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Myokymia After Snake Envenomation in Arizona

2005; Elsevier BV; Volume: 16; Issue: 2 Linguagem: Inglês

10.1580/1080-6032(2005)16[116

1545-1534

Frank LoVecchio, Anthony F. Pizon, Kevin L. Wallace, Donald B. Kunkel,

Trigeminal Neuralgia and Treatments

To the Editor: Myokymia is characterized as spontaneous, fine, involuntary undulating waves (ripples) of muscle fibers known to be reversed by calcium administration.1.Brick J.F. Gutmann L. Brick J. Apelgren K.N. Riggs J.E. Timber rattlesnake venom-induced myokymia: evidence for peripheral nerve origin.Neurology. 1987; 37: 1545-1546Crossref PubMed Google Scholar Myokymia is associated with intra-axial and peripheral demyelinating diseases (eg, multiple sclerosis and Guillain-Barré syndrome, respectively) and rattlesnake envenomation from the timber rattlesnake (Crotalus horridus horridus).2.Gutmann L. Gutmann L. Myokymia and neuromyotonia 2004.J Neurol. 2004; 251: 138-142Crossref PubMed Scopus (90) Google Scholar To our knowledge, these are the first 2 cases of envenomation-induced myokymia involving southwestern rattlesnake species. In case 1, a 14-year-old boy was bitten on his dorsal left foot by an unidentified rattlesnake while walking along the shore of Lake Powell near Page, AZ. He presented to the local emergency department with myokymia involving his face, arms, and legs and received 10 vials of Antivenin (Crotalidae) Polyvalent (Wyeth-Ayerst Laboratories, Philadelphia, PA) before transfer. At our institution, he continued to exhibit continuous, symmetrical, involuntary movements of the lower face, deltoid, and calf muscles. These movements were low amplitude, vermicular, and undulating, consistent with myokymia. Peak creatine kinase elevation was 1800 IU/L. No significant coagulopathy or progression of tissue swelling was observed, and no additional antivenom was administered before discharge, at which time the boy's myokymia had resolved. In case 2, a 40-year-old woman in the Lake Powell area was envenomated on her left index finger. The snake was captured and killed and was transported along with the patient for identification. The patient was transferred to our institution, where she was noted to have peripheral and perioral paresthesias and myokymia. She received intravenous (2 g of 10%) calcium chloride with prompt resolution of her facial myokymia. Her clinical course was remarkable for local swelling at the envenomation site, rhabdomyolysis (peak creatine kinase = 17 000 IU/L), and mild hypofibrinogenemia (105 mg/dL), for which she received 30 vials of Antivenin (Crotalidae) Polyvalent. She was discharged 1 day later after marked clinical improvement. Herpetologists identified the snake as the midget faded rattlesnake (Crotalus viridis decolor). In addition to causing local tissue injury and hematologic derangements, rattlesnake venom is known to occasionally have neurologic effects.3.Clement J.F. Pietrusko R.G. Pit viper snakebite envenomation in the United States.Clin Toxicol. 1979; 14: 515-538Crossref Scopus (5) Google Scholar The neurotoxicity usually manifests itself as weakness, slurred speech, cranial nerve palsies, and muscle fasciculations. These systemic neurotoxic effects are most commonly observed in envenomations from the Mojave rattlesnake (Crotalus scutulatus scutulatus).4.Jansen P.W. Perkin R.M. Van Stralen D. Mojave rattlesnake envenomation: prolonged neurotoxicity and rhabdomyolysis.Ann Emerg Med. 1992; 21: 322-325Abstract Full Text PDF PubMed Scopus (40) Google Scholar Myokymia, however, is a unique neurotoxic manifestation previously reported in the timber rattlesnake.1.Brick J.F. Gutmann L. Brick J. Apelgren K.N. Riggs J.E. Timber rattlesnake venom-induced myokymia: evidence for peripheral nerve origin.Neurology. 1987; 37: 1545-1546Crossref PubMed Google Scholar Clinically, myokymia may be difficult to distinguish from fasciculations, but there are distinct differences. Myokymia is derived from the Greek word for wave. Clinically, it appears as rippling muscles under the skin. When associated with rattlesnake envenomation, myokymia has been described in muscle groups close to the site of envenomation or in facial muscles.1.Brick J.F. Gutmann L. Brick J. Apelgren K.N. Riggs J.E. Timber rattlesnake venom-induced myokymia: evidence for peripheral nerve origin.Neurology. 1987; 37: 1545-1546Crossref PubMed Google Scholar In contrast, fasciculations occur randomly as single muscle twitches. Electrophysiologically, myokymia is a series of brief axonal bursts of a single motor unit that fires at rates of 5 to 150 Hz.2.Gutmann L. Gutmann L. Myokymia and neuromyotonia 2004.J Neurol. 2004; 251: 138-142Crossref PubMed Scopus (90) Google Scholar Each burst occurs as a doublet or multiplet. Fasciculations also occur as a single motor unit but do not fire in tetanic bursts and do not occur cyclically.2.Gutmann L. Gutmann L. Myokymia and neuromyotonia 2004.J Neurol. 2004; 251: 138-142Crossref PubMed Scopus (90) Google Scholar The etiology of myokymia is poorly described but thought to occur as a result of an interaction between the venom and the calcium or voltage-gated potassium channels on nerve axons.1.Brick J.F. Gutmann L. Brick J. Apelgren K.N. Riggs J.E. Timber rattlesnake venom-induced myokymia: evidence for peripheral nerve origin.Neurology. 1987; 37: 1545-1546Crossref PubMed Google Scholar,2.Gutmann L. Gutmann L. Myokymia and neuromyotonia 2004.J Neurol. 2004; 251: 138-142Crossref PubMed Scopus (90) Google Scholar,5.Lewis R.L. Gutmann L. Snake venoms and the neuromuscular junction.Semin Neurol. 2004; 24: 175-179Crossref PubMed Scopus (60) Google Scholar Axonal anatomy is unaffected, but a biochemical alteration in the microenvironment around an axon is likely the cause of myokymic discharge. Rapid resolution of myokymia with intravenous calcium or CroFab antivenom has been described and is consistent with the proposed biochemical etiology.1.Brick J.F. Gutmann L. Brick J. Apelgren K.N. Riggs J.E. Timber rattlesnake venom-induced myokymia: evidence for peripheral nerve origin.Neurology. 1987; 37: 1545-1546Crossref PubMed Google Scholar Myokymia, a neuromuscular disorder reported with eastern US rattlesnake envenomations (C horridus horridus), is now reported as a feature of envenomation by at least 1 species of southwestern rattlesnake (C viridis decolor).