A Young Woman With Intermittent Weakness and a Positive Family History of Similar Problems

A 22-year-old woman presented with a 3-month history of weakness, some instability of gait, and difficulty swallowing. The symptoms fluctuated somewhat during the day.

Past medical history was negative. She was on birth control pills. Family history was positive for her mother having myasthenia gravis (MG).

General physical examination and mentation were normal. Cranial nerve examination revealed mild ptosis on the left without clear evidence of fatigue. There was no diplopia or ophthalmoplegia, but there was mild orbicularis oculi and oris weakness. She had proximal weakness of about 4+/5 in the neck and shoulder muscles and 4/5 in the hips. Reflexes, sensation, and coordination were normal. There were no Babinski signs. The examination was otherwise unremarkable.

What is the Differential Diagnosis?

The differential diagnosis of this patient with limb and facial weakness includes facioscapulohumeral dystrophy, because of the family history, as her mother presented with similar weakness, but for that diagnosis, one has to assume that the mother’s diagnosis of MG was incorrect. Furthermore, this patient did not have prominent muscle atrophy, and there was no history of progression; ptosis is not common in that disorder. Another hereditary disease of similar presentation is oculopharyngeal dystrophy, but this usually presents at an older age, and she did not have the typical astronomers’ sign or prominent ptosis, and her symptoms fluctuated during the day. An acquired endocrine or autoimmune myopathy should be considered, but they usually do not present with facial weakness, nor does periodic paralysis, which presents with episodic rather than a fluctuating weakness as in this case. Hyperthyroidism could present with weakness and ophthalmoplegia, but the symptoms usually do not fluctuate.

Patients with sarcoidosis might have proximal weakness from myositis and develop bilateral seventh and sixth nerve palsies, but these do not fluctuate. Mitochondrial myopathies might present with ophthalmoplegia and ptosis, but in those patients the symptoms are usually slowly progressive.

The apparent autosomal-dominant inheritance could also suggest slow channel syndrome, but in that disease weakness is more prominent in the finger extensors.

Autoimmune MG is not hereditary, but there are reports of the disease occurring in families, and thus MG should be considered in the differential diagnosis because of the recently acquired weakness and asymmetric ptosis.

An EMG Test was Performed

Motor Nerve Studies

Nerve and Site	Latency (ms)	Amplitude (mV)	Conduction Velocity (m/s)
Median Nerve R.	Normal ≤4.2	Normal ≥ 6	Normal ≥ 50
Wrist	4.1	16	–
Elbow	8.4	16	59

Nerve and Site	Latency (ms)	Amplitude (mV)	Conduction Velocity (m/s)
Ulnar Nerve R.	Normal ≤ 3.6	Normal ≥ 8	Normal ≥ 50
Wrist	2.9	11	–
Below elbow	6.3	10	63

F-Wave Studies

Nerve	Latency (ms)	Normal Latency ≤ (ms)
Median nerve R.	27.4	30
Ulnar nerve R.	27.6	30

Sensory Nerve Studies

Nerve	Onset Latency (ms)	Normal Onset Latency ≤ (ms)	Peak Latency (ms)	Normal Peak Latency ≤ (ms)	Amp (μV)	Normal Amp ≥ (μV)	Conduction Velocity (m/s)	Normal Conduction Velocity ≥ (m/s)
Median nerve R.	2.6	2.6	3.1	3.1	38	20	50	50
Ulnar nerve R.	1.9	2.6	2.4	3.1	20	13	63	50

Repetitive Stimulation of Ulnar Nerve R.

Potential Number	Amp (mV)	Amp Decrement (%)	Area Decrement (%)
Before Exercise
1	8.19	0	0
2	7.68	6	7
3	7.09	13	15
4	6.82	17	20
5	6.80	17	20
Posttetanic
1	8.22	0	0
2	8.17	1	0
3	8.11	1	2
4	8.11	1	1
5	8.15	1	0
1 min Postexercise
1	8.35	0	0
2	7.92	5	3
3	7.39	11	12
4	7.18	14	15
5	7.25	13	15

Potential Number	Amp (mV)	Amp Decrement (%)	Area Decrement (%)
2 min Postexercise
1	8.63	0	0
2	8.26	4	7
3	7.61	12	17
4	7.34	15	22
5	7.29	16	22
3 min Postexercise
1	8.63	0	0
2	8.09	6	4
3	7.40	14	15
4	7.27	16	18
5	7.30	15	18

EMG Data

Muscle	Insrt Activity	Fibs	Pos Waves	Fasc	Amp	Dur	Poly	Pattern
Extensor digitorum com. R.	Norm	None	None	None	Variable	Norm	None	Full
Deltoid R.	Norm	None	None	None	Variable	Norm	Few	Full

What were the EMG Findings?

This test showed normal nerve conduction velocities and normal amplitudes and latencies of the CMAPs and SNAPs. She had the characteristic decrement of the CMAP area upon repetitive stimulation of the ulnar nerve at 3 Hz with posttetanic facilitation and postactivation exhaustion, diagnostic of MG ( Fig. 72B-1 ). There was no evidence of repetitive after-discharges on the CMAP as seen in congenital slow channel syndrome. There were also some polyphasic motor units and motor unit variability in the deltoid muscle, as occur in disorders of neuromuscular transmission.