Published on 03/03/2015 by admin
Filed under Neurology
Last modified 22/04/2025
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70 Other Peripheral Motor Hyperactivity Syndromes
H. Royden Jones, Jr.
This is another hyperkinetic presumed peripheral nerve hyperexcitability syndrome that occurs even more rarely than the stiff person syndrome. As noted above, there are a number of synonyms. It is a very unusual neurologic disorder also characterized by the continuous firing of peripheral nerves, and thus muscle fibers. It is much more subtle in its presentation. Typically the age of onset is in the teenage years, but it may be seen in adulthood. This appears to be autoimmune in origin (anti–potassium channel antibodies are positive in some patients.) This is a nonspecific finding per se as these antibodies may be found in a variety of neurologic syndromes.
Here the patient first notes what appears to be an almost continuous firing of groups of muscle fibers usually appearing to mimic fasciculations. These occur concomitantly in both agonists and antagonists. The patient often notes a sense of fatigue. Usually there are no significant associated muscle spasms, cramping, or pain; however, in some instances such may occur. The patient may note a degree of weakness because of the inconsistent firing of opposing muscle groups, leading to ineffective mechanical function. Some patients have excessive sweating.
Other than the apparent pseudo-fasciculations and a hint of muscle weakness, the neurologic examination is normal. Diagnosis is suggested when electromyography (EMG) demonstrates continuous firing of normal motor unit potentials without any abnormalities on insertion. These persist even during sleep. A therapeutic trial of phenytoin or carbamazepine most always leads to a complete cessation of all aspects of this syndrome. This therapeutic result serves to confirm the suspected diagnosis. This is a particularly gratifying diagnosis to make as some of these extremely unusual patients may have received an earlier clinical diagnosis of amyotrophic lateral sclerosis (ALS).
This is a rare peripheral nerve hyperexcitability syndrome that has similarities to Isaac syndrome, but with the added feature of pain and cramps. When EMG is performed, motor nerve stimulation results in a sustained contraction of the activated muscle. Some but not all of these patients may have potassium channel antibodies. Treatment is similar to that for Isaac syndrome.
These patients seemingly develop the relatively rapid onset of scattered fasciculations that occur much less frequently than those seen in Isaac syndrome. This is obvious by both clinical examination and EMG. The major issue here is the patient who is medically knowledgeable and makes an association between fasciculations and ALS. However, if there is no concomitant clinical weakness and/or muscle atrophy, this almost always is a benign idiopathic entity. Its importance lies in the need for the neurologist to respond immediately to the concern of their medical colleague by performing a very careful clinical evaluation followed by an EMG. If both areas are normal, the patients can be told that they have no greater risk to have ALS than any other healthy individual. Often such physicians and nurses have waited to seek expert attention for a matter of months while trying to ignore their symptoms. Finally they become so overwhelmed that they seek help often calling to say that they do have ALS. We look upon this request as a neurologic social emergency in order to immediately allay the angst of our colleagues. The emotional relief that is expressed by these colleagues when they hear the good news is heart warming.
This very uncommon finding is a subcutaneous vermification, or worm-like activity, of just a few muscles. These patients often rarely note these muscle twitches. They are very rhythmic and are basically a quivering under the skin. Typically these are seen in two neurologic conditions.
The most common setting is the patient with prior breast cancer who had a mastectomy many years earlier that was followed soon after surgery by radiation. After a many-year postradiation delay, sometimes as much as 20+ years, the patient begins to note weakness and atrophy in the adjacent arm and hand muscles. With careful inspection one may rarely note the myokymia. However, EMG will demonstrate the spontaneous rhythmic firing of grouped motor unit potentials. These are so unique that they provide strong evidence that the patient’s plexopathy is a consequence of radiation.
Subtle peripheral facial nerve lesions associated with a contiguous pontine glioma may also be characterized by myokymia. Here the subcutaneous tissue is so thin that the myokymia is clinically evident to the patient. They may note these adventitious movements when shaving or applying lipstick. In this instance, a magnetic resonance imaging is indicated to search for a pontine mass. On rare occasions, myokymia occurs subsequent to Bell palsy or spontaneously without a specific pathologic lesion.
Additional Resources
Benerud MD, Windebank A, Daube J. Long term follow-up of 121 patients with benign fasciculations. Neurology. 1993;34:622.
Newsom-Davis J. Neuromyotonia. Rev Neurol. 2004;160:85.
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