a, 10-
I
9-
-10
5
2
-0 d
0
c
IIA
0,
c
--I0
8-
U6
7-
- -20
6-
$
6
i
n
5-
- -30%
After Tetonic
Stimulation
F i g 1 . Mean responses on the repetitive nerve stimulation test
according to severity of myasthenia gravzs (MG). As =
asymptomatii MG IN = 20); R = M G in remission f N = 7);
N = normal controls iN = 21); bars repyesent normal range
(mean 2 2 SD); I = ocular MG (N = 29); IIA = mild generalized M G iN = 32); IIB = moderately severe generalized
M G (N = 25); IIC = severe generalized M G (N= 17).
350 Annals of Neurology
Vol 12 No 4
O c t o b e r 1982
2
Total
46
tion phenomenon with 37.0% (Table 4 ) . Both
phenomena were most commonly seen with mild
M G (I and IIA) and least often with severe M G (IIC).
Quantitative analysis of the response to repetitive
nerve stimulation showed good correlation between
the test responses and clinical severity of disease
(Figs 1-4). Normal responses were noted in ocular
MG, in M G in remission, and in the asymptomatic
M G group. In generalized M G the amplitude of the
muscle potential was within normal limits in all subgroups. At low stimulation rates a statistically
significant decremental response was noted in IIB
and IIC MG, the decremental responses being more
prominent in the more severe cases of generalized
MG. At high stimulation rates a definite decremental
response was noted in patients with IIC MG. Posttetanic facilitation and exhaustion phenomena were
observed in IIA and IIB MG. In IIC MG, however,
there was minor posttetanic facilitation but no posttetanic exhaustion.
We compared the various data between the noanticholinesterase group and anticholinesterase-off
group. No difference was noted in diagnostic sensitivity (72% in the off group versus 61% in the noanticholinesterase group), in frequency of the various
types of responses, or in posttetanic facilitation or
exhaustion phenomena. In contrast, in 23 patients
with IIA M G in whom the test was performed within
4 hours of removing anticholinesterases, only 1
2000~Vl
lsec
2000pv
1
I sec
F i g 2. Responses in mild generalized myasthenia gravis (IIA):
(A) normal amplitude (10,000 p V ) of muscle potential upon
single supramaximal stimulation; ( B ) 10 9% increment i n
amplitude of muscle potential after 30 seconds of exercise (this
may be due t o better positioning of the active electrode); (C)
13.5 % decremental response at 3lsec stimulation; (0)12.0 %
decremental response at 5 lsec stimulation; ( E ) normal (8 9% decremental) response at 50lsec stimulation and posttetanic
facilitation phenomenon; ( F ) posttetanic exhaustion phenomenon: 21 9% decremental response at 5 lsec stimulation 4 minutes
after 50lsec tetanic stimulation.
showed the decremental response. Quantitative analysis of the responses in this group showed a curve
similar to that of the asymptomatic M G group (see
Fig 1).These findings indicate that anticholinesterase
drugs can modify the results of the repetitive nerve
stimulation test and that a 12-hour anticholinesterase-off period is essential in obtaining accurate responses to the test in mild MG.
In 6 patients with M G crisis who were taking anticholinesterase medications, the drugs did not modify the diagnostic sensitivity or other variables. However, posttetanic exhaustion in IIC M G occurred
only in this group.
F i g 3 . Responses i n moderate generalized myasthenia gravis
(IIB): (A) normal amplitude (8,000 p V ) of muscle potential;
(B) 5 % increment in amplitude of muscle potential after 30
seconds of exercise; iC) 26 % decremental response at 3 lsec
stimulation; (0)26 % decremental response at 5 lsec stimulation; ( E ) 7 % decremental response at 50lsec stimulation and
posttetanic facilitation phenomenon ( 10 % decremental response); ( F ) 26 % decremental response at 5 lsec stimulation 4
minutes after 50lsec stimulation.
Discussion
In recent years, many new electrophysiological diagnostic tests for M G have been introduced [29]. These
include repetitive nerve stimulation tests on the deltoid and orbicularis muscles [23, 241 and on the abductor digiti quinti under regional curarization [ 13,
141, the double step test [9], and single-fiber electromyography [301. Even though these tests increase
diagnostic sensitivity, the repetitive nerve stimulation test on the abductor digiti quinti muscle (the
Harvey-Masland method) remains the most commonly used basic test in MG.
T o obtain a technically adequate response on the
repetitive nerve stimulation test, a few precautions
should be observed. The most troublesome source of
error is the artifact produced by movement of the
recording electrode. This error is best minimized by
fixing the forearm and hand on a stand with a heavy
base and using disc surface electrodes. The artifact is
Oh et al: EMG and Clinical Findings in MG
351
A
B
5OOOYV[
I sac
F i g 4. Responses in severe generalized myasthenia gravis (IIC):
(A) normal amplitude (1 1,000 p V ) of muscle potential; (8)
50% decremental response at 3 isec stimulation; (C) 50% decremental response at 5 isec stimulation; (0)54 % decremental
response at 50lsec stimulation; ( E ) 43 % decremental response
at 5 isec stimulation immediately after 50isec stimulation; ( F )
50 % decremental response at 5 isec stimulation 4 minutes after
50lsec stimulation.
easily detected by a sudden change in amplitude and
shape of the muscle potential. The genuine response
is characterized by a gradual decremental or incremental change in the muscle potential.
We used the peak-to-peak amplitude of the muscle
potential because it is easier to measure than the
baseline-to-negative peak amplitude. Our analysis of
the accuracy of the two methods in calculating the
response change at low stimulation rates in 2 1 normal
subjects did not show any significant difference ( t =
0.172).
The responses in the normal controls in our series
are comparable to those obtained by Botelho and associates [ l ] and Slomic et a1 [28], who presented
statistical data among normal controls. A decrement
of 7.0% at low stimulation rates in our controls lies
between the decrements of 59% reported by Botelho
et a1 [ 11 and 8% found by Slomic et a1 [281. Without
presenting the statistical data, many have used 8 to
10% decrement as the lower limit of normal response at low stimulation rates [8, 12, 14, 16, 19, 23,
24, 271. At the high rate of stimulation, we calculated
352 Annals of Neurology Vol 12 N o 4 October 1982
the increment or decrement by comparing the first
response with the lowest or highest response in the
first second. We observed a wider range of response
at high stimulation rates: 17.6% decremental to
42.4% incremental responses. Botelho et a1 [l] reported a 16.0% decremental response at 25/sec
stimulation for 0.25 second, and Slomic e t a1 [28],
47% decremental response at 50/sec stimulation for
1.5 seconds. This indicates that the use of 8 to 10%
decrement as the lower limit of normal at high
stimulation rates is not acceptable. The upper limit of
normal response at high stimulation rates is also critical because the abnormal incremental response at
those rates is used as a diagnostic index in EatonLambert syndrome and botulism 117, 221. Based o n
the upper limit for incremental response found in our
series, an incremental response greater than 42.4% is
abnormal.
To test posttetanic facilitation and exhaustion
phenomena, we chose one 5/sec stimulation immediately after and again 4 minutes after tetanic
stimulation, since Desmedt [4] reported that
maximum exhaustion usually occurred 4 minutes
after exercise in M G patients. We preferred tetanic
stimulation to exercise because tetanic stimulation is
better standardized.
Regarding the diagnostic sensitivity of the repetitive nerve stimulation test, a clear distinction should
be made between generalized M G and ocular MG.
This is because of the widely varying diagnostic sensitivity between the two types. In generalized MG,
85.1% of the tests showed an abnormal decremental
response typical of M G in our series. This is comparable to decremental responses of 92% reported by
Slomic et a1 I281 and 87% found by Botelho and
colleagues [ l ] , but higher than Horowitz et al's 72%
[14]. In contrast, in ocular MG, 17.2% of the tests
showed an abnormal decremental response in our series. This is comparable to the 14% reported by
Horowitz et a1 [ 141. Thus, in the diagnosis of ocular
MG, the repetitive nerve stimulation test o n the abductor digiti quinti muscle is least helpful. Our study
showed that diagnostic sensitivity increased with severity of disease. The incidence of normal response
progressively declined for ocular M G (82.8%!),
through mild (28.1%), and moderate (8.0%), to severe M G (0%).A similar experience was reported by
Horowitz and associates 1141 and by Slomic et a1 1281.
The variability of responses to the repetitive nerve
stimulation test in M G has not been emphasized in
the literature [19, 271. In our study the most common response was type 1, most prominent in IIA and
IIB MG. Type 2 and 5 responses were predominantly observed in IIC MG, indicating that the decremental response at high rates of stimulation occurs additionally in severe MG. Type 3 response,
which does not occur in M G according to Desmedt
[7], was found in our series and has also been observed by Mayer et a1 [ 191. The most unusual finding
was the type 6 response, which is thought to be diagnostic of Eaton-Lambert syndrome. Schwartz and
Stalberg [26] reported similar electrophysiological
findings in a case of MG, and electrophysiological
abnormalities similar to but not diagnostic of EatonLambert syndrome have been reported in several
cases of MG [2, 18, 21, 251. Thus, it is clear that the
incremental response at high stimulation rates may
occur in some cases of MG, and therefore the incremental response at high stimulation does not per se
establish a diagnosis of Eaton-Lambert syndrome.
However, certain quantitative differences seem to
exist between M G and Eaton-Lambert syndrome: in
MG, the amplitude of the muscle potential is usually
normal and the incremental response at high rates of
stimulation is usually smaller (less than 150%). Also,
as Table 3 shows, the best diagnostic yield for revealing the abnormal decremental response in MG is
achieved with a low stimulation rate, as the abnormal
response was revealed in 86.4% at low stimulation.
This observation agrees with previous reports [6, 8,
14, 23, 281.
Posttetanic facilitation and exhaustion phenomena
were described by Desmedt in 1957 [4] and a similar
effect was observed by various workers thereafter
[14, 281. Posttetanic facilitation was seen in 87.0%
and posttetanic exhaustion in 37.0% of our patients
with MG who showed an abnormal decremental response at 5/sec stimulation. Desmedt [51 recorded
posttetanic exhaustion in all clinically weak myasthenic muscles and believed its prominence to be “a
direct function of the amount of clinical involvement
of the tested muscles.” It did not occur in uninvolved
muscles of his patients with localized MG. In contrast, Slomic et a1 [28] observed the posttetanic
exhaustion phenomenon in 50% of their M G patients, regardless of clinical weakness of the abductor
pollicis muscle. Contrary to Desmedt [5], we have
observed posttetanic exhaustion in the abductor digiti quinti muscle in 4 patients with ocular M G and,
less frequently, in severe M G (IIC).
Ozdemir and Young [24] stated that the repetitive
nerve stimulation test is of little value in evaluating
the therapeutic response. This is contrary to our observation of a good correlation between electrophysiological and clinical assessment of disease
severity. As expected, a normal response was noted
in ocular MG, in MG in remission, and in
asymptomatic MG. With increasing severity of MG,
decremental responses became more marked at both
low and high stimulation rates. One exception to this
rule has been that posttetanic facilitation and exhaustion phenomena were prominent in milder MG.
O u r study documented two distinct responses to
the repetitive nerve stimulation test in patients with
MG: (1) in mild MG, an abnormal decremental response at low stimulation, normal response at high
stimulation, and prominent posttetanic facilitation
and exhaustion phefiomena; and (2) in severe MG, an
abnormal decremental response at low as well as high
rates of stimulation with less common posttetanic
facilitation and rare posttetanic exhaustion phenomena.
To account for these different responses between
patients with mild and those with severe MG, we
offer the following explanations. Under normal conditions, about 100 or 200 quanta of acetylcholine are
released in response to a single nerve impulse [3, 71.
During stimulation at low rates (less than 5/sec), presynaptic stores of readily available quanta of acetylcholine are depleted quickly and the number of
quanta released declines, resulting in a gradual decline in amplitude of the end-plate potential. However, under normal conditions, this decline in acetylcholine release does not produce a decremental
response at low stimulation rates in the repetitive nerve
stimulation test since the end-plate potential is always
above the threshold-the
safety factor. During
stimulation at high rates (more than 5/sec), mobilization of acetylcholine is enhanced and stores of readily
available acetylcholine are increased, with subsequent increase in acetylcholine release. This produces a higher amplitude of the end-plate potential,
resulting in an incremental response and posttetanic
facilitation lasting up to 2 minutes. This facilitation is
followed by postactivation exhaustion lasting up to
20 minutes, probably due to receptor desensitization
C291.
In MG, the decline in release of acetylcholine
during low-rate stimulation becomes important when
the potential produced by a single quantum is less
than normal, as expressed by the low amplitude
of miniature end-plate potentials [lo, 151. Low
amplitude of miniature end-plate potentials is
thought to be due to a reduced number of functioning acetylcholine receptors [ 101. Because of this diminished safety factor in MG, the end-plate potential
amplitude falls below threshold during low-rate
stimulation and some muscles fail to contract, producing the decremental response characteristic of
MG [7]. In mild MG, the normal mechanism of
increase in acetylcholine release during high-rate
stimulation can compensate for the minimally diminished safety factor, producing a normal response
at high stimulation rates and subsequent posttetanic
facilitation and exhaustion phenomena. In severe
MG, however, the neuromuscular block is so severe
that the normal mechanism of increased acetylcholine release during high-rate stimulation cannot
O h et al: EMG and Clinical Findings in MG
353
compensate for the marked!y diminished safety factor, thus producing a decremental response at high
rates of stimulation, less common postactivation
facilitation, and subsequent lack of posttetanic
exhaustion phenomena.
Supported by grants from the Alabama Chapter of the Myasthenia
Gravis Foundation.
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