The Eye and Vision — Page 88, Lesson 129

I want to walk you through the next part of the eye and vision chapter. We've covered the basic refractive errors — now let's look at a set of conditions and regulations that are directly relevant to you as a pilot. First, presbyopia. The word comes from Greek for 'old eye'. The lens of your eye changes shape to focus on near or far objects — that ability is called accommodation, and it depends on the lens's elasticity. As you age, that elasticity is gradually lost. Normally, after the age of 40 to 50, the lens can no longer accommodate fully. That produces a form of long-sightedness called presbyopia. The first effect you'll notice is difficulty reading small print in poor light. The condition usually requires only a minor correction with a weak convex lens — the same type of lens that corrects hypermetropia, but weaker. Half lenses or look-over spectacles are often sufficient. Next, astigmatism. A healthy cornea has a spheroidal shape — like part of a sphere. Astigmatism is usually caused by a misshapen or oblong cornea. Instead of being evenly curved, the cornea has different curvatures in different axes. As a result, objects appear irregularly shaped — they may look stretched or distorted. Astigmatism can be corrected with cylindrical lenses, also called toric lenses. Modern surgical techniques can also reshape the cornea using a scalpel or, more commonly, with laser techniques. Now, a critical regulatory point: it is a requirement that aircrew who have to wear correcting spectacles in order to exercise the privileges of their licence must carry a spare pair during flight. That spare must be easily accessible — not stowed away in a checked bag. This is a JAA requirement and it applies to you. Let's move to cataracts. Cataracts are normally associated with the ageing process, though some diseases can cause them at any age. Over time, the lens of the eye becomes cloudy, causing a marked loss of vision. For severe cases, traditional surgery removes a section of the lens and replaces it with an artificial substitute. The surgery uses local anaesthesia on an outpatient basis. Following successful treatment, pilots are normally allowed to return to flying. Now, glaucoma. This is a disease of the eye that causes a pressure rise of the liquid inside the eye — that liquid is called the aqueous humour. The aqueous humour protects the lens and nourishes the cornea. It passes through a small shutter — that's the drainage channel. That shutter can be flawed from birth or can become jammed, causing a rise in pressure inside the eye. The normal pressure range is 10 to 20 mm Hg — millimetres of mercury. Glaucoma damages the optic nerve and may cause severe pain. If left untreated, it leads to blindness. Part of the JAA medical examination includes a test for glaucoma. The symptoms can be treated either by eye drops — the drug named here is timitol — or by an operation in which a hole is made in the shutter. Glaucoma can be inherited or may result from the ageing process. The main symptoms of glaucoma are: acute pain in the eye — in extreme cases; blurred vision; sensitivity to high light levels; visual field deterioration; and red discolouration of the eye. Finally, let's talk about contact lenses. Contact lenses provide better peripheral vision than spectacles, and they are not subject to misting. So many pilots are attracted to wearing them instead of spectacles. But there are problems. The cornea does not have its own blood supply — it obtains its oxygen from the ambient air. Mild hypoxia and dehydration, caused by low humidity on the flight deck, can affect the cornea's oxygen supply. That's where the excerpt cuts off, but the point is clear: the flight deck environment creates challenges for contact lens wear that you need to consider. Let me show you the figure that accompanies this section. It illustrates the correction of myopia with a concave lens and the correction of hypermetropia with a convex lens.