Neuromyotonia entities

Clinic

• Neuromyotonia (NMT) is a form of peripheral nerve hyperexcitability that causes spontaneous muscular activity resulting from repetitive motor unit action potentials of peripheral origin.
• NMT along with Morvan's syndrome are the most severe types in the Peripheral Nerve Hyperexciteability spectrum.
• Example of two more common and less severe syndromes in the spectrum are
  1. Cramp Fasciculation Syndrome
  2. Benign Fasciculation Syndrome.

• Neuromyotonia refers to muscle twitching and cramping at rest that is exacerbated with exercise.
• It is caused by sustained or repetitive spontaneous muscle activity of peripheral nerve origin.
• Myokymia = Spontaneous rippling and twitching movements of muscles, is a visible component of neuromyotonia.
• Electromyography (EMG) discloses spontaneous, repetitive motor unit or single fiber discharges firing in irregular rhythmic bursts at high intraburst frequencies.
• Some of the muscles exhibiting twitching include the bilateral gastrocnemii, quadriceps femoris, biceps brachii, and right masseter. In vivo electrophysiological studies suggest at least some dysfunction of the muscle cell membrane. In the examined muscles, no abnormal insertional activity or fibrillation potentials were noted. Nerve conduction studies were normal.

 

Signs / Symptoms

• Muscle cramps, stiffness
• Myotonia-like symptoms (slow relaxation)
• Walking difficulties
• Hyperhidrosis
• Myokymia
• Fasciculations
• Fatigue, exercise intolerance
• Myoclonic jerks and other related symptoms.

Causes

The three causes of NMT are Acquire, Paraneoplastic and Hereditary

• Acquired form (80%), usually caused by antibodies against the neuromuscular junction.
• Autoreactive antibodies can be detected in a variety of peripheral (e.g. myasthenia gravis, Lambert–Eaton myasthenic syndrome) and CNS (e.g. paraneoplastic cerebellar degeneration, paraneoplastic limbic encephalitis) disorders. Their causative role has been established in some of these diseases but not all. Neuromyotonia is considered to be one of these with accumulating evidence for autoimmune origin over the last few years.
• Autoimmune neuromyotonia is typically caused by antibodies that bind to potassium channels on the motor nerve resulting in continuous/hyper-excitability.