People in the UK have been experiencing unusually hot weather this June, with a high of 33.2°C recorded on Saturday, 21 June 2025, in Charlwood near Gatwick — the hottest UK temperature of the year so far, according to a BBC news article. Whilst hot weather is often associated with pleasant activities like day trips, eating ice cream, and time spent outdoors, it can also be uncomfortable — especially for those with health conditions made worse by heat.
One such condition is orthostatic intolerance, a common issue for people with ME/CFS. Orthostatic intolerance occurs when the body fails to properly compensate for moving to an upright position, which is thought to result in reduced blood flow to the brain and symptoms such as lightheadedness, fatigue, and weakness. A subset of those with orthostatic intolerance also meet the criteria for postural orthostatic tachycardia syndrome (PoTS), which includes an excessive increase in heart rate as one of the symptoms.
Many factors can worsen orthostatic intolerance, but heat is a particularly relevant trigger during the summer months.
In a review, Associate Professor Zachary Schlader and colleagues explored the mechanisms behind orthostatic intolerance under heat stress. They describe the underlying causes as multifactorial and involving complex interactions between multiple physiological systems. These may include changes in blood vessel resistance, blood distribution, and the regulation of cardiac output (the amount of blood the heart pumps per minute).
The Bateman Horne Center has published an informative video about orthostatic intolerance, including symptoms and management.
Here are some of our articles exploring orthostatic intolerance (including PoTS):
- Orthostatic Intolerance, Cerebral Blood Flow, and ME/CFS
- ME/CFS, Orthostatic Intolerance, and the Tilt Table Test
- What Is PoTS?
- Can Treatment for Individuals with ME/CFS and PoTS Be More Targeted?
- Standing Up for ME: Cardiovascular Mechanisms of Orthostatic Intolerance
