LEMS is often associated with lung cancer (50–70%), specifically small-cell carcinoma, making LEMS a paraneoplastic syndrome. Of the people with small-cell lung cancer, 1–3% have LEMS. In most of these cases, LEMS is the first symptom of the lung cancer, and it is otherwise asymptomatic.

LEMS may also be associated with autoimmune diseases, such as hypothyroidism (an underactive thyroid gland) or diabetes mellitus type 1. Myasthenia gravis, too, may happen in the presence of tumors (thymoma, a tumor of the thymus in the chest); people with MG without a tumor and people with LEMS without a tumor have similar genetic variations that seem to predispose them to these diseases. HLA-DR3-B8 (an HLA subtype), in particular, seems to predispose to LEMS.

Lambert–Eaton myasthenic syndrome (LEMS) is a rare autoimmune disorder characterized by muscle weakness of the limbs. It is the result of an autoimmune reaction in which antibodies are formed against presynaptic voltage-gated calcium channels, and likely other nerve terminal proteins, in the neuromuscular junction (the connection between nerves and the muscle that they supply). The prevalence is 3.4 cases per million. Around 60% of those with LEMS have an underlying malignancy, most commonly small-cell lung cancer; it is therefore regarded as a paraneoplastic syndrome (a condition that arises as a result of cancer elsewhere in the body).

LEMS usually occurs in people over 40 years of age, but may occur at any age. The diagnosis is usually confirmed with electromyography and blood tests; these also distinguish it from myasthenia gravis, a related autoimmune neuromuscular disease.

If the disease is associated with cancer, direct treatment of the cancer often relieves the symptoms of LEMS. Other treatments often used are steroids, azathioprine, which suppress the immune system, intravenous immunoglobulin, which outcompetes autoreactive antibody for Fc receptors, and pyridostigmine and 3,4-diaminopyridine, which enhance the neuromuscular transmission. Occasionally, plasma exchange is required to remove the antibodies.

Neurotoxin may act on the neuromuscular junction either post synaptically or presynaptically as there are several different forms of toxins that the NMJ is sensitive to.(reference 14) Common mechanisms of action include blockage of acetylcholine release at the synapse thus causing the NMJ to become abnormal in function.(reference 12)

Myasthenia gravis is the most common neuromuscular disease affecting function of the end plate in patients. It is present in 100 people out of 1,000,000 in the population, and its onset is usually in either younger or older individuals.(reference 14)

Acquired myasthenia gravis is the most common neuromuscular junction disease.(reference 7) Important observations were made by Patrick and Lindstrom in 1973 when they found that antibodies attacking the acetylcholine receptors were present in around 85% of cases of myasthenia gravis.(reference renamed form 13)(reference 36) The remaining diseases were also a result of antibody attacks on vital proteins, but instead of the acetylcholine receptor, the culprits were MuSK, a muscle-specific serum kinase, and lipoprotein receptor-related protein.(reference 36) So these mechanisms describe myasthenia gravis that is acquired, and not congenital, affecting these vital proteins by an immunological response against self-antigens. The cases not caused by antibodies against the acetylcholine receptors became by convention called seronegative myasthenia gravis.(reference 37) The term seronegative came about because scientists would be testing for acetylcholine receptor antibodies in patients that had myasthenia gravis resulting in negative tests in the serum. This does not imply that there are no antibodies present, but this terminology only became present because scientists were testing for the wrong antigen.(reference 36)(reference 38)

Neonatal myasthenia gravis is a very rare condition in which a mother with myasthenia gravis passes down her antibodies to her infant through the placenta, causing the it to be born with antibodies that will attach self-antigens.(reference 12)

Drug-induced myasthenia gravis is also a very rare condition in which pharmacological drugs cause a blockade or disruption of the NMJ machinery.(reference 12) Robert W. Barrons summarizes the possible causes of drug-induced myasthenia gravis: "Prednisone was most commonly implicated as aggravating myasthenia gravis, and D-penicillamine was most commonly associated with myasthenic syndrome. The greatest frequency of drug-induced neuromuscular blockade was seen with aminoglycoside-induced postoperative respiratory depression. However, drugs most likely to impact myasthenic patients negatively are those used in the treatment of the disease. These include overuse of anticholinesterase drugs, high-dose prednisone, and anesthesia and neuromuscular blockers for thymectomy."(reference 39)

The prognosis of MG patients is generally good, as is quality of life, given very good treatment. In the early 1900s, the mortality associated with MG was 70%; now, that number is estimated to be around 3–5%, which is attributed to increased awareness and medications to manage symptoms. Monitoring of a person with MG is very important, as at least 20% of people diagnosed with it will experience a myasthenic crisis within two years of their diagnosis, requiring rapid medical intervention. Generally, the most disabling period of MG might be years after the initial diagnosis.

Myasthenia gravis occurs in all ethnic groups and both sexes. It most commonly affects women under 40 and people from 50 to 70 years old of either sex, but it has been known to occur at any age. Younger patients rarely have thymoma. The prevalence in the United States is estimated at between 0.5 and 20.4 cases per 100,000, with an estimated 60,000 Americans affected. Within the United Kingdom, an estimated 15 cases of MG occur per 100,000 people.

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.

The three causes of NMT are:

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. 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. 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. Moreover, these antibodies have been demonstrated to reduce potassium channel function in neuronal cell lines.

Prostate cancer is the second most common urological malignancy to be associated with paraneoplastic syndromes after renal cell carcinoma. Paraneoplastic syndromes of this nature tend to occur in the setting of late stage and aggressive tumors with poor overall outcomes (endocrine manifestations, neurological entities, dermatological conditions, and other syndromes). A vast majority of prostate cancer cases (over 70%) document paraneoplastic syndrome as a major clinical manifestation of prostate cancer; and interestingly (under 20%), the syndrome as an initial sign of disease progression to the castrate-resistant state. Urologist researchers identify serum markers that are associated with the syndrome in order to specific what type of therapies may work most effectively.

Paraneoplastic neurological syndromes may be related immune checkpoint inhibitors (ICIs), one of the underlying causes in inflammatory central nervous system diseases (CNS). The central idea around such research pinpoints treatment strategies to combat cancer related outcomes in the clinical arena, specifically ICIs. Research suggests that patients who are treated with ICIs are more susceptible to CNS disease (since the mechanism of ICIs induces adverse effects on the CNS due to augmented immune responses and neurotoxicity). The purpose of this exploration was to shed light on immunotherapies and distinguishing between neurotoxicity and brain metastasis in the early stages of treatment. In other research, scientists have found that paraneoplastic peripheral nerve disorders (autoantibodies linked to multifocal motor neuropathy) may provide important clinical manifestations. This is especially important for patients who experience inflammatory neuropathies since solid tumors are often associated with peripheral nerve disorders. CV2 autoantibodies, which target dihydropyriminase-related protein 5 (DRP5, or CRMP5) are also associated with a variety of paraneoplastic neurological syndromes, including sensorimotor polyneuropathies. Interestingly, patients undergoing immune therapies or tumor removal respond very well to antibodies that target CASPR2 (to treat nerve hyperexcitability and neuromyotonia).

In the case of paraneoplastic Cushing's syndrome arising from a small cel carcinoma of the endometrium, paraneoplastic syndrome has been seen to interfere with standard treatments and lead to unexpected complications and clinical course. The purpose of this clinical case demonstrates the aggressive nature of the neuroendocrine small cell carcinoma with rapid invasion and extra-uterine spread. The researchers raise recognition for timely recognition of paraneoplastic syndrome, which in this particular case use a combinatorial therapy of etoposide and cisplatin chemotherapy to save the 32-year old female patient's life (presented with persistent migraine-like headache, palpitations, progressive nausea and vomiting, photo- and sonobia, menometrorrhagia and concomitant general fatigue).

The mechanism for paraneoplastic syndrome varies from case to case. However, pathophysiological outcomes usually arise from when a tumor arises. Paraneoplastic syndrome often occurs alongside associated cancers as a result of activated immune systems. In this scenario, the body may produce antibodies to fight off the tumor by directly binding and destroying the tumor cell. Paraneoplastic disorders may arise in that antibodies would cross-react with normal tissues and destroy them.

In the United States, sarcoidosis has a prevalence of approximately 10 cases per 100,000 whites and 36 cases per 100,000 blacks. Heerfordt syndrome is present in 4.1–5.6% of those with sarcoidosis.

Harlequin syndrome is not debilitating so treatment is not normally necessary. In cases where the individual may feel socially embarrassed, contralateral sympathectomy may be considered, although compensatory flushing and sweating of other parts of the body may occur. In contralateral sympathectomy, the nerve bundles that cause the flushing in the face are interrupted. This procedure causes both sides of the face to no longer flush or sweat. Since symptoms of Harlequin syndrome do not typically impair a person’s daily life, this treatment is only recommended if a person is very uncomfortable with the flushing and sweating associated with the syndrome.

One possible cause of Harlequin syndrome is a lesion to the preganglionic or postganglionic cervical sympathetic fibers and parasympathetic neurons of the ciliary ganglion. It is also believed that torsion (twisting) of the thoracic spine can cause blockage of the anterior radicular artery leading to Harlequin syndrome. The sympathetic deficit on the denervated side causes the flushing of the opposite side to appear more pronounced. It is unclear whether or not the response of the undamaged side was normal or excessive, but it is believed that it could be a result of the body attempting to compensate for the damaged side and maintain homeostasis.

Since the cause and mechanism of Harlequin syndrome is still unknown, there is no way to prevent this syndrome.

Congenital myasthenic syndrome (CMS) is an inherited neuromuscular disorder caused by defects of several types at the neuromuscular junction. The effects of the disease are similar to Lambert-Eaton Syndrome and myasthenia gravis, the difference being that CMS is not an autoimmune disorder.

Neuromuscular disease is a very broad term that encompasses many diseases and ailments that impair the functioning of the muscles, either directly, being pathologies of the voluntary muscle, or indirectly, being pathologies of nerves or neuromuscular junctions.

Neuromuscular diseases are those that affect the muscles and/or their direct nervous system control, problems with central nervous control can cause either spasticity or some degree of paralysis (from both lower and upper motor neuron disorders), depending on the location and the nature of the problem. Some examples of central disorders include cerebrovascular accident, Parkinson's disease, multiple sclerosis, Huntington's disease and Creutzfeldt–Jakob disease. Spinal muscular atrophies are disorders of lower motor neuron while amyotrophic lateral sclerosis is a mixed upper and lower motor neuron condition.

In patients that have already been diagnosed with sarcoidosis, Heerfordt syndrome can be inferred from the major symptoms of the syndrome, which include parotitis, fever, and facial nerve palsy. In cases of parotitis, ultrasound-guided biopsy is used to exclude the possibility of lymphoma. There are many possible causes of facial nerve palsy, including Lyme disease, HIV, Melkersson–Rosenthal syndrome, schwannoma, and Bell's palsy. Heerfordt syndrome exhibits spontaneous remission. Treatments for sarcoidosis include corticosteroids and immunosuppressive drugs.

Neuromuscular disease can be caused by autoimmune disorders, genetic/hereditary disorders and some forms of the collagen disorder Ehlers–Danlos Syndrome, exposure to environmental chemicals and poisoning which includes heavy metal poisoning. The failure of the electrical insulation surrounding nerves, the myelin, is seen in certain deficiency diseases, such as the failure of the body's system for absorbing vitamin B-12

Diseases of the motor end plate include myasthenia gravis, a form of muscle weakness due to antibodies against acetylcholine receptor, and its related condition Lambert-Eaton myasthenic syndrome (LEMS). Tetanus and botulism are bacterial infections in which bacterial toxins cause increased or decreased muscle tone, respectively.Muscular dystrophies, including Duchenne's and Becker's, are a large group of diseases, many of them hereditary or resulting from genetic mutations, where the muscle integrity is disrupted, they lead to progressive loss of strength and decreased life span.

Further causes of neuromuscular diseases are :

Inflammatory muscle disorders

- Polymyalgia rheumatica (or "muscle rheumatism") is an inflammatory condition that mainly occurs in the elderly; it is associated with giant-cell arteritis(It often responds to prednisolone).

- Polymyositis is an autoimmune condition in which the muscle is affected.

- Rhabdomyolysis is the breakdown of muscular tissue due to any cause.

Tumors

- Smooth muscle: leiomyoma (benign)

- Striated muscle: rhabdomyoma (benign)

CMS is associated with genetic defects that affect proteins of the neuromuscular junction. Postsynaptic defects are the most frequent cause of CMS and often result in abnormalities in the acetylcholine receptor (AChR). In the neuromuscular junction there is a vital pathway that maintains synaptic structure and results in the aggregation and localization of AChR on the postsynaptic folds. This pathway consists of agrin, muscle-specific tyrosine kinase (MuSK), acetylcholine receptors (AChRs) and the AChR-clustering protein rapsyn, encoded by the RAPSN gene. The vast majority of mutations causing CMS are found in the AChR subunits and rapsyn genes.

Out of all mutations associated with CMS, more than half are mutations in one of the four genes encoding the adult acetylcholine receptor (AChR) subunits. Mutations of the AChR often result in endplate deficiency. Most of the mutations of the AChR are mutations of the CHRNE gene. The CHRNE gene codes for the epsilon subunit of the AChR. Most mutations are autosomal recessive loss-of-function mutations and as a result there is endplate AChR deficiency. CHRNE is associated with changing the kinetic properties of the AChR. One type of mutation of the epsilon subunit of the AChR introduces an Arginine into the binding site at the α/ε subunit interface of the receptor. The addition of a cationic Arg into the anionic environment of the AChR binding site greatly reduces the kinetic properties of the receptor. The result of the newly introduced Arg is a 30-fold reduction of agonist affinity, 75-fold reduction of gating efficiency, and an extremely weakened channel opening probability. This type of mutation results in an extremely fatal form of CMS.

Another common underlying mechanism of CMS is the mutation of the rapsyn protein, coded by the RAPSN gene. Rapsyn interacts directly with the AChRs and plays a vital role in agrin-induced clustering of the AChR. Without rapsyn, functional synapses cannot be created as the folds do not form properly. Patients with CMS-related mutations of the rapsyn protein typically are either homozygous for N88K or heterozygous for N88K and a second mutation. The major effect of the mutation N88K in rapsyn is to reduce the stability of AChR clusters. The second mutation can be a determining factor in the severity of the disease.

Studies have shown that most patients with CMS that have rapsyn mutations carry the common mutation N88K on at least one allele. However, research has revealed that there is a small population of patients who do not carry the N88K mutation on either of their alleles, but instead have different mutations of the RAPSN gene that codes for rapsyn on both of their alleles. Two novel missense mutations that have been found are R164C and L283P and the result is a decrease in co-clustering of AChR with raspyn. A third mutation is the intronic base alteration IVS1-15C>A and it causes abnormal splicing of RAPSN RNA. These results show that diagnostic screening for CMS mutations of the RAPSN gene cannot be based exclusively on the detection of N88K mutations

Dok-7 is a postsynaptic protein that binds and activates MuSK protein, which then leads to AChR clustering and typical folding of the postsynaptic membrane. Mutations of Dok-7 are another underlying mechanism of postsynaptic CMS.

Peripheral Myelin Protein 22 gene encodes a 22-kD protein that comprises 2 to 5% of peripheral nervous system myelin, it is located on chromosome locus 17p12

Overlap with Charcot-Marie-Tooth disease type 1A has been found in "Gly94fsX222 (c.281_282insG)", due to point mutations of PMP 22 that occur in a minority of cases of hereditary neuropathy with liability to pressure palsy. The point mutations -missense, nonsense and splice-site have each been alluded to in HNPP.

Hereditary neuropathy with liability to pressure palsy is an autosomal dominant genetic disease (which means one parent must be affected). A mutation in one copy of the gene PMP-22 (Peripheral myelin protein 22, 17p11.2) that makes the peripheral myelin protein causes haploinsufficiency, where the activity of the normal gene is insufficient to compensate for the loss of function of the other gene.

Hyporeflexia refers to below normal or absent reflexes (areflexia). It can be detected through the use of a reflex hammer. It is the opposite of hyperreflexia.

Hyporeflexia is generally associated with a lower motor neuron deficit (at the alpha motor neurons from spinal cord to muscle), whereas hyperreflexia is often attributed to upper motor neuron lesions (along the long, motor tracts from the brain). The upper motor neurons are thought to inhibit the reflex arc, which is formed by sensory neurons from intrafusal fibers of muscles, lower motor neurons (including alpha and gamma motor fibers) and appurtenant interneurons. Therefore, damage to lower motor neurons will subsequently result in hyporeflexia and/or areflexia.

Note that, in spinal shock, which is commonly seen in the transection of the spinal cord (Spinal cord injury), areflexia can transiently occur below the level of the lesion and can , after some time, become hyperreflexic. Furthermore, cases of severe muscle atrophy or destruction could render the muscle too weak to show any reflex and should not be confused with a neuronal cause.

Hyporeflexia may have other causes, including hypothyroidism, electrolyte imbalance (e.g. excess magnesium), drug induced (e.g. the symptoms of benzodiazepine intoxication include confusion, slurred speech, ataxia, drowsiness, dyspnea, and hyporeflexia).

Diseases associated with hyporeflexia include

- Centronuclear myopathy

- Guillain–Barré syndrome

- Lambert-Eaton myasthenic syndrome

- Polyneuropathy (Achilles and plantar reflexes)

The Kocher–Debré–Semelaigne syndrome is hypothyroidism in infancy or childhood characterised by lower extremity or generalized muscular hypertrophy, myxoedema, short stature and cretinism. The absence of painful spasms and pseudomyotonia differentiates this syndrome from its adult form, which is Hoffmann syndrome.

The syndrome is named after Emil Theodor Kocher, Robert Debré and Georges Semelaigne.

Also known as Debre–Semelaigne syndrome or cretinism-muscular hypertrophy, hypothyroid myopathy, hypothyroidism-large muscle syndrome, hypothyreotic muscular hypertrophy in children, infantile myxoedema-muscular hypertrophy, myopathy-myxoedema syndrome, myxoedema-muscular hypertrophy syndrome, myxoedema-myotonic dystrophy syndrome.

Kocher-Debre-Semelaigne syndrome gives infant a Hercules appearance.

Romano–Ward syndrome is inherited in an autosomal dominant pattern. It is the most common form of inherited long QT syndrome, affecting an estimated 1 in 7,000 people worldwide. It should be mentioned that "long QT syndrome" has 6 different variations, therefore Romano–Ward syndrome is one of many

Respiratory complications are often cause of death in early infancy.

Romano–Ward syndrome is the major variant of "long QT syndrome". It is a condition that causes a disruption of the heart's normal rhythm. This disorder is a form of long QT syndrome, which is a heart condition that causes the cardiac muscle to take longer than usual to recharge between beats; if untreated, the irregular heartbeats can lead to fainting, seizures, or sudden death