Aphakia

APHAKIA (from Greek a- [negative particle] and phakos [lens]), the absence of the lens at its usual place in the area of the pupil. Objective signs of an aphakic eye: 1) the absence of Purkinje images from the lens surfaces; 2) an abnormally deep anterior chamber; 3) tremor of the iris (not always observed); 4) deep blackness in the area of the pupil, due to the absence of physiological reflection of rays from the lens; if this reflection is present, especially in old people, the pupil appears grayish; 5) in the case of aphakia of operative origin—traces of a former operation: scars on the periphery of the cornea, remnants of the lens capsule and cortical masses in the area of the pupil, coloboma of the iris, partial or complete (may also be absent), etc.; 6) in the case of dislocation of the lens—the finding of it in a new place: in the vitreous body, in the anterior chamber, under the conjunctiva, etc. In optical terms, the aphakic eye is a very simple system, since the refractive indices of the aqueous humor and the vitreous body are equal to each other, and the refractive index of the cornea is only slightly higher than that of the aqueous humor; thus, it can be considered that the aphakic eye has only one single refracting surface—the anterior surface of the cornea, separating two media: in front air with a refractive index of 1, and behind—aqueous humor and vitreous body with a refractive index of 1.336. Such a simplest system is convenient for performing various optical calculations. In connection with the absence of the lens, the refractive power of the eye is significantly reduced. The change in the refractive power of the eye after removal of the lens from it is presented in the following table (according to Hess): Length of Eye with Lens Aphakic Eye Correction Correction on Glass for Distance on Glass for Near Ametropia of Eye at Principal Point 12 mm from Aphakic Eye at Principal Point 12 mm from + 16.6 D + 13.8 H = 27.3 D +20.5 D + 8.3 + 7.3 H = 20.6 + 16.5 + 1 + 1 H = 15.1 + 12.6 - 1 - 1 H = 12.7 + 11 - 4 - 4.2 H = 10.5 + 9.3 - 8.5 - 9.5 II = 6.5 + 6.0 -12.5 -14.7 II = 3 + 2.9 -15.9 -19.8 -19 -24.6 M = 3.5 - 3.65 -21.5 -29 M = 4.8 - 5.1 From this table, among other things, it follows that a myopic eye 31 mm in length, which is corrected with a glass of about 20 D, becomes emmetropic after removal of the lens from it. If, however, the eye before removal of the lens had emmetropic refraction, 23–24 mm in length, then now it requires a corrective convex glass of 11 or 12 D. Due to the correction of the aphakic eye with such a strong biconvex glass located in front of the cornea at a distance of about 12 mm, the size of the image obtained on the retina will be increased, approximately, by 1/5. In connection with this, one might expect a corresponding increase in visual acuity. If, however, in practice visual acuity after removal of senile cataract is rarely higher than one, this is explained by the presence of a large number of various random obstacles both connected with the operation and inherent in the preceding state of the eye and especially its light-sensitive apparatus. Therefore, if visual acuity in an aphakic eye reaches 0.5, such cases are already considered successful. In view of the fact that in an aphakic eye the lens is absent, there is a necessity to correct such eyes not only for distance, but also with a second glass—for work. Since we usually work at a distance of 25–30 cm, this second glass must be, obviously, stronger by 3–4 D. For the correction of aphakic eyes, instead of ordinary biconvex glasses, it is much better to prescribe aspheric glasses (see), which give good vision even with oblique views through the glass. To eliminate astigmatism of the cornea, which usually develops after the operation, it is necessary to prescribe combined correction with a spherical glass and a cylindrical (or better to combine an aspheric glass with a toric one). As for the question of the "accommodation" of an aphakic eye, such accommodation cannot exist due to the absence of the lens; by increasing the distance of the glass from the eye, an aphakic person is able to replace his missing accommodation to a certain extent; however, good vision for work can be provided only with the help of a second glass.