Functional neurological symptom disorder can mimic many other conditions. Some alternative diagnoses for FND include:

- Hemiplegic migraine

- Multiple sclerosis

- Motor neurone disease

- Parkinson's

- Autoimmune disorders

- Ehlers–Danlos syndrome

- Stroke

- Vitamin B12 deficiency or pernicious anaemia

- Myasthenia gravis

Functional neurological disorder is a common problem, with estimates suggesting that up to a third of neurology outpatients having functional symptoms. In Scotland, around 5000 new cases of FND are diagnosed annually. Furthermore, non-epileptic seizures account for 1 in 7 referrals to neurologists after an initial seizure, and functional weakness has a similar prevalence to multiple sclerosis.

Functional somatic syndromes may occur in 6 to 36% of the population.

Whether a given medical condition is termed a "functional disorder" depends in part on the state of knowledge. Some diseases, including epilepsy, schizophrenia, and migraine headaches were once considered functional disorders, but are no longer generally classified that way.

Treatment may involve investigation, reassurance and explanation, and possibly specialist treatment such as antidepressants or cognitive behavioral therapy.

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.

A functional disorder is a medical condition that impairs the normal function of a bodily process, but where every part of the body looks completely normal under examination, dissection or even under a microscope. This stands in contrast to a structural disorder (in which some part of the body can be seen to be abnormal) or a psychosomatic disorder (in which symptoms are caused by psychological or psychiatric illness). Definitions vary somewhat between fields of medicine.

Generally, the mechanism that causes a functional disorder is unknown, poorly understood, or occasionally unimportant for treatment purposes. The brain or nerves are often believed to be involved. It is common that a person with one functional disorder will have others.

Empirical studies have found that the prognosis for conversion disorder varies widely, with some cases resolving in weeks, and others enduring for years or decades. There is also evidence that there is no cure for Conversion Disorder, and that although patients may go into remission, they can relapse at any point. Furthermore, many patients who are 'cured' continue to have some degree of symptoms indefinitely.

Information on the frequency of conversion disorder in the West is limited, in part due to the complexities of the diagnostic process. In neurology clinics, the reported prevalence of unexplained symptoms among new patients is very high (between 30 and 60%). However, diagnosis of conversion typically requires an additional psychiatric evaluation, and since few patients will see a psychiatrist it is unclear what proportion of the unexplained symptoms are actually due to conversion. Large scale psychiatric registers in the US and Iceland found incidence rates of 22 and 11 newly diagnosed cases per 100,000 person-years, respectively. Some estimates claim that in the general population, between 0.011% and 0.5% of the population have conversion disorder.

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.

Although in itself neither degenerative nor fatal, the chronic pain of fibromyalgia is pervasive and persistent. Most people with fibromyalgia report that their symptoms do not improve over time. An evaluation of 332 consecutive new people with fibromyalgia found that disease-related factors such as pain and psychological factors such as work status, helplessness, education, and coping ability had an independent and significant relationship to FM symptom severity and function.

Fibromyalgia is estimated to affect 2–8% of the population. Female are affected about twice as often as male based on criteria as of 2014.

Fibromyalgia may not be diagnosed in up to 75% of affected people.

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.

ADHD often precedes the onset of ODD, and approximately half of children with ADHD, Combined Type also have ODD. ODD is a risk factor for CD and frequently precedes the onset of CD symptoms. Children with an early onset of CD symptoms, with at least one symptom before age 10 years, are at risk for more severe and persistent antisocial behavior continuing into adulthood. Youth with early-onset conduct problems are particularly at risk for ASPD (note that an onset of CD prior to age 15 is part of the diagnostic criteria for ASPD), whereas CD is typically limited to adolescence when youth's CD symptoms begin during adolescence.

Externalizing disorders are frequently comorbid or co-occurring with other disorders. Individuals who have the co-occurrence of more than one externalizing disorder have homotypic comorbidity, whereas individuals who have co-occurring externalizing and Internalizing disorders have heterotypic comorbidity. It is not uncommon for children with early externalizing problems to develop both internalizing and further externalizing problems across the lifespan.

Functional weakness is weakness of an arm or leg due to the nervous system not working properly. It is not caused by damage or disease of the nervous system. Patients with functional weakness experience symptoms of limb weakness which can be disabling and frightening such as problems walking or a ‘heaviness’ down one side, dropping things or a feeling that a limb just doesn’t feel normal or ‘part of them’. Functional weakness may also be described as functional neurological symptom disorder (FNsD), Functional Neurological Disorder (FND) or functional neurological symptoms. If the symptoms are caused by a psychological trigger, it may be diagnosed as 'dissociative motor disorder' or conversion disorder (CD).

To the patient and the doctor it often looks as if there has been a stroke or have symptoms of multiple sclerosis. However, unlike these conditions, with functional weakness there is no permanent damage to the nervous system which means that it can get better or even go away completely.

The diagnosis should usually be made by a consultant neurologist so that other neurological causes can be excluded. The diagnosis should be made on the basis of positive features in the history and the examination (such as Hoover's sign). It is dangerous to make the diagnosis simply because tests are normal. Neurologists usually diagnose wrongly about 5% of the time (which is the same for many other conditions.)

Many patients with functional weakness suffer from not being believed. Although psychological factors can be important for a some patients, for the majority of individuals the cause of their weakness has a physical trigger such as a virus, injury or other medical condition. The symptoms of functional weakness are real, and are as disabling and distressing as Multiple Sclerosis or Parkinson's.

The most effective treatment is physiotherapy, however it is also helpful for patients to understand the diagnosis, and some may find CBT helps them to cope with the emotions associated with being unwell. For those with conversion disorder, psychological therapy is key to their treatment as it is emotional or psychological factors which are causing their symptoms.

Functional gastrointestinal disorders are very common. Globally, irritable bowel syndrome and functional dyspepsia alone may affect 16–26% of the population.

The exact cause of the disorder remains unknown, and relatively few studies have focused exclusively on the etiology of schizophreniform disorder. Like other psychotic disorders, a diathesis–stress model has been proposed, suggesting that some individuals have an underlying multifactorial genetic vulnerability to the disorder that can be triggered by certain environmental factors. Schizophreniform disorder is more likely to occur in people with family members who have schizophrenia or bipolar disorder.

Major Depression is a mental disorder characterized by an all-encompassing low mood accompanied by low self-esteem, and loss of interest or pleasure in normally enjoyable activities.Nearly 5 million of the 31 million Americans who are 65 years or older are clinically depressed, and 1 million have major depression. Approximately 3 percent of healthy elderly persons living in the community have major depression. Recurrence may be as high as 40 percent. Suicide rates are nearly twice as high in depressed patients as in the general population. Major depression is more common in medically ill patients who are older than 70 years and hospitalized or institutionalized. Severe or chronic diseases associated with high rates of depression include stroke (30 to 60 percent), coronary heart disease (8 to 44 percent), cancer (1 to 40 percent), Parkinson's disease (40 percent), Alzheimer's disease (20 to 40 percent), and dementia (17 to 31 percent).

Minor depression is a clinically significant depressive disorder that does not fulfill the duration criterion or the number of symptoms necessary for the diagnosis of major depression. Minor depression, which is more common than major depression in elderly patients, may follow a major depressive episode. It also can be a reaction to routine stressors in older populations. Fifteen to 50 percent of patients with minor depression develop major depression within two years.

Schizophreniform disorder is equally prevalent among men and women. The most common ages of onset are 18–24 for men and 18–35 for women. While the symptoms of schizophrenia often develop gradually over a period of years, the diagnostic criteria for schizophreniform disorder require a much more rapid onset.

Available evidence suggests variations in incidence across sociocultural settings. In the United States and other developed countries, the incidence is low, possibly fivefold less than that of schizophrenia. In developing countries, the incidence is substantially higher, especially for the subtype "With Good Prognostic Features". In some of these settings schizophreniform disorder may be as common as schizophrenia.

The specific molecular mechanism that underpins this movement disorder is not well known. However, most researchers suggest that it follows an autosomal dominant genetic inheritance pattern in which mutations in certain genes give rise to structural abnormalities in nervous system networks responsible for voluntary skeletal muscle movement, which, in turn, result in the functional movement abnormalities seen in patients. Despite being autosomal dominant, it is important to note that the disease has variable expressivity. That is, patients who have inherited a mutated dominant allele, along with their genetically affected parent, can be symptomatic or asymptomatic for CMM disorder. The genes that currently have evidence to be associated with CMM disorder include "DCC" (deleted in colorectal carcinoma), "DNAL4" (dynein axonemal light chain 4), and "RAD51 (recombination protein A)".

"DCC" encodes a receptor for "NTN1" (netrin-1), a protein thought to be responsible for axon guidance and neuronal cell migration during development. A mutation of this gene (including nonsense, splice site mutation, insertions, frameshift) has been identified as a possible cause for CMM disorder. Experiments in mice also support the claim that CMM disorder is associated with genetic mutations in "DCC". "Kanga" mice, lacking the P3 intracellular domain of the "DCC" receptor, show a hopping gait, moving their hind legs in a strictly paired fashion, as do kangaroos.

"DNAL4" encodes a component of dynein motor complex in commissural neurons of the corpus callosum. In contrast to "DCC", "DNAL4" is thought to have a recessive inheritance pattern for the CMM disorder. In CMM disorder patients, researchers found splice site mutations on "DNAL4", which caused skipping of exon 3, and thereby omission of 28 amino acids from "DNAL4" protein. This mutant "DNAL4" protein, in turn, could lead to faulty cross-hemisphere wiring, resulting in CMM.

"RAD51" maintains genome integrity by repairing DNA double-strand breaks through homologous recombination. "RAD51" heterozygous mutations, specifically premature termination codons, have been found in many CMM disorder patients through genome-wide linkage analysis and exome sequencing. In a mouse model, researchers also found "RAD51" products in corticospinal tract axons at the pyramidal decussation. They therefore suggest that "RAD51" might be a gene that, when haploinsufficient, causes CMM disorder in humans.

Despite identification of three prospective genes, no genotype-phenotype correlations have yet been found. That is, the severity of clinical signs and symptoms does not correlate with the type of genetic variant. Mutations in the above genes account for a total of about 35 percent of cases. Mutations in other genes that have not been identified likely account for the other cases of this disorder.

CMM has clear severe impacts on a patient’s ability to carry out daily manual tasks. It is recommended that children be placed under more forgiving school environments, allowing more time for written evaluations and limiting handwritten assignments, to ease the burden of the movement disability. Furthermore, because of patients’ inability to perform pure unilateral movements and their difficulty with tasks requiring skilled bimanual coordination, young and new members to the workforce are encouraged to consider professions that do not require complex bimanual movements, repetitive or sustained hand movements, or extensive handwriting, to reduce overuse, pain, and discomfort in upper limbs.

Because of its pronounced and obviously noticeable signs and symptoms, CMM patients can suffer social stigma, however physicians need to make it clear to parents, family, and friends that the disorder bears no relation to intellectual abilities. However, the rarity of this neurologic disease, found in one in a million people, makes its societal and cultural significance quite limited.

Late life depression refers to a major depressive episode occurring for the first time in an older person (usually over 50 or 60 years of age). Concurrent medical problems and lower functional expectations of elderly patients often obscure the degree of impairment. Typically, elderly patients with depression do not report depressed mood, but instead present with less specific symptoms such as insomnia, anorexia, and fatigue. Elderly persons sometimes dismiss less severe depression as an acceptable response to life stress or a normal part of aging.

There are three main causes of PVS (persistent vegetative state):

1. Acute traumatic brain injury

2. Non-traumatic: neurodegenerative disorder or metabolic disorder of the brain

3. Severe congenital abnormality of the central nervous system

Medical books (such as Lippincott, Williams, and Wilkins. (2007). In A Page: Pediatric Signs and Symptoms) describe several potential causes of PVS, which are as follows:

- Bacterial, viral, or fungal infection, including meningitis

- Increased intracranial pressure, such as a tumor or abscess

- Vascular pressure which causes intracranial hemorrhaging or stroke

- Hypoxic ischemic injury (hypotension, cardiac arrest, arrhythmia, near-drowning)

- Toxins such as uremia, ethanol, atropine, opiates, lead, colloidal silver

- Trauma: Concussion, contusion

- Seizure, both nonconvulsive status epilepticus and postconvulsive state (postictal state)

- Electrolyte imbalance, which involves hyponatremia, hypernatremia, hypomagnesemia, hypoglycemia, hyperglycemia, hypercalcemia, and hypocalcemia

- Postinfectious: Acute disseminated encephalomyelitis (ADEM)

- Endocrine disorders such as adrenal insufficiency and thyroid disorders

- Degenerative and metabolic diseases including urea cycle disorders, Reye syndrome, and mitochondrial disease

- Systemic infection and sepsis

- Hepatic encephalopathy

In addition, these authors claim that doctors sometimes use the mnemonic device AEIOU-TIPS to recall portions of the differential diagnosis: Alcohol ingestion and acidosis, Epilepsy and encephalopathy, Infection, Opiates, Uremia, Trauma, Insulin overdose or inflammatory disorders, Poisoning and psychogenic causes, and Shock.

In the United States, it is estimated that there may be between 15,000 and 40,000 patients who are in a persistent vegetative state, but due to poor nursing home records exact figures are hard to determine.