Friday, December 21, 2007

7.1 Cold fish eye

A patient once remarked that her post-LASIK vision seemed to change for the worse but only during the winter time. An examination and a comparison with past records indeed confirmed her self-observation. Since no other similar reports have surfaced, we have regarded this case as anecdotal. Although, it is entirely possible that structural change of a thinner cornea in response to a lower ambient temperature did occur. It'll be interesting to study post-LASIK Alaskans' visual stability, for example.

So how does the eyes keep warm - or cool, for that matter. Human corneal temperature is normally 34.2°C. Then there is a temperature gradient from the cornea all the way deep into the orbit, reaching the body temp. This indicates that the anterior segment, including the cornea, iris, and the crystalline lens, is kept cool through blood and aqueous circulation. Normal corneas seem to be quite comfortable with the temperature change and can stay transparent. Even solar keratitis affects mostly only the epithelium, the rest is still clear.

The crystalline lens is a different story. In a simulation, when a model human eye is exposed to an infrared radiation source of 1500°C, the lens temperature increase can reach 1-2°C, which seems enough to cause infrared cataracts.

Hmm, what if you live in a cold environment? There are the experimental "cold cataracts" seen in the lenses of newborn cows, rats, and mice; although, luckily, not in that of human babies. What about fish living in the -2°C Arctic/Antarctic ocean. Well, they have a glycoprotein antifreeze in their body systems and, most interestingly, cold-resistant lens proteins. These proteins are actually gamma-crystallins. It seems that in some cold-adapted fish, these crystallins are more tightly packed than the mammalian kinds, and are therefore far more resistant to protein-water phase separation (hence the opacities). That does make sense, or the chance of survival for a cataractous fish would not be so great.

Next time, when you look at a fish in the eye, do remember the remarkable biochemistry contained within.

What if you are an eagle flying up in the cold air and needs to hunt for a rabbit a mile away? The eagles do have an extra eyelid, as that in a rabbit, called nictating membrane. In the eagles, this membrane is transparent which swings into action during hunt. Their retinal cell density is about 5 times that of the humans. So we can safely assume the eagles have much better visual acuity than the humans. Cold cataracts? Not a chance. In the eagle eye, within the vitreous, there is a structure called pecten body. It is a vascularized feathery structure with the function of a heat radiator. That is how the eagle eye is kept warm even flying through frigid air, and, some say, pecten body also provides oxygen to the ocular structures. It turns out that all birds, including the lowly chicken, have it, too. Surprise, surprise.

It pays to strike up a conversation with a veterinarian sometimes.

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