Neuromyotonia (NMT), also known as Isaacs Syndrome and Isaacs-Merton syndrome, is a form of peripheral nerve hyperexcitability that causes spontaneous muscular activity resulting from repetitive motor unit action potentials of peripheral origin. Prevalence is unknown but 100–200 cases have been reported so far.
Signs and symptomsNMT is a diverse disorder. As a result of muscular hyperactivity, patients may present with muscle cramps, stiffness, myotonia-like symptoms (slow relaxation), associated walking difficulties, hyperhidrosis (excessive sweating), myokymia (quivering of a muscle), fasciculations (muscle twitching), fatigue, exercise intolerance, myoclonic jerks and other related symptoms. The symptoms (especially the stiffness and fasciculations) are most prominent in the calves, legs, trunk, and sometimes the face and neck, but can also affect other body parts. NMT symptoms may fluctuate in severity and frequency. Symptoms range from mere inconvenience to debilitating. At least a third of people also experience sensory symptoms.
CausesThe three causes of NMT are:
DiagnosisDiagnosis is clinical and initially consists of ruling out more common conditions, disorders, and diseases, and usually begins at the general practitioner level. A doctor may conduct a basic neurological exam, including coordination, strength, reflexes, sensation, etc. A doctor may also run a series of tests that include blood work and MRIs. From there, a patient is likely to be referred to a neurologist or a neuromuscular specialist. The neurologist or specialist may run a series of more specialized tests, including needle electromyography EMG/ and nerve conduction studies (NCS) (these are the most important tests), chest CT (to rule out paraneoplastic) and specific blood work looking for voltage-gated potassium channel antibodies, acetylcholine receptor antibody, and serum immunofixation, TSH, ANA ESR, EEG etc. Neuromyotonia is characterized electromyographically by doublet, triplet or multiplet single unit discharges that have a high, irregular intraburst frequency. Fibrillation potentials and fasciculations are often also present with electromyography. Because the condition is so rare, it can often be years before a correct diagnosis is made. NMT is not fatal and many of the symptoms can be controlled. However, because NMT mimics some symptoms of motor neuron disease (ALS) and other more severe diseases, which may be fatal, there can often be significant anxiety until a diagnosis is made. In some rare cases, acquired neuromyotonia has been misdiagnosed as amyotrophic lateral sclerosis (ALS)Rowland LP, Shneider NA. Amyotrophic lateral sclerosis. N Engl J Med 2001; 344: 1688–700. particularly if fasciculations may be evident in the absence of other clinical features of ALS. However, fasciculations are rarely the first sign of ALS as the hallmark sign is weakness.Hirota N, Eisen A, Weber M. Complex fasciculations and their origin in amyotrophic lateral sclerosis and Kennedy’s disease. Muscle Nerve 2000; 23: 1872–5. Similarly, Multiple sclerosis has been the initial misdiagnosis in some NMT patients. In order to get an accurate diagnosis see a trained neuromuscular specialist.
TypesThere are three main types of NMT:
- Monophasic (symptoms that resolve within several years of onset; postinfection, postallergic)
- Relapsing Remitting