The hallmark sign of POTS is a measured increase in heart rate by at least 30 beats per minute within 10 minutes of assuming an upright position. For people aged between 12 and 19, the minimum increase for diagnosis is 40 beats per minute. This symptom is known as orthostatic (upright) tachycardia (fast heart rate). It occurs without any coinciding drop in blood pressure, as that would indicate orthostatic hypotension. It should be noted, however, that certain medications to treat POTS may cause orthostatic hypotension. It is accompanied by other features of orthostatic intolerance—symptoms which develop in an upright position and are relieved by reclining. These orthostatic symptoms include palpitations, light-headedness, chest discomfort, shortness of breath, nausea, weakness or "heaviness" in the lower legs, blurred vision and cognitive difficulties. Symptoms may be exacerbated with prolonged sitting, prolonged standing, alcohol, heat, exercise, or eating a large meal.

In up to one third of people with POTS, fainting occurs in response to postural changes or exercise. Migraine-like headaches are common, sometimes with symptoms worsening in an upright position (orthostatic headache). Some people with POTS develop acrocyanosis, or blotchy, red/blue skin upon standing, especially over the feet (indicative of blood pooling). 48% of people with POTS report chronic fatigue and 32% report sleep disturbances. Others exhibit only the cardinal symptom of orthostatic tachycardia.

POTS can co-occur in all types of Ehlers–Danlos syndrome (EDS), a hereditary connective tissue disorder marked by loose hypermobile joints prone to subluxations and dislocations, skin that exhibits moderate or greater laxity, easy bruising, and many other symptoms. A trifecta of POTS, EDS, and Mast Cell Activation Syndrome (MCAS) is becoming increasingly more common, with a genetic marker common among all three conditions. POTS is also often accompanied by vasovagal syncope, with a 25% overlap being reported. There is significant overlap between POTS and chronic fatigue syndrome, with evidence of POTS in 25–50% of CFS cases. Fatigue and reduced exercise tolerance are prominent symptoms of both conditions, and dysautonomia may underlie both conditions.

Postural orthostatic tachycardia syndrome (POTS) is a condition in which a change from lying to standing causes an abnormally large increase in heart rate. This occurs with symptoms that may include lightheadedness, trouble thinking, blurry vision, or weakness. Other commonly associated conditions include irritable bowel disease, insomnia, chronic headaches, Ehlers-Danlos syndrome, fibromyalgia.

The cause of POTS is poorly understood. Often it begins after a viral infection, surgery, or pregnancy. Risk factors include a family history of the condition. Diagnosis in adults is based on an increase in heart rate of more than 30 beats per minute within ten minutes of standing up which is accompanied by symptoms. Low blood pressure with standing, however, does not occur. Other conditions which can cause similar symptoms, such as prolonged bedrest, dehydration, hyperthyroidism, anemia, and certain medications, must not be present.

Treatment may include avoiding factors that bring on symptoms, increasing dietary salt and water, compression stockings, exercise, cognitive behavioral therapy, and medications. Medications used may include beta blockers, pyridostigmine, midodrine, or fludrocortisone. More than 50% of people whose condition was triggered by a viral infection get better within five years. About 90% get better with treatment. It is estimated that 0.5 to 3 million people are affected in the United States. The average age of onset is 20 years old and it occurs more often in females.

Symptoms of Da Costa's syndrome include fatigue upon exertion, shortness of breath, palpitations, sweating, and chest pain. Physical examination reveals no physical abnormalities causing the symptoms.

Da Costa's syndrome is generally considered a physical manifestation of an anxiety disorder.

The symptoms of Dysautonomia, which are numerous and vary widely for each individual, are due to inefficient or unbalanced efferent signals sent via both systems. The primary symptoms in individuals with dysautonomia include

Dysautonomia or autonomic dysfunction is a condition in which the autonomic nervous system (ANS) does not work properly. This may affect the functioning of the heart, bladder, intestines, sweat glands, pupils, and blood vessels. Dysautonomia has many causes, not all of which may be classified as neuropathic. A number of diseases can feature dysautonomia, such as Parkinson's disease, multiple system atrophy, autonomic failure, postural orthostatic tachycardia syndrome, and autonomic neuropathy.

The diagnosis is achieved through functional testing of the ANS, focusing on the affected organ system. Investigations may be performed to identify underlying disease processes that may have led to the development of symptoms or autonomic neuropathy. Symptomatic treatment is available for many symptoms associated with dysautonomia, and some disease processes can be directly treated .

Upper airway resistance syndrome or UARS is a common sleep disorder characterized by the narrowing of the airway that can cause disruptions to sleep. The primary symptoms include excessive fatigue and chronic insomnia. UARS can be diagnosed by polysomnography, and can be treated with lifestyle changes, dental devices, or CPAP therapy. UARS is similar to certain types of sleep apneas.

Symptoms of UARS are similar to those of obstructive sleep apnea, but are usually less severe. Fatigue, daytime sleepiness, unrefreshing sleep, and frequent awakenings during sleep are the most common symptoms.

Many patients experience chronic insomnia that creates both a difficulty falling asleep and staying asleep. As a result, patients typically experience frequent sleep disruptions. Loud snoring also serves as a possible indicator of the syndrome, but is not a symptom required for diagnosis.

Some patients experience hypotension, which may cause lightheadedness, and patients with UARS are also more likely to experience headaches and irritable bowel syndrome.

For many people cardiomegaly is asymptomatic. For others, if the enlarged heart begins to affect the body's ability to pump blood effectively, then symptoms associated with congestive heart failure may arise.

- Heart palpitations – irregular beating of the heart, usually associated with a valve issue inside the heart.

- Severe shortness of breath (especially when physically active) – irregularly unable to catch one's breath.

- Chest pain

- Fatigue

- Swelling in legs

- Increased abdominal girth

- Weight gain

- Edema – swelling

- Fainting

Functional somatic syndrome is characterized by ambiguous, non-specific symptoms that appear in otherwise-healthy populations. Overlap exists in different syndromes, including gastrointestinal issues, pain, fatigue, cognitive difficulties, and sleep difficulties. Some have proposed to group symptoms into clusters or into one general functional somatic disorder given the finding of correlations between symptoms and underlying etiologies.

Functional somatic syndrome is a term used to refer to physical symptoms that are poorly explained. It encompass disorders such as chronic fatigue syndrome, fibromyalgia, chronic widespread pain, temporomandibular disorder, irritable bowel syndrome, lower back pain, tension headache, atypical face pain, non-cardiac chest pain, insomnia, palpitation, dyspepsia, and dizziness. General overlap exists between this term, somatization, and somatoform.

The symptoms and signs of hypertensive heart disease will depend on whether or not it is accompanied by heart failure. In the absence of heart failure, hypertension, with or without enlargement of the heart (left ventricular hypertrophy) is usually symptomless.

Symptoms, signs and consequences of Congestive heart failure can include:

- Fatigue

- Irregular pulse or palpitations

- Swelling of feet and ankles

- Weight gain

- Nausea

- Shortness of breath

- Difficulty sleeping flat in bed (orthopnea)

- Bloating and abdominal pain

- Greater need to urinate at night

- An enlarged heart (cardiomegaly)

- Left ventricular hypertrophy and left ventricular remodeling

- Diminished coronary flow reserve and silent myocardial ischemia

- Coronary heart disease and accelerated atherosclerosis

- Heart Failure With Normal Left Ventricular Ejection Fraction (HFNEF), often termed diastolic heart failure

- Atrial fibrillation, other cardiac arrhythmias, or sudden cardiac death

Heart failure can develop insidiously over time or patients can present acutely with acute heart failure or acute decompensated heart failure and pulmonary edema due to sudden failure of pump function of the heart. Sudden failure can be precipitated by a variety of causes, including myocardial ischemia, marked increases in blood pressure, or cardiac arrhythmias.

Cardiomegaly is a medical condition in which the heart is enlarged. It is more commonly referred to as an enlarged heart. The causes of cardiomegaly may vary. Many times this condition results from high blood pressure (hypertension) or coronary artery disease. An enlarged heart may not pump blood effectively, resulting in congestive heart failure. Cardiomegaly may improve over time, but many people with an enlarged heart need lifelong treatment with medications. Having an immediate family member who has or had cardiomegaly may indicate that a person is more susceptible to getting this condition. Cardiomegaly is not a disease but rather a condition that can result from a host of other diseases such as obesity or coronary artery disease. Recent studies suggest that cardiomegaly is associated with a higher risk of sudden cardiac death (SCD).

Affected individuals typically experience limited relief from standard pain relieving medication, with the exception of some neuroleptic agents. Patients frequently experience 'pseudovisceral' phenomena or symptoms of altered autonomic nervous system function including nausea, bloating, abdominal swelling, loss of appetite with consecutively lowered body weight or an altered defecation process.

Pain is typically related to tensing the abdominal wall muscles, so any type of movement is prone to aggravate pain. Lying quietly can be the least painful position. Most patients report that they cannot sleep on the painful side.

Other conditions can share features with hypertensive heart disease and need to be considered in the differential diagnosis. For example:

- Coronary artery disease or ischemic heart disease due to atherosclerosis

- Hypertrophic cardiomyopathy

- Left ventricular hypertrophy in athletes

- Congestive heart failure or heart failure with normal ejection fraction due to other causes

- Atrial fibrillation or other disorders of cardiac rhythm due to other causes

- Sleep apnea

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.

Irritable Bowel Syndrome (IBS),

Fibromyalgia (FMS),

Chronic Fatigue Syndrome (CFS),

Chronic Pelvic Pain (CPP),

Interstitial Cystitis (IC),

Temporomandibular Joint Pain (TMJ), Functional Neurological Symptom Disorder (FNsD),

Non-Cardiac Chest Pain (NCCP),

Post-Traumatic Stress Disorder (PTSD),

Dysuria (Pain On Urination),

and Multiple Chemical Sensitivity

Anterior cutaneous nerve entrapment syndrome (ACNES) is a nerve entrapment condition that causes chronic pain of the abdominal wall. It occurs when nerve endings of the lower thoracic intercostal nerves (7–12) are 'entrapped' in abdominal muscles, causing a severe localized nerve (neuropathic) pain that is usually experienced at the front of the abdomen.

ACNES syndrome is frequently overlooked and unrecognized, although the incidence is estimated to be 1:2000 patients.

The relative unfamiliarity with this condition often leads to significant diagnostic delays and misdiagnoses, often resulting in unnecessary diagnostic interventions and futile procedures. Physicians often misdiagnose ACNES as irritable bowel syndrome or "functional disorders", as symptoms of the condition are not dispositive.

The defining symptoms of fibromyalgia are chronic widespread pain, fatigue, sleep disturbance, and heightened pain in response to tactile pressure (allodynia). Other symptoms may include tingling of the skin (paresthesias), prolonged muscle spasms, weakness in the limbs, nerve pain, muscle twitching, palpitations, and functional bowel disturbances.

Many people experience cognitive dysfunction (known as "fibrofog"), which may be characterized by impaired concentration, problems with short and long-term memory, short-term memory consolidation, impaired speed of performance, inability to multi-task, cognitive overload, and diminished attention span. Fibromyalgia is often associated with anxiety and depressive symptoms.

Other symptoms often attributed to fibromyalgia that may be due to a comorbid disorder include myofascial pain syndrome, also referred to as chronic myofascial pain, diffuse non-dermatomal paresthesias, functional bowel disturbances and irritable bowel syndrome, genitourinary symptoms and interstitial cystitis, dermatological disorders, headaches, myoclonic twitches, and symptomatic hypoglycemia. Although fibromyalgia is classified based on the presence of chronic widespread pain, pain may also be localized in areas such as the shoulders, neck, low back, hips, or other areas. Many sufferers also experience varying degrees of myofascial pain and have high rates of comorbid temporomandibular joint dysfunction. 20–30% of people with rheumatoid arthritis and systemic lupus erythematosus may also have fibromyalgia.

Fibromyalgia is classed as a disorder of pain processing due to abnormalities in how pain signals are processed in the central nervous system. The American College of Rheumatology classifies fibromyalgia as being a functional somatic syndrome. The expert committee of the European League Against Rheumatism classifies fibromyalgia as a neurobiological disorder and as a result exclusively give pharmacotherapy their highest level of support. The International Classification of Diseases (ICD-10) lists fibromyalgia as a diagnosable disease under "Diseases of the musculoskeletal system and connective tissue," under the code M79-7, and states that fibromyalgia syndrome should be classified as a functional somatic syndrome rather than a mental disorder. Although mental disorders and some physical disorders commonly are co-morbid with fibromyalgia – especially anxiety, depression, irritable bowel syndrome, and chronic fatigue syndrome – the ICD states that these should be diagnosed separately.

Differences in psychological and autonomic nervous system profiles among affected individuals may indicate the existence of fibromyalgia subtypes. A 2007 review divides individuals with fibromyalgia into four groups as well as "mixed types":

1. "extreme sensitivity to pain but no associated psychiatric conditions" (may respond to medications that block the 5-HT3 receptor)

2. "fibromyalgia and comorbid, pain-related depression" (may respond to antidepressants)

3. "depression with concomitant fibromyalgia syndrome" (may respond to antidepressants)

4. "fibromyalgia due to somatization" (may respond to psychotherapy)

The winter-over syndrome is a condition found in individuals who "winter-over" throughout the Antarctic (or Arctic) winter. It has been observed in inhabitants of research stations in Antarctica, as well as in polar bases such as Thule, Alert and Eureka. It consists of a variety of behavioral and medical disturbances, including irritability, depression, insomnia, absentmindedness, aggressive behavior, and irritable bowel syndrome.

Possible contributing causes of winter-over syndrome include stress, social isolation, subsyndromal seasonal affective disorder and polar T syndrome.

The effects of the disease can have profound effects on everyday life. As well, the recurring side effects of excessive belching, dizziness, dry mouth, hangovers, disorientation, irritable bowel syndrome, and chronic fatigue syndrome can lead to other health problems such as depression, anxiety and poor productivity in employment. The random state of intoxication can lead to personal difficulties, and the relative obscurity of the condition can also make it hard to seek treatment.

Chronic idiopathic constipation is similar to constipation-predominant irritable bowel syndrome (IBS-C); however, people with CIC do not have other symptoms of IBS, such as abdominal pain. Diagnosing CIC can be difficult as other syndromes must be ruled out as there is no physiological cause for CIC. Doctors will typically look for other symptoms, such as blood in stool, weight loss, low blood count, or other symptoms.

To be considered functional constipation, symptoms must be present at least a fourth of the time.

Possible causes are:

- Anismus

- Descending perineum syndrome

- Other inability or unwillingness to control the external anal sphincter, which normally is under voluntary control

- A poor diet

- An unwillingness to defecate

- Nervous reactions, including prolonged and/or chronic stress and anxiety, that close the internal anal sphincter, a muscle that is not under voluntary control

- Deeper psychosomatic disorders which sometimes affect digestion and the absorption of water in the colon

There is also possibility of presentation with other comorbid symptoms such as headache, especially in children.

Auto-brewery syndrome, also known as gut fermentation syndrome, is a rare medical condition in which intoxicating quantities of ethanol are produced through endogenous fermentation within the digestive system. One gastrointestinal organism, "Saccharomyces cerevisiae", a type of yeast, has been identified as a pathogen for this condition.

Claims of endogenous fermentation of this type have been used as a defense against drunk driving charges.

One case went undetected for 20 years.

It has also been investigated, but eliminated, as a possible cause of sudden infant death syndrome.

A variant occurs in persons with liver abnormalities that prevent them from excreting or breaking down alcohol normally. Patients with this condition can develop symptoms of auto-brewery syndrome even when the gut yeast produces a quantity of alcohol that is too small to intoxicate a healthy individual.

Symptoms can sometimes come and go with wave-like reoccurrences or fluctuations in severity of symptoms. Common symptoms include impaired cognition, irritability, depressed mood, and anxiety; all of which may reach severe levels which can lead to relapse.

The protracted withdrawal syndrome from benzodiazepines can produce symptoms identical to generalized anxiety disorder as well as panic disorder. Due to the sometimes prolonged nature and severity of benzodiazepine withdrawal, abrupt withdrawal is not advised.

Common symptoms of post-acute withdrawal syndrome are:

- Psychosocial dysfunction

- Anhedonia

- Depression

- Impaired interpersonal skills

- Obsessive-compulsive behaviour

- Feelings of guilt

- Autonomic disturbances

- Pessimistic thoughts

- Impaired concentration

- Lack of initiative

- Craving

- Inability to think clearly

- Memory problems

- Emotional overreactions or numbness

- Sleep disturbances

- Physical coordination problems

- Stress sensitivity

- Increased sensitivity to pain

- Panic disorder

- Generalized anxiety disorder

- Sleep disturbance (dreams of using, behaviors associated with the life style)

Symptoms occur intermittently, but are not always present. They are made worse by stress or other triggers and may arise at unexpected times and for no apparent reason. They may last for a short while or longer. Any of the following may trigger a temporary return or worsening of the symptoms of post-acute withdrawal syndrome:

- Stressful and/or frustrating situations

- Multitasking

- Feelings of anxiety, fearfulness or anger

- Social conflicts

- Unrealistic expectations of oneself