Pseudophakia refers to the presence of an intraocular lens (IOL) implant. The IOL may be inserted at the time of cataract surgery for the removal of crystalline lens or it may be put as a secondary procedure to correct aphakia. The IOL may be placed in the anterior chamber (anterior-chamber IOL) or posterior chamber (posterior-chamber IOL).
Refractive power of the eye is determined predominantly by variables like power of the cornea, power of the crystalline lens, and axial length of the eyeball. In emmetropia, these three components of refractive power combine to produce normal refraction to the eye.
Emmetropia is the condition where the eye has no refractive error and requires no correction for distance vision. In an emmetropic eye, rays of light parallel to the optical axis focuses on the retina. The far point in emmetropia (point conjugate to retina in non- accommodating state) is optical infinity, which is 6 meters. Ametropia (refractive error) results when cornea and lens inadequately focus the light rays. The measuring unit for refractive error is dioptre (D), which is defined as the reciprocal of the focal length in meters.
The term ametropia (refractive error) describes any condition where light is poorly focused on light sensitive layer of eye, resulting in blurred vision. This is a common eye problem and includes conditions such as myopia (near- sightedness), hypermetropia (far- sightedness), astigmatism, and presbyopia (age- related diminution of vision). A person who is able to see without the aid of spectacles or contact lenses is emmetropic.
Prevalence and distribution of ametropia vary greatly with age. Majority of children in early infancy are found to be somewhat hypermetropic. During the school years, children begin to become myopic in increasing numbers.
IOLs are made up of acrylic or quality Perspex i.e. poly-methyl-methacrylate (PMMA). The lenses are about 4- 6 mm in diameter and are biconvex or plano- convex. Lens power calculations for primary implantation necessitate axial length measurement with ultrasonography, keratometry and the use of standard tables. A stronger lens may be required in children. Lens loops are usually made up of flexible methyl-methacrylate. Foldable IOLs made up of silicone or various polymers of acrylic are also available for insertion through a small incision following cataract surgery by phacoemulsification.
A patient who needs 12.5 D in aphakic spectacles would need about 21 D of an IOL in posterior chamber of eye. The average magnification of an IOL in posterior chamber is about 1.5%, compared with the original crystalline lens. For an anterior chamber IOL the average power is about 18 D, and the magnification is about 2 %. Some patients may detect this disparity by alternately covering each eye. Almost everyone can achieve binocular vision with one eye pseudophakia and other phakic (eye with crystalline lens). An IOL may be implanted at the time of cataract surgery or as secondary implantation at a later date. Advantages of IOL includes
- Minimal after- care of patients.
- Rapid return of binocular vision.
- Minimal aniseikonia (different image size seen by the eyes).
- Normal peripheral vision.
Fedorov SN, Kolinko AL, Kolinko AL. Estimation of optical power of the intraocular lens. Vestn Oftalmol. 1967; 80: 27- 31.
Holladay JT, Prager TC, Chandler TY, et al. A three-part system for refining intraocular lens power calculations. J Cataract Refract Surg. 1988; 13: 17- 24.
Retzlaff JA, Sanders DR, Kraff MC. Development of the SRK/T intraocular lens implant power calculation formula. J Cataract Refract Surg. 1990; 16: 333- 340.
Hoffer KJ. The Hoffer Q formula: a comparison of theoretic and regression formulas. J Cataract Refract Surg. 1993; 19: 700- 712.
Pseudophakia patients are generally asymptomatic.
Pseudophakia is the condition being produced by implantation of an intraocular lens in the eye.
- Multifocal IOLs produce an image that is about 30% reduced in contrast to mono-focal lenses.
- Toric IOL lenses are spherocylindrical glasses similar to spectacles. The magnitude of the cylinder in the IOL should be about 1.4 times the astigmatism in the cornea to neutralise corneal astigmatism completely.
- Aspheric IOL lenses are designed to minimise spherical aberration and to restore asphericity of eyes in young patients.
Calculation of IOL power:
Prior to cataract surgery, the power of the IOL needed to give desired postoperative refraction is determined by measuring corneal curvature and axial length in a mathematical formula.
Corneal curvature is measured commonly by keratometry. Small variability of the keratometry reading gives an IOL power to within 0.5 D. Variability in the measurement of the axial length tends to be the main source of discrepancy in the calculation of IOL power.
When the cornea is irregular (due to corneal pathology or previous corneal or refractive procedures), a better prediction of the required IOL power may be obtained using corneal topography (photokeratoscopy or videokeratography) rather than keratometry to measure the corneal curvature. Corneal topography generates many more data points to be used in calculation of IOL power. Corneal topography may be used to assess the magnitude, location and regularity of pre- existing astigmatism. Incision in the peripheral part of steep axis produces a central flattening effect and reduces astigmatism. Change in corneal contour is less for the more peripheral incisions in the sclera or limbus.
IOL calculations that require axial length:
Theoretical formulas: These formulas for IOL power calculation has not changed since its original description by Fyoderov et al. in 1967. Several investigators have presented these formulas in different forms. Six variables used in the formula are
Several additional measurements of the eye are taken, but only seven preoperative variables (axial length, corneal power, horizontal corneal diameter, anterior chamber depth, lens thickness, preoperative refraction, and age) were found to improve significantly the prediction of effective lens position. Third generation formulas Holladay 1, Hoffer Q and SRK/T and newer Holladay 2 are much more accurate than previous formulas. Older formulas such as SRK1, SRK2, and Binkhorst 1do not give desired result if the central corneal power is measured incorrectly.
Refractive lens exchange (RLE) for high myopia and hypermetropia with normal cornea and no previous keratorefractive surgery:
Since there is loss of accommodation and the patients are relatively young, a small degree of myopia (minus 0.50 D) may be desirable so that dependence on spectacles may be reduced.
For myopia, third- generation theoretical formulas give excellent predictions if the axial length is stable and the measurements are accurate.
For hypermetrpia, the Holladay 2 formula is used. The use of this formula may reduce the prediction error in these cases to less than 1 D. Accurate measurements of axial length and corneal power are especially important in these cases.
Methods to determine axial length: Axial length may be determined by optical methods and ultrasonic methods. Ultrasonic measurement is less dependent on the density of the cataract.
Patients with previous keratorefractive surgery:
The number of patients who had keratorefractive surgery (radial keratotomy, photorefractive keratectomy, or laser assisted in-situ keratomileusis) is increasing steadily. These groups of patients present a difficult challenge. Instruments used to measure corneal power make incorrect assumptions for corneas that have irregular astigmatism. Due to this, the calculation method and the trial hard contact lens method are the most accurate, followed by corneal topography, automated keratometry, and manual keratometry.
IOL calculations using net corneal power values and preoperative refraction:
In a standard cataract surgery with IOL implantation, the preoperative refraction is not very helpful for calculation of the power of an implant, because as the crystalline lens is removed, so the diopteric power is being removed and then replaced. Cases in which power is not reduced in the eye (such as secondary implant in aphakia, piggy-back IOL in pseudophakia, or a minus IOL in the anterior chamber of a myopic phakic eye), the necessary IOL power for a desired postoperative refraction may be calculated from the corneal power and pre- operative refraction. Measurement of axial length is not necessary.
The formula used for calculation of IOL power uses following variables
Secondary piggy-back IOL in pseudophakia:
In patients who have significant residual refractive error following primary IOL implant, it is often easier surgically and more predictable optically, to leave the primary implant in place and calculate the secondary piggy-back IOL power to achieve the desired refraction. This method does not require knowledge of the power of the primary implant or of the axial length. This formula works for both plus or minus lenses.
Primary minus IOL in the anterior chamber of a myopic phakic eye:
The calculation for a minus IOL in the anterior chamber is the same as for the aphakic calculation of an anterior chamber lens, with the exception that the power of the lens is negative. In the past, these lenses were reserved for high myopia that could not be corrected by radial keratotomy or photorefractive keratectomy. Because successful laser assisted in-situ keratomileusis procedures have been performed in myopias up to minus 20 D, these lenses may be reserved for myopia that exceeds this power.
Optical refractive state in pseudophakia:
It depends upon the power of the Implanted IOL. Postoperatively, patient may have
Diagnosis depends upon
History: It documents whether previous cataract surgery was performed, and whether were there any associated complications or not.
Examination of eyes: Eyes are examined for
Usually no management is required in pseudophakia.
Pseudophakic patients may require correction of residual refractive power for distance, but only reading glasses may be needed.
Management of any complication as and when required.
Eliminating need for spectacles for both near and distance is an obtainable goal in many patients. Accommodative potential of either a pseudophakic or aphakic eye resulting in the combination of functional distance and near vision is described as pseudo- accommodation or apparent accommodation.
Intraocular lenses designed to improve accommodative amplitude are divided into
Prognosis is usually good.
There may be increased risk of retinal detachment, especially in high myopes and if posterior capsule is not intact.
A pseudophakic patient may develop complications like