In the presence of a right-to-left shunt of the heart, such as a patent foramen ovale, venous bubbles may enter the arterial system, resulting in an arterial gas embolism. A similar effect, known as ebullism, may occur during explosive decompression, when water vapour forms bubbles in body fluids due to a dramatic reduction in environmental pressure. Inert gases
The main inert gas in air is nitrogen, but nitrogen is not the only gas that can cause DCS. Breathing gas mixtures such as trimix and heliox include helium, which can also cause decompression sickness. Helium both enters and leaves the body faster than nitrogen, so different decompression schedules are required, but, since helium does not cause narcosis, it is preferred over nitrogen in gas mixtures for deep diving. There is some debate as to the decompression requirements for helium during short-duration dives. Most divers do longer decompressions; however, some groups like the WKPP have been experimenting with the use of shorter decompression times by including deep stops. The balance of evidence as of 2020 does not indicate that deep stops increase decompression efficiency. Any inert gas that is breathed under pressure can form bubbles when the ambient pressure decreases. Very deep dives have been made using hydrogen-oxygen mixtures (hydrox), but controlled decompression is still required to avoid DCS. Isobaric counterdiffusion
DCS can also be caused at a constant ambient pressure when switching between gas mixtures containing different proportions of inert gas. This is known as isobaric counterdiffusion, and presents a problem for very deep dives.
Congenital myasthenic syndrome (CMS) is an inherited neuromuscular disorder caused by defects of several types at the neuromuscular junction. The effects of the disease are similar to Lambert-Eaton Syndrome and myasthenia gravis, the difference being that CMS is not an autoimmune disorder. There are only 600 known family cases of this disorder and it is estimated that its overall frequency in the human population is 1 in 200,000. Types
The types of CMS are classified into three categories: presynaptic, postsynaptic, and synaptic. Presynaptic symptoms include brief stops in breathing, weakness of the eye, mouth, and throat muscles. These symptoms often result in double vision and difficulty chewing and swallowing. Postsynaptic symptoms in infants include severe muscle weakness, feeding and respiratory problems, and delays in the ability to sit, crawl, and walk. Synaptic symptoms include early childhood feeding and respiratory problems, reduced mobility, curvature of the spine, and weakness, which causes a delay in motor milestones. Presentation
The onset of symptoms for all ages may include droopy eyelids. A particular form of postsynaptic CMS (slow-channel CMS) includes severe weakness beginning in infancy or childhood that progresses to respiratory problems, and leads to loss of mobility in adolescence or later life. Mechanisms
Postsynaptic CMS
CMS is associated with genetic defects that affect proteins of the neuromuscular junction. Postsynaptic defects are the most frequent cause of CMS and often result in abnormalities in the acetylcholine receptor (AChR).