Neuromyotonia

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Neuromyotonia
Other namesIsaacs syndrome, Isaacs-Mertens syndrome
Specialty Neurology, neuromuscular medicine   OOjs UI icon edit-ltr-progressive.svg

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 cramp fasciculation syndrome and benign fasciculation syndrome. [1] NMT can have both hereditary and acquired (non-inherited) forms. The prevalence of NMT is unknown. [2]

Contents

Signs and symptoms

NMT 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. These neuromyotonic discharges can cause bursts of spontaneous motor activity that present either continuously or in recurring, decrementing clusters. They can start and stop abruptly, typically waning in strength and are unaffected by voluntary activity. [3]

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.

Causes

The three causes of NMT are:[ citation needed ]

  1. Acquired
  2. Paraneoplastic
  3. Hereditary

The acquired form is the most common, accounting for up to 80 percent of all cases and is suspected to be autoimmune-mediated, which is usually caused by antibodies against the neuromuscular junction.

The exact cause is unknown. However, autoreactive antibodies can be detected in a variety of peripheral (e.g. myasthenia gravis, Lambert–Eaton myasthenic syndrome) and central nervous system (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. [4] Autoimmune neuromyotonia is typically caused by antibodies that bind to potassium channels on the motor nerve resulting in continuous/hyper-excitability. Onset is typically seen between the ages of 15–60, with most experiencing symptoms before the age of 40. [5] Some neuromyotonia cases do not only improve after plasma exchange but they may also have antibodies in their serum samples against voltage-gated potassium channels. [6] Moreover, these antibodies have been demonstrated to reduce potassium channel function in neuronal cell lines.

It is suspected that the peripheral nerve hyperexcitability associated with Isaacs' and Morvan's syndromes is a result of a potassium channel defect in the motor nerve membrane. [3]

Diagnosis

Diagnosis 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. [7]

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) [8] 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. [9] Similarly, multiple sclerosis has been the initial misdiagnosis in some NMT patients. In order to get an accurate diagnosis see a trained neuromuscular specialist.

People diagnosed with benign fasciculation syndrome or enhanced physiological tremor may experience similar symptoms as NMT, although it is unclear today whether BFS or EPT are weak forms of NMT.

Types

There are three main types of NMT:[ citation needed ]

Peripheral nerve hyperexcitability

Neuromyotonia is a type of peripheral nerve hyperexcitability. Peripheral nerve hyperexcitability is an umbrella diagnosis that includes (in order of severity of symptoms from least severe to most severe) benign fasciculation syndrome, cramp fasciculation syndrome, neuromyotonia and morvan's syndrome. Some doctors will only give the diagnosis of peripheral nerve hyperexcitability as the differences between the three are largely a matter of the severity of the symptoms and can be subjective. However, some objective EMG criteria have been established to help distinguish between the three.

Moreover, the generic use of the term peripheral nerve hyperexcitability syndromes to describe the aforementioned conditions is recommended and endorsed by several prominent researchers and practitioners in the field. [10]

Treatments

There is no known cure for neuromyotonia, but the condition is treatable. Anticonvulsants, including phenytoin and carbamazepine, usually provide significant relief from the stiffness, muscle spasms, and pain associated with neuromyotonia. Plasma exchange and IVIg treatment may provide short-term relief for patients with some forms of the acquired disorder. [5] It is speculated that the plasma exchange causes an interference with the function of the voltage-dependent potassium channels, one of the underlying issues of hyper-excitability in autoimmune neuromyotonia. [11] Botox injections also provide short-term relief. Immunosuppressants such as Prednisone may provide long term relief for patients with some forms of the acquired disorder.

Prognosis

The long-term prognosis is uncertain, and has mostly to do with the underlying cause; i.e. autoimmune, paraneoplastic, etc. However, in recent years increased understanding of the basic mechanisms of NMT and autoimmunity has led to the development of novel treatment strategies. NMT disorders are now amenable to treatment and their prognoses are good. Many patients respond well to treatment, which usually provide significant relief of symptoms. Some cases of spontaneous remission have been noted, including Isaac's original two patients when followed up 14 years later.

While NMT symptoms may fluctuate, they generally don't deteriorate into anything more serious, and with the correct treatment the symptoms are manageable.

A very small proportion of cases with NMT may develop central nervous system findings in their clinical course, causing a disorder called Morvan's syndrome, and they may also have antibodies against potassium channels in their serum samples. Sleep disorder is only one of a variety of clinical conditions observed in Morvan's syndrome cases ranging from confusion and memory loss to hallucinations and delusions. However, this is a separate disorder.

Some studies have linked NMT with certain types of cancers, mostly lung and thymus, suggesting that NMT may be paraneoplastic in some cases. In these cases, the underlying cancer will determine prognosis. However, most examples of NMT are autoimmune and not associated with cancer.

Related Research Articles

<span class="mw-page-title-main">Lambert–Eaton myasthenic syndrome</span> Medical condition

Lambert–Eaton myasthenic syndrome (LEMS) is a rare autoimmune disorder characterized by muscle weakness of the limbs.

Morvan's syndrome is a rare, life-threatening autoimmune disease named after the nineteenth century French physician Augustin Marie Morvan. "La chorée fibrillaire" was first coined by Morvan in 1890 when describing patients with multiple, irregular contractions of the long muscles, cramping, weakness, pruritus, hyperhidrosis, insomnia and delirium. It normally presents with a slow insidious onset over months to years. Approximately 90% of cases spontaneously go into remission, while the other 10% of cases lead to death.

<span class="mw-page-title-main">Benign fasciculation syndrome</span> Involuntary muscle twitching in the voluntary muscles

Benign fasciculation syndrome (BFS) is characterized by fasciculation (twitching) of voluntary muscles in the body. The twitching can occur in any voluntary muscle group but is most common in the eyelids, arms, hands, fingers, legs, and feet. The tongue can also be affected. The twitching may be occasional to continuous. BFS must be distinguished from other conditions that include muscle twitches.

Myotonia is a symptom of a small handful of certain neuromuscular disorders characterized by delayed relaxation of the skeletal muscles after voluntary contraction or electrical stimulation, and the muscle shows an abnormal EMG.

<span class="mw-page-title-main">Neuromuscular junction</span> Junction between the axon of a motor neuron and a muscle fiber

A neuromuscular junction is a chemical synapse between a motor neuron and a muscle fiber.

<span class="mw-page-title-main">POEMS syndrome</span> Paraneoplastic syndrome

POEMS syndrome is a rare paraneoplastic syndrome caused by a clone of aberrant plasma cells. The name POEMS is an acronym for some of the disease's major signs and symptoms, as is PEP.

Neuromyelitis optica spectrum disorders (NMOSD) are a spectrum of autoimmune diseases characterized by acute inflammation of the optic nerve and the spinal cord (myelitis). Episodes of ON and myelitis can be simultaneous or successive. A relapsing disease course is common, especially in untreated patients.

<span class="mw-page-title-main">Stiff-person syndrome</span> Human medical condition

Stiff-person syndrome (SPS), also known as stiff-man syndrome, is a rare neurologic disorder of unclear cause characterized by progressive muscular rigidity and stiffness. The stiffness primarily affects the truncal muscles and is characterised by spasms, resulting in postural deformities. Chronic pain, impaired mobility, and lumbar hyperlordosis are common symptoms.

<span class="mw-page-title-main">Neuritis</span> Inflammation of a nerve or generally any part of the nervous system

Neuritis, from the Greek νεῦρον), is inflammation of a nerve or the general inflammation of the peripheral nervous system. Inflammation, and frequently concomitant demyelination, cause impaired transmission of neural signals and leads to aberrant nerve function. Neuritis is often conflated with neuropathy, a broad term describing any disease process which affects the peripheral nervous system. However, neuropathies may be due to either inflammatory or non-inflammatory causes, and the term encompasses any form of damage, degeneration, or dysfunction, while neuritis refers specifically to the inflammatory process.

<span class="mw-page-title-main">Chronic inflammatory demyelinating polyneuropathy</span> Medical condition

Chronic inflammatory demyelinating polyneuropathy (CIDP) is an acquired autoimmune disease of the peripheral nervous system characterized by progressive weakness and impaired sensory function in the legs and arms. The disorder is sometimes called chronic relapsing polyneuropathy (CRP) or chronic inflammatory demyelinating polyradiculoneuropathy. CIDP is closely related to Guillain–Barré syndrome and it is considered the chronic counterpart of that acute disease. Its symptoms are also similar to progressive inflammatory neuropathy. It is one of several types of neuropathy.

<span class="mw-page-title-main">Limbic encephalitis</span> Inflammation involving the limbic system in the brain

Limbic encephalitis is a form of encephalitis, a disease characterized by inflammation of the brain. Limbic encephalitis is caused by autoimmunity: an abnormal state where the body produces antibodies against itself. Some cases are associated with cancer and some are not. Although the disease is known as "limbic" encephalitis, it is seldom limited to the limbic system and post-mortem studies usually show involvement of other parts of the brain. The disease was first described by Brierley and others in 1960 as a series of three cases. The link to cancer was first noted in 1968 and confirmed by later investigators.

A paraneoplastic syndrome is a syndrome that is the consequence of a tumor in the body. It is specifically due to the production of chemical signaling molecules by tumor cells or by an immune response against the tumor. Unlike a mass effect, it is not due to the local presence of cancer cells.

Neuromuscular junction disease is a medical condition where the normal conduction through the neuromuscular junction fails to function correctly.

Ian Kirkland Hart FRCP was a lecturer and consultant in neurology at the Walton Centre in Liverpool. He ran a clinic for neurological paraneoplastic syndromes, myasthenia gravis, neuromyotonia, Lambert–Eaton myasthenic syndrome and autoimmune encephalitis. He was also the founder member of the Walton Centre Clinical Neuroimmunology Group researching on autoantibody-associated neurological diseases.

Cramp fasciculation syndrome (CFS) is a rare peripheral nerve hyperexcitability disorder. It is more severe than the related disorder known as benign fasciculation syndrome; it causes fasciculations, cramps, pain, fatigue, and muscle stiffness similar to those seen in neuromyotonia. Patients with CFS, like those with neuromyotonia, may also experience paresthesias. Most cases of cramp fasciculation syndrome are idiopathic.

Multifocal motor neuropathy (MMN) is a progressively worsening condition where muscles in the extremities gradually weaken. The disorder, a pure motor neuropathy syndrome, is sometimes mistaken for amyotrophic lateral sclerosis (ALS) because of the similarity in the clinical picture, especially if muscle fasciculations are present. MMN is thought to be autoimmune. It was first described in the mid-1980s.

<span class="mw-page-title-main">Autoimmune encephalitis</span> Type of encephalitis

Autoimmune encephalitis (AIE) is a type of encephalitis, and one of the most common causes of noninfectious encephalitis. It can be triggered by tumors, infections, or it may be cryptogenic. The neurological manifestations can be either acute or subacute and usually develop within six weeks. The clinical manifestations include behavioral and psychiatric symptoms, autonomic disturbances, movement disorders, and seizures.

<span class="mw-page-title-main">Autoimmune autonomic ganglionopathy</span> Medical condition

Autoimmune autonomic ganglionopathy is a type of immune-mediated autonomic failure that is associated with antibodies against the ganglionic nicotinic acetylcholine receptor present in sympathetic, parasympathetic, and enteric ganglia. Typical symptoms include gastrointestinal dysmotility, orthostatic hypotension, and tonic pupils. Many cases have a sudden onset, but others worsen over time, resembling degenerative forms of autonomic dysfunction. For milder cases, supportive treatment is used to manage symptoms. Plasma exchange, intravenous immunoglobulin, corticosteroids, or immunosuppression have been used successfully to treat more severe cases.

Anti-VGKC-complex encephalitis are caused by antibodies against the voltage gated potassium channel-complex (VGKC-complex) and are implicated in several autoimmune conditions including limbic encephalitis, epilepsy and neuromyotonia.

Facial onset sensory and motor neuronopathy, often abbreviated FOSMN, is a rare disorder of the nervous system in which sensory and motor nerves of the face and limbs progressively degenerate over a period of months to years. This degenerative process, the cause of which is unknown, eventually results in sensory and motor symptoms — the former consisting mainly of paresthesia followed by numbness, and the latter in muscle weakness, atrophy, and eventual paralysis. FOSM is characterized by sensory and motor loss beginning in the face and spreading to involve an increasingly larger area including the scalp, upper arms and trunk. The muscles or respiration and swallowing are commonly affected. In many ways, it is reminiscent of the much better known condition amyotrophic lateral sclerosis, with which it is closely related. There is no cure; treatment is supportive. Life expectancy may be shortened by respiratory complications arising from weakness of the muscles that aid breathing and swallowing. It was first described in four patients by Vucic and colleagues working at the Massachusetts General Hospital in the United States; subsequent reports from the United Kingdom, Europe and Asia point to a global incidence of the disease. It is thought to be exceptionally rare, with only approximately 100 individuals described to date in the medical literature.

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