The components of the breathing gas mixture, the rate of compression and the time for adaptation influence the presentation. Breathing helium-oxygen, mild effects are seen at pressures of 100 metres, and these effects progress to become debilitating at depths greater than 300 metres. Adding 5 to 10 per cent nitrogen (or hydrogen) to the helium-oxygen respiratory gas mixture reduces symptoms and may permit useful work to be performed after more rapid compression and/or deeper exposure (see later). There is a marked individual variation in susceptibility.
Many different symptoms have been reported from various studies, but most symptoms seem to involve a disturbance of central nervous system function. Effects reported include tremor of the hands and arms that may extend to the whole body, occasional muscle jerks, light-headedness or dizziness, headache, euphoria, drowsiness and loss of consciousness. There is a tendency to fall asleep if not stimulated. Dysphoria and even paranoia are possible. Gastro-intestinal symptoms such as nausea sometimes progressing to vomiting, epigastric sensations, diarrhoea, loss of appetite and aversion to food (leading to weight loss in prolonged exposures) and abdominal cramps may result from a disturbance of the vestibulo-ocular reflex.
Dyspnoea at depths in excess of 300 metres may be a manifestation of HPNS, but this can be difficult to separate from the effects of dense gas and increases in the work of breathing. It can develop or intensify suddenly and may be precipitated by exercise. The distress is greater during inspiration, but surprisingly it is ameliorated by using nitrogen in the breathing mixture, which paradoxically increases gas density and thus the work of breathing.
Tremor may appear in depths as shallow as 150 metres (16 ATA), and it progressively intensifies with increasing depth and pressure. This sign is increasingly reported on deep technical dives where the compressions are extremely fast. The tremor is seen both at rest and on movement. The tremor frequency is 8 to 12 Hz, which differs from that caused by Parkinson’s disease and cerebellar disease, which have a frequency of 3 to 8 Hz. It may be thought of as an extension or exaggeration of the normal physiological resting tremor. The amplitude but not the frequency of the tremor increases with faster rates of compression or increasing absolute pressure. There is a gradual return toward normal following cessation of compression, but it may not be complete until the diver is decompressed. Divers learn to adapt to the tremor, thus leading to an apparent improvement after a day or two.
Opsoclonus is an involuntary, constant, random jittering of the eyes. It is said to be one of the earliest signs of HPNS, and it develops at a depth of 160 metres (17 ATA).
Disturbances of long-term memory and decreases in psychomotor performance have been reported following exposures that produced HPNS. The performance impairment abates somewhat during a stay at constant pressure, but at depths greater than 300 metres, full recovery has not been recorded. Other neuropsychological changes have been reported in some divers. The question remains as to what degree of cognitive performance decrement is acceptable from an occupational and safety standpoint.
Psychomotor tests involving manual dexterity reveal a considerable performance decrement, correlated with the tremor, and averaging 1 per cent, for each 20 metres of depth. Manual dexterity gradually starts returning toward normal levels after 1½ hours at a constant pressure.
The EEG records during exposure of divers reveal an increase in theta activity and a decrease in alpha waves. Increased theta activity may be seen from depths of 60 metres while breathing air or from 150 metres while breathing helium-oxygen mixtures.
Sleep disruptions such as an increase of awake periods and a decrease in sleep stages 3 and 4 and rapid eye movement sleep has been reported at depths of 450 metres.
Somatosensory evoked potentials increase in amplitude, but they are accompanied by an increase in threshold for sensory stimulation. Shortened latency of peaks following the initial cortical P1 is consistent with a state of hyperexcitability in the brain.
The evoked cortical responses may also be altered during deep dives. A progressive decline in the auditory evoked response, by as much as 50 per cent at 457 metres, has been observed. This may be the result of increased sound conduction in high-density gas. Visual evoked responses have not shown any consistent changes.