Orthostatic intolerance
Orthostatic intolerance (OI) is a subcategory of Dysautonomia, a disorder of the autonomic nervous system. 
OI can also be defined as "the development of symptoms during upright standing relieved by recumbency," or by sitting back down again.  Over 500,000 Americans have been diagnosed with OI.  It affects more women than men (female-to-male ratio is at least 4:1), usually under the age of 35.
Orthostatic intolerance occurs in humans because standing upright is a fundamental stressor and requires rapid and effective circulatory and neurologic compensations to maintain blood pressure, cerebral blood flow, and consciousness.  When a human stands, approximately 750 mL of thoracic blood is abruptly translocated downward.  People who suffer from OI lack the basic mechanisms to compensate for this deficit.  Changes in heart rate, blood pressure, and cerebral blood flow that produce OI "may be related to abnormalities in the interplay between blood volume control, the cardiovascular system, the autonomic nervous system and local circulatory mechanisms that regulate these basic physiological functions."

# Symptoms
Symptoms of OI are triggered by the following:
1. An upright posture for long periods of time (i.e., standing in line, standing in a shower, or even sitting at a desk. 
2. A warm environment (such as in hot summer weather, a hot crowded room, a hot shower or bath) after exercise. 
3. Emotionally stressful events (seeing blood or gory scenes, being scared or anxious). 
4. Inadequate fluid and salt intake.
Orthostatic intolerance is divided, roughly based on patient history, in two variants: acute and chronic, and have symptoms that increase in severity.

### Acute OI
Patients who suffer from acute OI usually manifest the disorder by a temporary loss of consciousness and posture, with rapid recovery (simple faints, or syncope), as well as remaining conscious during their loss of posture.  This is different than a syncope caused by cardiac problems because there are known precipitants to the fainting spell (standing, heat, emotion) and prodromal symptoms (nausea, blurred vision, headache).  As  Dr. Julian M. Stewart, an expert in OI from New York Medical College states, "Many syncopal patients have no intercurrent illness; between faints, they are well."
Symptoms:
1. Lightheadedness
2. Headache              
3. Fatigue    
4. Sleep Disorders 
5. Visual Disturbances (black/white/spots)  
6. Exercise intolerance
7. Weakness    
8. Hyperpnea/Dyspnea 
9. Tremulousness    
10. Sweating  
11. Anxiety/Heart Palpitations

### Chronic OI
Patients with chronic orthostatic intolerance are ill on a daily basis.  Defining symptoms of chronic OI include day-to-day dizziness in all patients, with high incidence of the following symptoms:
1. Altered vision (blurred, "white outs," black outs)
2. Fatigue
3. Exercise intolerance (frequently postexercise malaise)
4. Nausea
5. Neurocognitive deficits 
6. Sleep problems 
7. Heat
8. Palpitations
A large proportion of patients also experience the following symptoms: 
1. Headache 
2. Tremulousness 
3. Difficulty breathing or swallowing 
4. Sweating 
5. Pallor 
6. Other vasomotor symptoms.

# Management and Prognosis
OI is "notoriously difficult to diagnose."  As a result, many patients have gone undiagnosed or misdiagnosed and either untreated or treated for other disorders.  Current tests for OI (tilt-testing, autonomic assessment, and vascular integrity) can also specify and simply treatment.  (See Dr. Julian Stewart's article, "Orthostatic Intolerance: An Overview" for a more detailed description of OI tests.)
Most patients experience in an improvement of their symptoms, but for some, OI can be gravely disabling and can be progressive in nature.  The ways in which symptoms present themselves vary greatly from patient to patient; as a result, individualized treatment plans are necessary.
OI is treated both pharmacologically and non-pharmacologically.  Treatment does not cure OI; rather, it controls symptoms.  Often, a combination of medications are used.  The following is a list of the medications used, and what they specifically focus on:
Medications that increase blood volume:
Fludrocortisone (Florinef)
Oral contraceptive pills
Clonidine
Erythropoietin
Medications that interfere with the release or action of epinephrine and norepinephrine:
Beta-blockers (e.g., atenolol, propranolol)
Disopyramide (Norpace)
Angiotensin converting enzyme inhibitors
Medications that improve vasoconstriction:
Stimulants: (e.g., Ritalin or Dexedrine)
Midodrine (Proamatine)
Ephedrine and pseudoephedrine (Sudafed)
Theophylline (low-dose)
Selective serotonin reuptake inhibitors (Prozac, Zoloft, and Paxil)
Behavioral changes that patients with OI can make are avoiding prolonged sitting and quiet standing, warm environments, and vasodilating medications, using postural maneuvers and pressure garments, treating co-existing medical conditions, increasing salt and fluid intake, and physical therapy and exercise.  (For a more detailed description of non-pharmacological treatments, see Dr. Peter C. Rowe's article, "General Information Brochure on Orthostatic Intolerance and its Treatment").
A notable sufferer of orthostatic intolerance is Greg Page, formerly of The Wiggles. It is due to this illness that Page chose to leave the group in 2006.