A 71-year-old woman is evaluated for difficulty holding her head upright. She notes that her head feels heavy and reports intermittent difficulty with swallowing and speech that is worse in the evening. She has had no pain, sensory changes, weakness in the extremities, or cognitive or visual symptoms. She has no other medical problems and takes no medication.
On physical examination, vital signs are normal. Speech is mildly dysarthric. Cervical extension is weak. No ptosis, ophthalmoplegia, sensory deficit, or weakness in the extremities is noted.
Results of laboratory studies show a normal serum creatine kinase level; no acetylcholine receptor antibodies are detected.
Findings from routine nerve conduction and needle electromyography studies of the limbs are unremarkable, but a repetitive stimulation protocol reveals a decremental response. An MRI of the brain is normal.
Which of the following is the most likely diagnosis?
A. Bulbar amyotrophic lateral sclerosis
B. Inclusion body myositis
C. Multiple sclerosis
D. Myasthenia gravis
MKSAP Answer and Critique
The correct answer is D. Myasthenia gravis. This content is available to MKSAP 18 subscribers as Question 232 in the Neurology section. More information about MKSAP is available online.
Myasthenia gravis can present with pronounced weakness of cervical or bulbar muscles. Fluctuation in weakness, with a fatigable pattern that worsens later in the day, suggests this neuromuscular junction disorder. In the setting of probable myasthenia gravis, seronegative status for acetylcholine receptor antibodies should trigger testing for muscle-specific kinase (MuSK) antibodies, which are positive in half of seronegative patients. Weakness of cervical extension, such as this patient exhibits, is a hallmark of MuSK antibody–positive myasthenia gravis. Although nerve conduction studies using a special protocol for repetitive stimulation can reveal a pattern of decrementing weakness in neuromuscular junction disorders, this protocol is not part of routine electromyography. Therefore, if suspicion for myasthenia gravis is high, a repetitive stimulation protocol should be specifically requested.
Although amyotrophic lateral sclerosis can present with prominent bulbar and cervical weakness, the fluctuating nature of the weakness, the absence of tongue weakness or fasciculations and of upper motor neuron signs, and the negative electromyography (EMG) findings make this an unlikely diagnosis.
Inclusion body myositis is a progressive inflammatory myopathy that can present with bulbar, forearm flexor, and quadriceps weakness. The absence of additional muscle involvement, the normal creatine kinase level, and the normal limb findings on needle EMG rule out this entity.
Multiple sclerosis (MS) can cause bulbar weakness secondary to central demyelination. Focal neck extensor weakness is atypical. The patient's history and normal MRI exclude MS.
Polymyositis also can present with head drop, but often proximal limb muscles are involved. In addition, a normal creatine kinase level and normal results on routine limb EMG rule out this entity.
- In a patient with fluctuating weakness of the cervical muscles that worsens in the evening, myasthenia gravis should be suspected; weakness of cervical extension is a hallmark of muscle-specific kinase antibody–positive myasthenia gravis.