Monday, February 11, 2008

7.20 Mr Sulu, take us home

(Star Trek IV, The Voyage Home, 1986)

You all remember the Star Trek movie, "The Wrath of Khan (1982)", in which Captain James Tiberius Kirk receives a pair of reading glasses as a birthday present from Dr Leonard (Bones) H McCoy. Kirk proceeds to lament rather unconvincingly the onslaught of old age. Presumably, by the 23rd Century, presbyopia has already been eradicated. Unfortunately, Capt Kirk is allergic to Retinax V - the medicine for treating Presbyopia, or "老花眼old blurry eyes" in Chinese and "老眼old eyes" in Japanese. The keyword: "old".

Funny the script did not follow the more creative vein as that for Captain Jean-Luc Picard who chooses to be bald (Ha!!) There is perhaps a lack of imagination on the screen writers' part. Because even now, there are invasive ways of manipulating presbyopia. Not that this is necessary, a pair of inexpensive OTC readers, bifocals including bifocal contacts, or monovision correction of various kinds (from LASIK to contact lenses, to spectacles) all can quickly resolve the issue.

Presbyopia is an age-related loss of crystalline lens deformability for focusing at near. This was described by Thomas Young (1773-1829) in 1793. To explain the process, he had invoked the elasticity theory. The Young's Modulus (stress = E x strain) is dependent on the material. In the case of accommodation, the lens capsule appears the candidate (see image below). Young's argument was that the capsule elasticity had diminished during aging. Indeed, recent measurements confirmed that the Modulus was about 6 × 107 dyn/cm2 in children which decreased to 3 × 107 dyn/cm2 at age 60, and to 1·5 × 107 dyn/cm2 in extreme old age. Similar to tired old rubber bands which cannot resume the original lengths when the tension is released.

(Lens capsule is the outer most layer which envelops the whole lens
- it is the faint band to the left (anterior to) the single-cell-layer epithelium in the above image)

That is not the whole story, though. There are three parts involved in the accommodation process (see below, 1-3):
(1: The zonules; 2: the crystalline lens; and 3: ciliary muscle)

The ciliary muscle must first contract to allow a decrease in the tension of the zonules, so the capsule can re-shape the lens. Does the ciliary muscle change with aging as well? Not the contractility. It appears the diameter of the ciliary muscle ring does decrease owing to a configurational change. The overall result is the zonules no longer have enough space to relax. And the weakened capsule also can no longer deform the lens as much as before. Did the lens diameter increase with aging also? Some say no; although the only way to make sure is to do a large-scale high-resolution MRI of the eye, because the lens is hiding behind the iris, inaccessible through optical means. The zonules? It is assumed, but never quantified, that the zonular fiber elasticity also changes with time.

Loose ends galore, indeed.

So, how can presbyopia be manipulated surgically? At least in theory, the ciliary muscle ring can be re-positioned to create more space for the zonules. This procedure has already been attempted; although the outcome is still unclear. Alternatively, Conductive Keratoplasty (CK), using radiofrequency to heat and shrink collagen fibers in the cornea, can be done. This to steepen the curvature of the appropriate area of the cornea, so that more plus power is produced for near tasks. CK takes about 3 min and is done to only one eye - a monovision correction as well. It seems to work for some, for a limited duration anyway (about 4-7 years). Interested parties are urged to consult their own eye doctors.

It's been 26 years since "The Wrath of Khan" and we still don't know the composition and the mechanism of effect of Retinax. Well, the sci-fi writers have already pointed the way, perhaps pharmacological researchers can now take us home - to a presbyopia-free world.

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