What is High Pressure Diving Nervous Syndrome?
High-pressure nervous syndrome (HPNS) is a condition that can occur when a diver goes deeper than 150m while breathing a mixture of helium and oxygen. This condition can cause changes in the functioning of the nervous system, the way a person thinks and feels (psychological effects), and their brain activity, as recorded by an EEG (a test that measures electrical activity in the brain). The term “neurological” is often used to describe this.
The signs and symptoms of HPNS can vary depending on how quickly the pressure increases during a dive and how deep the dive is. Essentially, if a diver goes deeper more quickly and at higher pressures, the symptoms can be more severe. Hence, HPNS is a significant risk for people attempting to dive very deep.
What Causes High Pressure Diving Nervous Syndrome?
High Pressure Nervous Syndrome (HPNS) is believed to occur due to an increase in pressure on the central nervous system, or the part of the body that involves the brain and spinal cord. This increased pressure can make the central nervous system overactive.
However, studies have shown that it’s not just the elevated pressure itself causing this condition. High-pressure helium, a type of gas, seems to be the main factor.
The overactivity of the central nervous system linked to HPNS seems to be largely caused by something called N-methyl-D-aspartate receptors. These receptors are proteins that help send signals in the brain. This finding gives us hope for future research to help improve this condition.
Risk Factors and Frequency for High Pressure Diving Nervous Syndrome
There isn’t any recorded data about HPNS (High Pressure Nervous Syndrome) in scientific literature.
Signs and Symptoms of High Pressure Diving Nervous Syndrome
High-pressure nervous syndrome (HPNS) primarily affects the central nervous system, causing it to become overactive. This can result in neurological and psychological changes, which can be seen in EEG recordings (a test that checks the electrical activity of the brain). HPNS is distinct from other conditions like nitrogen narcosis, decompression sickness, and oxygen toxicity.
The most common symptom of HPNS is tremors. These tremors often start in the hands and feet but can spread to the entire body. They usually occur while the person is at rest or moving, with a frequency of 8 to 12 Hz. The severity of the tremors increases with a faster speed of compression and higher hydrostatic pressure. Another early sign of HPNS is opsoclonus, a condition characterized by random eye movement in multiple directions. Other potential symptoms include headaches, dizziness, fatigue, sudden jerking muscle movements, muscle weakness, and feelings of intense happiness. While seizures have been reported in animals with HPNS, they have not been observed in humans.
- Overactivity of the central nervous system
- Tremors starting in the hands and feet
- Random eye movement in multiple directions
- Headaches
- Dizziness
- Fatigue
- Sudden jerking muscle movements
- Muscle weakness
- Feelings of intense happiness (Euphoria)
People with HPNS may also experience gastrointestinal symptoms such as:
- Nausea
- Vomiting
- Stomach cramps
- Diarrhea
- Loss of appetite
In some cases, the individual may have memory problems, cognitive deficits (issues with thinking and understanding), impaired motor skills, drowsiness, and sleep disturbances including vivid dreams or nightmares.
The specific symptoms a person experiences, and the severity of those symptoms, can be influenced by a variety of factors. These include the type of gases being breathed in, the rate of compression, and the level of hydrostatic pressure. For example, adding a certain amount of nitrogen or hydrogen to the helium-oxygen mixture a person is breathing can lessen the symptoms of HPNS. Faster compression rates and increased hydrostatic pressure can make the symptoms more severe.
It’s also important to note that everyone experiences HPNS differently. Some people may have a greater tolerance to certain symptoms or experience them to a lesser degree.
Testing for High Pressure Diving Nervous Syndrome
To understand how deep-sea diving can affect people, several tests were carried out. These included tests on memory, thinking ability, vision, motor skills, and even balance. For example, a hand steadiness test was done to see if divers developed any shake or tremor in their hands. Tests were also done on things like hand strength, vision, and coordination.
People filled out questionnaires at different depths to help evaluate how well they were performing. This can involve tests on motor skills (the ability to move the body accurately), visuomotor tests (the eyes and hands working together), and cognitive tests (tests of memory and thinking ability).
An Electroencephalogram (EEG), a test that records the electrical activity of the brain, was also used. In divers who developed a condition called high-pressure neurological syndrome (HPNS), which can cause tremors, dizziness, and nausea, the EEG tests showed an increase in a type of electrical activity in the brain known as theta waves and a decrease in alpha waves. These changes were particularly noted in frontal brain regions at depths of around 200 meters.
The EEG picks up on changes in sleep patterns too when at high pressure. In particular, they noted an increase in the initial, lighter stages of sleep (stages I and II), and a decrease in the deeper, more restorative stages of sleep (stages III and IV). The time spent in REM sleep, the stage where dreaming happens, was also reduced.
In a similar vein, pressure also seemed to affect what’s called ‘somatosensory evoked potentials’, which is how the brain responds to sensations. There seemed to be a sped-up reaction to sensations, suggesting that the brain was in a state of hyperexcitability.
Treatment Options for High Pressure Diving Nervous Syndrome
High-pressure nervous syndrome (HPNS) is a significant issue for deep diving. Despite not being fully preventable, certain approaches can delay the onset or modify the symptoms of HPNS. These include:
Reducing the Speed of Compression: If divers take longer to descend or pause during descent to acclimatize, they might experience milder symptoms of HPNS, or even avoid them altogether. However, this process needs to be really slow, which can be a disadvantage for technical dives that require quick descents. It’s also important to note that even with a slow descent, divers might experience symptoms of HPNS beyond 330 meters.
Changing the Breathing Gas Mixture: Adding a small percentage of nitrogen (5% to 10%) to the pure helium-oxygen gas mixture used by divers might help reduce symptoms of HPNS. Nitrogen is supposed to counteract some effects of HPNS thanks to its narcotic-like properties. This approach also has other benefits like reducing costs, increasing thermal comfort and reducing speech distortion. Yet, divers should be cautious of nitrogen narcosis, a kind of diving sickness caused by breathing in too much nitrogen. Alternatively, hydrogen has been used for similar purposes due to its favorable properties for deep dives. To manage breathability, hydrogen is often mixed with helium and oxygen in a specific ratio, which has led to successful deep dives without significant symptoms.
Medications: Currently, there are no specific medicines to treat HPNS. Anesthetics, barbiturates, and anticonvulsants have been investigated to manage the symptoms, but none have been conclusively proven to help. Some of these medications have been found effective in animal studies, but they have adverse side effects that affect their diving abilities. However, recent studies on a group of medications known as 5-HT1A receptor antagonists have shown some promise.
Diver Selection: Finally, choosing individuals who are less susceptible to HPNS could also be a solution. While this approach might lead to discovery of new symptoms or development of symptom-free pressure-related tissue injuries, further studies could provide more reliable conclusions.
What else can High Pressure Diving Nervous Syndrome be?
Diving deep underwater can put a lot of pressure on the human body, which can be particularly challenging. Because of this high pressure, other factors also change such as gas concentration, substance density, and even the typical behaviour of heat and sound. This can lead to neurological symptoms if a condition called High Pressure Neurological Syndrome or HPNS isn’t handled properly. Conditions related to deep diving include:
- Oxygen toxicity
- Intoxication with polluted breathing gases
- Nitrogen narcosis
- High Pressure Neurological Syndrome (HPNS)
- Decompression illness
- Carbon dioxide retention due to increased gas density
The particular symptoms experienced, the depth at which they start and stop, the way the diving is conducted (like how fast you ascend or descend, and where you stop for decompression) and the mix of gases you breathe can all help to differentiate between these conditions.
What to expect with High Pressure Diving Nervous Syndrome
The symptoms can continue, but tend to get better over time when the pressure remains the same. This was proven by a study conducted by Rostain et al, who found that the sleep patterns of divers began to improve after their first week under pressure. However, normal sleep patterns were only observed when divers were brought up from deep depths, specifically below 200 meters.
While most symptoms typically improve after divers are brought up to the surface (a process known as decompression), some like lethargy or tiredness, might persist for a while. Despite this, it’s important to note that divers who only experience High-Pressure Nervous Syndrome (or HPNS – a condition that can arise from diving at extreme depths) generally recover fully. Moreover, there haven’t been any permanent neurological damage or any damage to brain tissue identified as being related to HPNS.
Possible Complications When Diagnosed with High Pressure Diving Nervous Syndrome
All divers who suffer from High-Pressure Nervous Syndrome (HPNS) do recover. However, the condition can severely impair a diver’s ability during a dive, potentially leading to dangerous decisions or actions.
Key Points:
- All divers with HPNS recover effectively
- HPNS can severely impact the performance of a diver
- This performance impact may lead to risky decisions or actions
Preventing High Pressure Diving Nervous Syndrome
Professional divers should be well-informed about the diseases related to diving, how to prevent them, and the characteristics of the gases they breathe while diving. It’s crucial for divers to follow the safest diving procedures, like a slow rate of compression and a suitable mixture of gases for breathing. These safety rules are the best way to prevent illnesses associated with diving.
As such, doctors who evaluate whether divers are fit to dive should also pay attention to the diver’s mental wellness. Ensuring safe diving involves making sure that the divers are knowledgeable about the rules and that they follow them closely.
