Disabling Foot Cramping in a Runner Secondary to Paramyotonia Congenita: A Case Report

Introduction

Causes of muscle cramping and stiffness with various etiologies are common complaints in athletes. Among the causes of these complaints are the myotonias. Myotonic syndromes, which are characterized by abnormally slow relaxation after voluntary muscle contraction due to a muscle membrane abnormality, are frequently overlooked because, with the exception of myotonic dystrophy, these syndromes are relatively rare. The amount of myotonia and its distribution vary considerably among individual patients and with regard to the different myotonic syndromes. Additionally, patients frequently have difficulty describing the symptoms of myotonia. ⁵ We report on a runner whose condition became apparent when he felt cramping foot pain while increasing his training intensity.

Case report

An 18-year-old male runner was referred to our sports medicine clinic with a 1-year history of cramping left foot pain. He was a national high school champion and was recruited as a scholarship athlete for a division one collegiate running program. His symptoms developed insidiously when he increased his speed work training about 1 year earlier, with an onset of severe left foot pain and tightness that would develop after about 20 minutes of exercise. The patient also noted occasional cramps in his calves after excessive exercise. He did not notice any swelling, numbness, tingling, or burning sensations. In spite of this, the patient was able to achieve a high level of competitive long-distance running.

He saw numerous private physicians and tried physical therapy, interdigital nerve block, and was immobilized in a walking cast for more than 1 month for a presumed stress fracture without effect. One physician diagnosed foot compartment syndrome and recommended surgery. Results of two magnetic resonance imaging scans on two occasions showed no abnormalities. Results of blood studies, including cell count, electrolyte, and chemistry panel, were within normal limits.

The patient's past medical history was unremarkable, and he was not taking any medications. A more detailed review of symptoms, however, revealed that cold weather or immersing himself in cold water, such as a pool, could produce cramping of his hands, feet, or jaw. A family history revealed similar symptoms in his sister and maternal uncle. His mother also had very mild symptoms of hand cramping with exposure to cold weather (less than 25°C).

On physical examination, the patient had no atrophy or fasciculations of his extremities or tongue. Neurovascular status was intact. He had no pain associated with deep palpation of the bones or muscles of the foot. Manual muscle testing of all foot and leg muscles was 5/5 bilaterally, and there was no pain provocation. He had full range of motion in the cervical and thoracolumbar spine. There was no evidence of adverse neural tension with straight leg raise or slump tests. Tinel's test was negative at the fibular head and tarsal tunnel. Metatarsal compression test was negative. He had full, pain-free range of motion in the ankle, subtalar, and midtarsal joints.

Further neurologic testing, including full motor and sensory testing of the upper extremities, mental status screen, cranial nerves, coordination, and gait, were within normal limits, except for mild proximal muscle weakness noted at the deltoids and triceps rated at 4/5 bilaterally. No percussion myotonia was noted. There was no evidence of frontal balding or temporalis muscle wasting. Immersion of the patient's right hand in ice water for approximately 45 seconds revealed no myotonia or cramping.

To demonstrate his symptoms, he was asked to run. After 20 minutes, he began to experience cramping, which started with feeling of tightness on the plantar surface of the left foot. The more he continued to run, the more the symptoms were aggravated and evolved to the toes curling with intrinsic muscle spasm. The patient's symptoms resolved 10 minutes after stopping the run.

A Stryker examination of the medial, central, and deep posterior compartments of the left and the central compartment on the right leg was performed to rule out compartment syndrome. The results of our study revealed normal pressure in all compartments preexercise (<10 mm Hg) and 1 minute postexercise (<20 mm Hg).

Results of an eye examination, including cataract evaluation, were normal. On laboratory examination, serum creatinine phosphokinase (CPK) was 687 IU/L (normal range for men, 35-232 IU/L). Repeated CPK study and aldolase after 2 weeks rest from running was 669 IU/L and 9 (normal range, 1-8). Results of thyroid function tests were normal. Results of motor and sensory nerve conduction studies were normal. A needle electromyography (EMG) study at rest demonstrated abnormal sustained runs of positive sharp waves at virtually every needle site in all limbs studied. Motor unit potentials and recruitment were normal in all muscles. Following immersion of his hands in cold water for approximately 4-5 minutes, a repeat needle examination of his first dorsal interossei in the hand showed a large amount of myotonic discharges. He was referred to a neurologist for further consultation. The neurologist recommended a muscle biopsy to rule out a myopathy or other inflammatory muscle disorder, and the results of a biopsy specimen taken from the quadriceps muscle were normal.

According to the patient's history, examination, EMG findings, and laboratory test results, including muscle biopsy, it was determined that the patient had paramyotonia congenita. He was treated initially with carbamazepine and mexiletine; however, these were discontinued secondary to reported adverse effects of subtle cognitive slowing. He then was given phenytoin (400 mg q h), which lessened his symptoms and allowed him to continue running with minimal symptoms, provided he maintain his running mileage at less than 50 miles per week and that he run on softer surfaces and modify his workouts during colder weather. Eventually, he decided to give up running, as he was not able to maintain mileage high enough to compete successfully at an elite college level. Although he stopped running, he has been doing well with medication.

Discussion

Paramyotonia congenita is a rare autosomal dominant disease, characterized by cold-induced myotonic stiffness, that is increased by sustained muscular activity (paradoxical myotonia) and may be followed by a variable degree of weakness. ² Paramyotonia congenita is now classified together with hyperkalemic periodic paralysis, some forms of hypokalemic periodic paralysis, and potassium-aggravated myotonias in the group of hereditary sodium channelopathies. ^1,3,6

The differential diagnosis for paramyotonia congenita includes myotonia congenita and myotonia dystrophy. Myotonia congenita is distinguished by stiffness that decreases with warming-up exercises and does not show typical cold temperature provocation. Myotonic dystrophy, the most common myotonic syndrome, is a progressive, multisystemic disease associated with muscle atrophy, cataracts, cardiac arrhythmias, testicular atrophy, and other endocrine abnormalities. ⁵ These features were absent in our patient. The other specific myotonic disorder for differential diagnosis is a proximal myotonic myopathy (PROMM). This disease is predominantly characterized by proximal weakness without atrophy, muscle pain, cataracts, myotonia, cardiac disturbances, and gonadal dysfunction. ⁴ Even though our patient had subtle weakness in bilateral proximal upper extremities, other features did not bear out a diagnosis of PROMM. Given that foot-cramping pain was the initial presenting symptom in our patient, we considered a possibility of compartment syndrome; however, results of the patient's Stryker examination were not compatible with this diagnosis and also the patient denied any discomfort in his leg or more proximal sites with the exception of the foot. Other causes of muscle cramping or stiffness include continuous muscle-fiber activity like cramping-fasciculation syndrome or neuromyotonia, myokymia, stiff-man syndrome, tetany, and tetanus. ⁵ These diseases can be easily excluded with history and electrophysiologic studies.

Our patient had a relatively benign disorder that was long-standing and was clinically characterized by foot myotonia that worsened with repetitive muscle contraction but was not induced by abrupt movement following a period of rest, and hand stiffness that worsened with exposure to cold. For additional examination of myotonia, a repetitive nerve stimulation test after exercise can be helpful, because a postexercise decrement phenomenon is observed in patients with clinical myotonia. However, a repetitive nerve stimulation test was not performed in this case because the standard EMG findings were so dramatic. The patient's family history indicated an autosomal dominant inherited disorder. These features are characteristic of paramyotonia congenita. Although the typical paralysis after myotonia was absent in our patient, his myotonic symptom that was significantly prolonged after exercise and his cold-sensitive myotonia are more compatible with paramyotonia congenita than with any other myotonic syndromes.

Conclusion

We report on a patient whose clinical and electro-diagnostic findings are most consistent with paramyotonia congenita. Myotonia should be considered in the differential diagnosis of athletes with exercise-induced muscle stiffness or cramping pain, particularly if the course is protracted and initial examinations are unremarkable. It is important for physicians to be aware of myotonia to avoid additional costly and unnecessary testing, and to consult with a neurologic specialist when the diagnosis is being considered.