An 80-year-old woman was admitted to another hospital because of acute respiratory failure from myasthenia gravis (MG), diagnosed 2 years before with a positive edrophonium test and elevated acetylcholine receptor antibody titers. She was treated with corticosteroids early with improvement. However, her condition had deteriorated recently after a respiratory infection. She was intubated and maintained on methylprednisolone 60 mg every 6 hours and pyridostigmine 60 mg every 4 hours. She was transferred after 10 days because of a lack of improvement.
Past medical history included hypertension and a mastectomy secondary to carcinoma of the breast.
On examination the patient was intubated and on respiratory assistance. She was afebrile; blood pressure and pulse were normal. She was alert, and her cranial nerves were normal without ptosis or ophthalmoplegia. There was prominent drooling. Muscle strength showed neck flexors 2+/5, shoulders 3/5, biceps and triceps 4/5, grip 4/5, hand 4/5, iliopsoas 3−/5, and quadriceps 4/5. No clear fatigue was detected on repetitive testing. Reflexes were 1+ at the brachioradialis and ankles, trace at the knees and biceps, and 2+ at the triceps equally in both sides; she had normal sensory examination. There were no Babinski signs.
An edrophonium test produced no obvious improvement.
She received plasma exchange for 5 consecutive days without significant benefit.
What is the Differential Diagnosis?
This patient had been properly diagnosed with MG. She then developed severe weakness, was intubated, and received high doses of corticosteroids. She was transferred for evaluation and plasma exchanges because of a lack of improvement.
The causes of weakness in the ICU are outlined in Table 44-1 (see Fig. 43-1 in Case 43 for the workup of patients who develop weakness in the ICU). The generalized weakness, in this case, suggests that she was in a myasthenic crisis or that she was overmedicated with pyridostigmine which might explain the drooling. The dose, however, was low, and she did not improve when it was discontinued afterward. Edrophonium produced no improvement.
| Myelopathies |
| Amyotrophic lateral sclerosis and other motor neuron disorders |
| Myasthenia gravis and, rarely, Eaton–Lambert myasthenic syndrome and other myasthenic syndromes |
| Persistent pharmacological paralysis |
| Tick paralysis |
| Botulism |
| Organophosphate poisoning |
| Guillain–Barré syndrome |
| Toxic, vasculitic neuropathies (rarely) |
| Chronic inflammatory demyelinating polyneuropathy (rarely) |
| Porphyria |
| Critical illness neuropathy |
| Inflammatory myopathies |
| Toxic myopathies and other causes of rhabdomyolysis |
| Critical illness myopathy |
| Acid maltase deficiency |
| Muscular dystrophies, congenital myopathy, and mitochondrial myopathy |
| Endocrine and electrolyte imbalance |
The decreased reflexes could suggest another disorder or neuromuscular transmission, such as Eaton–Lambert myasthenic syndrome or botulism, but she had excessive salivation and no pupillary abnormalities, and the diagnosis of MG was well documented. The lack of long tract signs and fasciculations was against amyotrophic lateral sclerosis; the presence of reflexes was against Guillain–Barré syndrome and similar neuropathies, including critical illness polyneuropathy (CIP). There was no history of exposure, and she did not have muscle spasms or increased sweating to suggest organophosphate poisoning.
Myopathies appear to be a good possibility; among them, polymyositis, inclusion body myositis, and acid maltase deficiency can present with acute respiratory failure. Other possibilities include electrolyte imbalance and hypophosphatemia. Finally, critical illness myopathy (CIM) should be considered because she had been intubated and received high doses of corticosteroids. Although she did not receive paralyzing agents, MG itself could have caused weakness and predisposed her to CIM, which can also occur in patients on corticosteroids who are not receiving paralyzing agents.
What Other Tests Should Be Done?
Serum creatine kinase (CK) was 260 IU/L (normal, 80–200 IU/L); a complete blood chemistry profile and complete blood count were normal, except the hematocrit was 28% (normal, 35%–45%).
An EMG Test was Performed
| Nerve and Site | Latency(ms) | Amplitude (mV) | Conduction Velocity (m/s) |
|---|---|---|---|
| Peroneal Nerve R. | Normal ≤ 5.7 | Normal ≥ 3 | Normal ≥ 40 |
| Ankle | 4.9 | 0.7 | – |
| Fibular head | 11.3 | 0.7 | 41 |
| Tibial Nerve L. | Normal ≤ 5.3 | Normal ≥ 4 | Normal ≥ 40 |
|---|---|---|---|
| Ankle | 5.3 | 2 | – |
| Pop. fossa | 17.9 | 2 | 41 |
| Ulnar Nerve R. | Normal ≤ 3.6 | Normal ≥ 8 | Normal ≥ 50 |
|---|---|---|---|
| Wrist | 3.4 | 5 | – |
| Below elbow | 6.4 | 5 | 53 |
| Above elbow | 8.4 | 5 | 60 |
| Nerve and Site | Latency (ms) | Amplitude (mV) | Conduction Velocity (m/s) |
|---|---|---|---|
| Median Nerve R. | Normal ≤ 4.2 | Normal ≥ 6 | Normal ≥ 50 |
| Wrist | 3.1 | 1 | – |
| Elbow | 7.5 | 1 | 58 |
* Needle recording at tibialis ant. muscle. The amplitude was equal during nerve and muscle stimulation.
| Nerve | Latency (ms) | Normal Latency ≤ (ms) |
|---|---|---|
| Peroneal nerve R. | 53.6 | 54 |
| Tibial nerve L. | 52.8 | 54 |
| Ulnar nerve R. | 28.9 | 30 |
| Median nerve R. | 29.2 | 30 |
| 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) |
|---|---|---|---|---|---|---|---|---|
| Sural nerve R. | 3.3 | 3.5 | 3.8 | 4.0 | 15 | 11 | 42 | 40 |
| Ulnar nerve R. | 2.2 | 2.6 | 2.7 | 3.1 | 14 | 13 | 55 | 50 |
| Median nerve R. | 2.5 | 2.6 | 3.0 | 3.1 | 23 | 20 | 52 | 50 |
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