Understanding Anisometropia and Aphakia in Ophthalmology
Anisometropia is a condition where the refractive status of the eyes differs, with causes ranging from hereditary factors to acquired conditions. Classifications based on refractive error and dioptric differences determine the severity of symptoms and potential vision problems. Treatment options include LASIK, contact lenses, and iseikonic lenses, especially in cases of anisometropic amblyopia. Aphakia, on the other hand, refers to the absence of the crystalline lens, leading to optical challenges in the eye's focusing mechanism.
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ANISOMETROPIA It is a situation in which refractive status of the two eyes are different, i.e. unequal. However, an insignificant difference in refractive status of the eyes is quite common. AETIOLOGY a. Hereditary It is due to congenital cataract, congenital glaucoma, etc. b. Acquired It is due to surgical or nonsurgical trauma, unilateral aphakia and inequality in the rate of refractive changes in both eyes.
CLASSIFICATION I. Based on refractive error a. Isoanisometropia Here refractive status of both the eyes are either hypermetropic or myopic. b. Antimetropia Here refractive status of one eye is myopia and the other is hypermetropia. II. Based on dioptric difference Patients symptoms vary significantly with the degree of dioptric difference between the two eyes a. Low = 0 to 2.50D b. High = 2.50D to 6.00D c. Very high = > 6.00D
In low anisometropia binocular vision is easily achieved and full optical correction is well-tolerated. A difference of 0.25D produces 0.5% difference in size of the retinal images of the two eyes. A difference of up to 5% of retinal image size is well-tolerated and two images can be fused without strain. So, a dioptric difference of more than 2.50D will lead to binocular vision problem and eye strain. Often the vision becomes uniocular and the worse eye becomes lazy, i.e. amblyopic and convergent if corrective measures are not undertaken in childhood.
OPTICAL PROBLEMS/DIFFICULTIES OF ANISOMETROPIA a. Binocular vision: > 2.50D of difference in dioptric strength between the two eyes leads to eye strain due to effort of fusion. Binocular vision is not possible with spectacle correction if the anisometropia is > 4.00D. b. Amblyopia: Often a difference of > 2.00D in hypermetropic patient is sufficient to induce amblyopia in the more hypermetropic eye. However, in myopic patients with anisometropia amblyopia is less likely to develop unless the difference is very significant. c. Squinting: Convergent squint in childhood and divergent squint in adults. d. Diplopia: It develops due to difference in image size of > 8%. e. A difference in stimulus to the accommodation between the two eyes. f. A difference in prismatic affect and distortion between the two eyes on looking through the spectacles obliquely, i.e. away from the optical centres.
TREATMENT a. LASIK/LASEK b. Contact lenses c. Iseikonic lenses d. If the patient is amblyopic (anisometropic amblyopia) treatment of amblyopia.
Aphakia Aphakia means absence of the crystalline lens in it s normal anatomical position. OPTICS In aphakia the eye consists of a curved surface, i.e. cornea (radius of curvature 8.00mm) in between two media of different refractive indices (air = 1, aqueous and vitreous humour = 1.33). The anterior focal distance is 23 mm and the posterior 31 mm, as opposed to 15 mm and 24 mm respectively in an emmetropic eye. The absence of lens leads to extreme hypermetropia (Fig. 9-8A) and loss of accommodation. If the eye was previously emmetropic, the correcting convex lens in spectacle required to focus the image on the retina is estimated to be approximately +10.00D (Fig. 9-8B).
Figs 9-8A and B: (A) Optics of aphakia, (B) Correction of aphakia by + 10.00D convex lens (Cx) SYMPTOMS A. Blurring of vision for both distance and near b. History of cataract operation or injury c. Patient may wear very thick convex glass.
SIGNS If extra capsular (ECCE) /intra capsular (ICCE) cataract extraction is done: Vision is finger counting at few feet without glasses. Upper limbus Presence of linear scar with or without sutures (10 0 nylon Usually interrupted/continuous) may be seen. Anterior chamber depth Deep. Iridodonesis, i.e. tremulousness of iris due to lack of support. Peripheral button hole iridectomy (PBHI) may be seen. Pupil Jet black due to loss of IIIrd and IVth Purkinje image (in ICCE) and IIIrd image (in ECCE). Ophthalmoscopy The optic disc is very small.
TREATMENT Spectacles Spectacles are usually advised after 6 weeks of surgery. The time is required for complete wound healing and stabilisation of refractive error particularly astigmatism. If the patient was previously emmetropic usual prescription for glasses will be roughly as follows: Glasses advised Right Eye = +10.00DSPH with +2.00 DCYL 180 (astigmatism with-the-rule) Add: +3.00DSPH for near vision The +3.00DSPH near addition is due to loss of accommodation due to absence of the lens. Aspherical lenticular resin lens (CR 39) is ideal for aphakic patients than crown glass lens.
Optical disadvantages of aphakic glasses: i. Image magnification is 25 30%. So, in uniocular aphakia binocular vision is not possible due to aniseikonia. Hence, to avoid diplopia (where phakic eye vision is > 6/36), balanced (+10.00D) or frosted glass is dispensed for the phakic eye. The image magnification causes objects to appear closer to the eye then they are really. ii. Spherical aberration Pincushion distortion (see chapter - 7) iii. Jack-in-the-box phenomenon Due to prismatic aberration a ring scotoma is produced all around the edge of the lens. This causes an unseen object to suddenly pop up in front of the eyes or disappear into the ring scotoma, as the patient moves his eyes.
iv. Restricted visual field. v. Lack of physical coordination which results from image magnification, restricted visual field, pincushion distortion and Jack-in-the-box phenomenon. vi. Thick, heavy lenses are cosmetically deficient. vii. Loss of lens often leads to coloured vision due to lack of natural filter offered by the lens. UV-A (315 400 nm) protection offered by the lens is lost.
Contact Lens Disadvantages i. Inability of elderly patients to insert and remove contact lens efficiently. ii. Foreign body sensation. iii. Additional glasses required for reading correction. However, bifocal contact lenses are available and becoming increasingly popular. Advantages All the disadvantages of glasses are neutralised: i. Image magnification is 6 7%. Hence, binocular vision is possible in uniocular aphakia. ii. Aberrations are lessened, i.e. pincushion distortion, etc. iii. Increased visual field. iv. Better physical coordination. v. Cosmetically attractive.
Secondary IOL Implantation Advantages i. Image minification is 0 2%. Hence, quick return to binocularity is achieved due to minimum aniseikonia. ii. Absence of aberrations. iii. Restoration of normal peripheral field of vision. iv. Excellent physical coordination. Disadvantages It is significantly reduced to usual complications following primary IOL implantation surgery, e.g. corneal decompensation, infection, astigmatism, etc. Secondary IOL implantation in aphakia may be; a. AC IOL implantation in aphakia following intra capsular cataract extraction (ICCE ) If the aphakic eye was emmetropic earlier, AC IOL of +18.00D strength is required to focus the image on the retina. b. PC IOL Implantation in aphakia following extra capsular cataract extraction (ECCE) - +20.00D strength of PC IOL is required to focus the image on the retina. c. Hyperopic LASIK.
PSEUDOPHAKIA TYPES OF IOL It depends on location/support of the IOL (Fig. 9-9). Pseudophakia means replacement of the natural crystalline lens by a synthetic intraocular lens (IOL). MATERIALS OF IOL Polymethyl Methacrylate (PMMA) Silicon Acrylic. CALCULATION OF IOL POWER It is done by: Axial length measurement by A-scan Keratometry Standard calculation formulas
Fig. 9-9: Types of IOL (depending on support/location) Figs 9-10A to E: Showing different types of IOL depending on location/support. (A) Angle supported lens, (B) IRIS claw lens, (C) IRIS supported, (D) Ciliary sulcus supported, (E) Endocapsular lens
Residual Refractive Error Residual refractive error in pseudophakia consists of; Spherical error Accurate biometry overcomes this error. Astigmatism Phacoemulsification results in astigmatism against-the- rule, whereas PC IOL implantation with sutures results in astigmatism with-the-rule. Loss of accommodation However, nowadays multifocal IOL s, accommodative IOL s are increasingly available to correct this error. After phacoemulsification glasses may be advised after only one week and after small incision cataract surgery (SICS) glasses may be advised after 3 weeks. However, after ECCE with PC IOL implantation glasses are advised only after 6 weeks.
