Fig. 5.1
Normal atmospheric and intraocular pressures at sea level. Note that the transcorneal pressure difference is 15
Diving into a deep-sea environment will cause an increase in the ambient pressure acting on the body. How these changes in ambient pressure affect the body depends on the anatomy of the organs in question. A fluid-filled organ or solid organ will not change in size as the pressure changes because fluids are not compressible. A gas-filled space with elastic walls, however, will change in size and possibly shape. This is due to Boyle’s law , which states that the volume of a certain quantity of gas is inversely proportional to the absolute pressure, assuming the temperature remains the same. As an example, a balloon filled with gas at sea level would shrink to one-half its size at a depth of 33 ft. This law is important to divers, as most tissue spaces in the body that contain gas have limited capacity to alter their shape, especially the lungs and middle ear. Therefore, one must add gas to the middle ear on descent to prevent it from collapsing and exhale on ascent to avoid damaging lung tissue. Damage to tissues due to changes in pressure is called barotrauma.
Another applicable principle is described by Henry’s Law , which states that the amount of gas that will dissolve in a liquid at a given temperature is directly proportional to the partial pressure of that gas. As a diver descends, the increased pressure causes more nitrogen (the predominant atmospheric gas, comprising 78% of normal air) to enter into solution in his or her tissues than was present at sea level. If enough nitrogen enters the tissues, and the diver ascends too quickly, the excess gas will not have a chance to be eliminated gradually by the lungs. The gas will come out of solution and a gas phase, or bubbles, will form in the blood and body tissues. These bubbles can result in decompression sickness (DCS) , also known as “the bends,” and will be discussed in greater detail later in the chapter.
Refractive Changes in Water
Two-thirds of the refractive power of the eye is generated by the air-tear film interface. When submerged underwater without a face mask, this interface is changed to water-tear film, and the refractive power of the eye changes dramatically. This change in interface causes approximately five to six diopter hyperopic shift, which is responsible for the blurring of objects when underwater [10].
By wearing a pair of goggles or face mask, the air-tear film interface is restored, and the induced hyperopia is eliminated. However, the light that is traveling toward the eye will be refracted away from the normal as it exits the water and enters the air inside the face mask. This refraction of light will cause the object being viewed to be magnified by approximately 30% and appear closer than it actually is [7, 20].
Refractive Correction Underwater
For persons who need refractive correction while underwater, two options exist: contact lenses and prescription face masks. Soft contact lenses are the preferred lenses to be worn if needed for diving, as hard PMMA lenses can result in corneal edema [5–7, 11, 13]. The edema is due to the formation of nitrogen bubbles in the tear film, which interferes with normal tear physiology and causes epithelial edema [16, 17, 21]. When contact lenses are not an option, prescription face mask lenses provide another alternative. The prescription should be ground into the face mask, as lenses that are bonded onto the faceplate can eventually be displaced as the bonding material is eroded.
Numerous studies have shown that a hyperopic shift, sometimes severe and even permanent, can occur with high-altitude exposure after incisional keratorefractive surgery (see Mader’s chapter in this volume). Since part of the effect may be attributable to decreased atmospheric pressure in addition to hypoxia, the potential effect of increased pressure after refractive procedures has been studied in small cohorts of patients. When two subjects (four eyes) who were >1 year out from bilateral radial keratotomy (RK) were exposed to 3 atm in a hyperbaric chamber for 1 h, no changes in keratometry, cycloplegic refraction, and corneal pachymetry were found immediately post-exposure (Peters:1999vo). Additionally, subjects post-LASIK , photorefractive keratectomy (PRK) , and RK exhibited no clinically significant change in manifest refraction or best-corrected visual acuity during a simulated dive to 99 FSW (4 atm) in a hyperbaric chamber.
Light Transmission
As light travels through water , it is attenuated by both scattering and absorption. [12] As depth increases, the amount of available light decreases, and the water becomes progressively darker. Even in the clearest waters, only about 20% of incident light reaches a depth of 33 FSW and only 1% reaches 260 FSW [7]. During the day and in clear water, sufficient light for unaided vision is to a depth of approximately 400 FSW [18].
Color Vision
The deep-sea environment can also affect our perception of colors. As the visible light passes through increasing depths of water, selected wavelengths of light are absorbed. Clear water has a maximum transparency of a wavelength of 480 nm, which is near the blue end of the spectrum [7]. Longer wavelengths, such as red and yellow, are absorbed first. Red is usually not seen below 30 ft, and yellow disappears around 75 ft. Below 100 ft only blue and green colors remain without the use of artificial light [18]. Red colors at this depth are then perceived as black [20]. The use of photography or hand-held lights allows for objects to be seen in their actual color at these depths.
Surgical Considerations
Special circumstances exist for patients who have recently undergone ophthalmologic surgery and will be exposed to the increased pressures of the deep sea. Individuals who have had recent surgery must ensure all incisions are healed, as it is possible for pathogens in the environment to enter through the wounds. This could potentially lead to endophthalmitis or other unwanted complications. There are no specific recommendations for participation in diving after eye surgery, and in this situation, adherence to the surgeon’s advice about sports after eye surgery should be followed. Data suggest that current small-incision cataract surgery techniques, especially with a scleral tunnel approach, maintain IOP within 30 min after surgery and would not allow fluid ingress or egress thereafter. Mask pressure equilibration (see below) would be particularly important shortly after intraocular surgery, since a negative pressure environment in the mask could draw aqueous and/or vitreous out through any weak area in the healed incision. However, no reports of such occurrences have been published to date, suggesting this concern may be only theoretic. Similarly, while the US Navy considered radial keratotomy (RK) a disqualifier for service, no corneal rupture or wound dehiscence has been reported in recreational divers after RK.
Gas Precautions While Diving
As noted above, solid- or fluid-filled objects maintain a constant shape and size under various pressures. This is true for the eye as well as it is filled with noncompressible aqueous and vitreous. Once a face mask or goggle is placed over the eye, an air chamber is developed that can interact with the eye and ocular adnexa. As a diver descends, this air chamber develops a negative pressure relative to the surroundings, and the eye and surrounding tissues are drawn toward the goggle or mask (Fig. 5.2). This phenomenon has been termed “mask squeeze .” If the negative pressure gradient is high enough, it can result in marked edema and ecchymosis of the lids or a subconjunctival hemorrhage. Although these changes can have a very dramatic outward appearance and may cause the patient anxiety and distress, they typically resolve over hours to days with no permanent consequences (Fig. 5.2). Rare instances of orbital subperiosteal hemorrhage also have been described in association with inadequate mask pressure equalization. In order to prevent the negative pressure buildup, positive pressure must be added to this space. The diver can do this by simply blowing air through their nose to add air to the chamber. Because of this, only face masks should be worn for diving, as it is not possible to add air while diving with goggles that just cover the eyes. When the diver ascends, the air in the face mask or goggles will expand. This usually does not cause a problem, as the expanding gas will easily escape from beneath the mask edge where it contacts the skin.
