Occlusal factors as an etiologic factor in TMD is a controversial topic. Abnormalities of occlusion (problems with the bite) are often blamed for TMD but there is no evidence that these factors are involved. Occlusal abnormalities are incredibly common, and most people with occlusal abnormalities do not have TMD. Although occlusal features may affect observed electrical activity in masticatory muscles, there are no statistically significant differences in the number of occlusal abnormalities in people with TMD and in people without TMD. There is also no evidence for a causal link between orthodontic treatment and TMD. The modern, mainstream view is that the vast majority of people with TMD, occlusal factors are not related. Theories of occlusal factors in TMD are largely of historical interest. A causal relationship between occlusal factors and TMD was championed by Ramfjord in the 1960s. A small minority of dentists continue to prescribe occlusal adjustments in the belief that this will prevent or treat TMD despite the existence of systematic reviews of the subject which state that there is no evidence for such practices, and the vast majority of opinion being that no irreversible treatment should be carried out in TMD (see Occlusal adjustment).

TMD does not obviously run in families like a genetic disease. It has been suggested that a genetic predisposition for developing TMD (and chronic pain syndromes generally) could exist. This has been postulated to be explained by variations of the gene which codes for the enzyme catechol-O-methyl transferase (COMT) which may produce 3 different phenotypes with regards pain sensitivity. COMT (together with monoamine oxidase) is involved in breaking down catecholamines (e.g. dopamine, epinephrine, and norepinephrine). The variation of the COMT gene which produces less of this enzyme is associated with a high sensitivity to pain. Females with this variation, are at 2–3 times greater risk of developing TMD than females without this variant. However this theory is controversial since there is conflicting evidence.

In some cases Meige's syndrome can be reversed when it is caused by medication. It has been theorized that it is related to cranio-mandibular orthopedic misalignment, a condition that has been shown to cause a number of other movement disorders (Parkinon's, tourettes, and torticollis). This theory is supported by the fact that the trigeminal nerve is sensory for blink reflex, and becomes hypertonic with craniomandibular dysfunction. Palliative treatments are available, such as botulinum toxin injections.

The main symptoms involve involuntary blinking and chin thrusting. Some patients may experience excessive tongue protrusion, squinting, light sensitivity, muddled speech, or uncontrollable contraction of the platysma muscle. Some Meige's patients also have "laryngeal dystonia" (spasms of the larynx). Blepharospasm may lead to embarrassment in social situations, and oromandibular dystonia can affect speech, making it difficult to carry on the simplest conversations. This can cause difficulty in both personal and professional contexts, and in some cases may cause patients to withdraw from social situations.

The condition tends to affect women more frequently than men.

Craniomandibular osteopathy, also known as lion's jaw, is a developmental disease in dogs causing extensive bony changes in the mandible and skull. In this disease, a cyclical resorption of normal bone and replacement by immature bone occurs along the inner and outer surfaces of the affected bones. It usually occurs between the ages of 3 and 8 months. Breeds most commonly affected include the West Highland White Terrier, Scottish Terrier, Cairn Terrier, and Boston Terrier. It is rare in large-breed dogs, but it has been reported. Symptoms include firm swelling of the jaw, drooling, pain, and difficulty eating.

It is an inherited disease, especially in Westies, in which it has been recognized as an autosomal recessive trait. Canine distemper has also been indicated as a possible cause, as has "E. coli" infection, which could be why it is seen occasionally in large-breed dogs. Growth of lesions will usually stop around the age of one year, and possibly regress. This timing coincides with the normal completion of endochondral bone growth and ossification. If the disease is extensive, especially around the tympanic bulla (middle ear), then the prognosis is guarded.

A similar disease seen in young Bullmastiffs is known as calvarial hyperostotic syndrome. It is also similar to human infantile cortical hyperostosis. It is characterized by irregular, progressive bony proliferation and thickening of the cortical bone of the calvaria, which is part of the skull. Asymmetry of the lesions may occur, which makes it different from craniomandibular osteopathy. Symptoms include painful swelling of the skull, fever, and lymph node swelling. In most cases it is self-limiting.

Although the brain and spinal cord are surrounded by tough membranes, enclosed in the bones of the skull and spinal vertebrae, and chemically isolated by the blood–brain barrier, they are very susceptible if compromised. Nerves tend to lie deep under the skin but can still become exposed to damage. Individual neurons, and the neural networks and nerves into which they form, are susceptible to electrochemical and structural disruption. Neuroregeneration may occur in the peripheral nervous system and thus overcome or work around injuries to some extent, but it is thought to be rare in the brain and spinal cord.

The specific causes of neurological problems vary, but can include genetic disorders, congenital abnormalities or disorders, infections, lifestyle or environmental health problems including malnutrition, and brain injury, spinal cord injury or nerve injury. The problem may start in another body system that interacts with the nervous system. For example, cerebrovascular disorders involve brain injury due to problems with the blood vessels (cardiovascular system) supplying the brain; autoimmune disorders involve damage caused by the body's own immune system; lysosomal storage diseases such as Niemann-Pick disease can lead to neurological deterioration. The National Institutes of Health recommend considering the evaluation of an underlying celiac disease in people with unexplained neurological symptoms, particularly peripheral neuropathy or ataxia.

In a substantial minority of cases of neurological symptoms, no neural cause can be identified using current testing procedures, and such "idiopathic" conditions can invite different theories about what is occurring.

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.

BFRBs are among the most poorly understood, misdiagnosed, and undertreated groups of disorders. BFRBs may affect at least 1 out of 20 people. Trichotillomania alone is believed to affect 10 million people in the United States.

A neurological disorder is any disorder of the nervous system. Structural, biochemical or electrical abnormalities in the brain, spinal cord or other nerves can result in a range of symptoms. Examples of symptoms include paralysis, muscle weakness, poor coordination, loss of sensation, seizures, confusion, pain and altered levels of consciousness. There are many recognized neurological disorders, some relatively common, but many rare. They may be assessed by neurological examination, and studied and treated within the specialities of neurology and clinical neuropsychology.

Interventions for neurological disorders include preventative measures, lifestyle changes, physiotherapy or other therapy, neurorehabilitation, pain management, medication, or operations performed by neurosurgeons. The World Health Organization estimated in 2006 that neurological disorders and their sequelae (direct consequences) affect as many as one billion people worldwide, and identified health inequalities and social stigma/discrimination as major factors contributing to the associated disability and suffering.

There are a number of factors that could potentially contribute to the development of feeding and eating disorders of infancy or early childhood. These factors include:

- Physiological – a chemical imbalance effecting the child's appetite could cause a feeding or eating disorder.

- Developmental – developmental abnormalities in oral-sensory, oral-motor, and swallowing can impact the child's eating ability and elicit a feeding or eating disorder.

- Environmental – simple issues such as inconsistent meal times can cause a feeding or eating disorder. Giving the child food that they are not developmentally acquired for can also cause these disorders. Family dysfunction and sociocultural issues could also play a role in feeding or eating disorders.

- Relational – when the child is not securely attached to the mother, it can cause feeding interactions to become disturbed or unnatural. Other factors, such as parental emotional unavailability and parental eating disorders, can cause feeding and eating disorders in their children.

- Psychological and behavioral – these factors include one involving the child's temperament. Characteristics such as being anxious, impulsive, distracted, or strong-willed personality types are ones that could affect the child's eating and cause a disorder. The individual could have learned to reject food due to a traumatic experience such as choking or being force fed.

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)

Education, and a "watch and wait" strategy, are the only treatment needed for many, and the majority of individuals with tics do not seek treatment; treatment of tic disorders is similar to treatment of Tourette syndrome.

Osteoporosis is due to causal factors like atrophy of disuse and gonadal deficiency. Hence osteoporosis is common in post menopausal women and in men above 50 yrs. Hypercorticism may also be causal factor, as osteoporosis may be seen as a feature of Cushing's syndrome.

Metabolic bone disease is an umbrella term referring to abnormalities of bones caused by a broad spectrum of disorders.

Most commonly these disorders are caused by abnormalities of minerals such as calcium, phosphorus, magnesium or vitamin D leading to dramatic clinical disorders that are commonly reversible once the underlying defect has been treated. These disorders are to be differentiated from a larger group of genetic bone disorders where there is a defect in a specific signaling system or cell type that causes the bone disorder. There may be overlap. For example, genetic or hereditary hypophosphatemia may cause the metabolic bone disorder osteomalacia. Although there is currently no treatment for the genetic condition, replacement of phosphate often corrects or improves the metabolic bone disorder.

Physical and emotional changes are often the most indicative symptoms of feeding and eating disorders of infancy or early childhood. The child's growth and development may be delayed due to the lack of necessary nutrients. The child will usually weigh much less than other children. Withdrawal and irritability are often associated with children that are malnourished.

Tic disorders are more common among males than females.

A large, community-based study suggested that over 19% of school-age children have tic disorders; the children with tic disorders in that study were usually undiagnosed.

As many as 1 in 100 people may experience some form of tic disorder, usually before the onset of puberty. Tourette syndrome is the more severe expression of a spectrum of tic disorders, which are thought to be due to the same genetic vulnerability. Nevertheless, most cases of Tourette syndrome are not severe. Although a significant amount of investigative work indicates genetic linkage of the various tic disorders, further study is needed to confirm the relationship.

The cause of BFRBs is unknown.

Emotional variables may have a differential impact on the expression of BFRBs.

Research has suggested that the urge to repetitive self-injury is similar to a body-focused repetitive behavior but others have argued that for some the condition is more akin to a substance abuse disorder.

Researchers are investigating a possible genetic component.

Movement disorders are clinical syndromes with either an excess of movement or a paucity of voluntary and involuntary movements, unrelated to weakness or spasticity. Movement disorders are synonymous with basal ganglia or extrapyramidal diseases. Movement disorders are conventionally divided into two major categories- "hyperkinetic" and "hypokinetic".

Hyperkinetic movement disorders refer to dyskinesia, or excessive, often repetitive, involuntary movements that intrude upon the normal flow of motor activity.

Hypokinetic movement disorders refer to akinesia (lack of movement), hypokinesia (reduced amplitude of movements), bradykinesia (slow movement) and rigidity. In primary movement disorders, the abnormal movement is the primary manifestation of the disorder. In secondary movement disorders, the abnormal movement is a manifestation of another systemic or neurological disorder.

Genetic disorders may also be complex, multifactorial, or polygenic, meaning they are likely associated with the effects of multiple genes in combination with lifestyles and environmental factors. Multifactorial disorders include heart disease and diabetes. Although complex disorders often cluster in families, they do not have a clear-cut pattern of inheritance. This makes it difficult to determine a person’s risk of inheriting or passing on these disorders. Complex disorders are also difficult to study and treat, because the specific factors that cause most of these disorders have not yet been identified. Studies which aim to identify the cause of complex disorders can use several methodological approaches to determine genotype-phenotype associations. One method, the genotype-first approach, starts by identifying genetic variants within patients and then determining the associated clinical manifestations. This is opposed to the more traditional phenotype-first approach, and may identify causal factors that have previously been obscured by clinical heterogeneity, penetrance, and expressivity.

On a pedigree, polygenic diseases do tend to "run in families", but the inheritance does not fit simple patterns as with Mendelian diseases. But this does not mean that the genes cannot eventually be located and studied. There is also a strong environmental component to many of them (e.g., blood pressure).

- asthma

- autoimmune diseases such as multiple sclerosis

- cancers

- ciliopathies

- cleft palate

- diabetes

- heart disease

- hypertension

- inflammatory bowel disease

- intellectual disability

- mood disorder

- obesity

- refractive error

- infertility

Anorectal disorders are painful but common conditions like hemorrhoids, tears, fistulas, or abscesses that affect the anal region. Most people experience some form of anorectal disorder during their lifetime. Primary care physicians can treat most of these disorders, however, high-risk individuals include those with HIV, roughly half of whom need surgery to remedy the disorders.

Because these disorders afflict the rectum, people are often embarrassed or afraid to confer with a medical professional.

Treatment depends upon the underlying disorder. Movement disorders have been known to be associated with a variety of autoimmune diseases.

There is considerable research into the causes, diagnosis and treatments for FGIDs. Diet, microbiome, genetics, neuromuscular function and immunological response all interact. Heightened mast cell activation has been proposed to be a common factor among FGIDs, contributing to visceral hypersensitivity as well as epithelial, neuromuscular, and motility dysfunction.

Motor disorders are disorders of the nervous system that cause abnormal and involuntary movements. They can result from damage to the motor system.

Motor disorders are defined in the fifth edition of the "Diagnostic and Statistical Manual of Mental Disorders" (DSM-5) – published in 2013 to replace the fourth text revision (DSM-IV-TR) – as a new sub-category of neurodevelopmental disorders. The DSM-5 motor disorders include developmental coordination disorder, stereotypic movement disorder, and the tic disorders including Tourette syndrome.

In utero exposure to cocaine and other street drugs can lead to agenesis of corpus callosum.

Due to the wide range of genetic disorders that are presently known, diagnosis of a genetic disorder is widely varied and dependent of the disorder. Most genetic disorders are diagnosed at birth or during early childhood, however some, such as Huntington's disease, can escape detection until the patient is well into adulthood.

The basic aspects of a genetic disorder rests on the inheritance of genetic material. With an in depth family history, it is possible to anticipate possible disorders in children which direct medical professionals to specific tests depending on the disorder and allow parents the chance to prepare for potential lifestyle changes, anticipate the possibility of stillbirth, or contemplate termination. Prenatal diagnosis can detect the presence of characteristic abnormalities in fetal development through ultrasound, or detect the presence of characteristic substances via invasive procedures which involve inserting probes or needles into the uterus such as in amniocentesis.