Pseudophakia

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.

References:

Sihota Ramanjit, Tandon Radhika. Parsons’ Diseases of the Eye Twenty Second Edition. Elsevier 2015. P 76- 78.

Khurana AK. Theory and Practice of Optics and Refraction Second Edition. Reed Elsevier India Private Limited 2008. P 66- 70.

Mukherjee PK. Clinical Examination in Ophthalmology Second Edition. Elsevier Relx India Pvt. Ltd. 2016. P 176- 181.

Yanoff Myron, Duker Jay S. Ophthalmology Third Edition. Mosby Elsevier 2009. P 415- 422.

Chang David F, Dell Steven J, Hill Warren E, Lindstrom Richard L, Waltz Kevin L. Mastering Refractive IOLs- The Art and Science. Slack Incorporated 2008. P 264- 266.

Brightbill Frederick S, McDonnell Peter J, McGhee Charles NJ, Farjo Ayad A, Serdarevic Olivia. Corneal Surgery- Theory, Technique and Tissue Fourth Edition. Mosby Elsevier 2009. P 397- 405.

Friedman Neil J, Kaiser Peter K. Essentials of Ophthalmology First Edition. Saunders Elsevier 2007. P 229.

Kaiser Peter K, Friedman Neil J, Pineda Roberto. The Massachusetts Eye and Ear Infirmary Illustrated Manual of Ophthalmology Fourth Edition. Elsevier Saunders 2014. P 309- 310.

Rosenfield Mark, Logan Nicola, Edwards Keith. Optometry- Science, Techniques and Clinical Management Second edition. Butterworth Heinemann Elsevier 2009. P 361- 362.

http://lomalindahealth.org/media/health-care/pdfs/ophthalmology/aphak.pdf

http://www.medscape.com/viewarticle/870276?nlid=110047_450&src=WNL_mdplsfeat_161014_mscpedit_opth&uac=245425CZ&spon=36&impID=1215419&faf=1

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.

• Patients with mono-focal IOL implants experience loss of accommodation and can see clearly at one distance only without correction.
• Most lenses may cause positive (bright) or negative (dark) visual phenomenon (dysphotopsias) from internal reflections within the IOL that occur in certain lighting conditions.
• Edge glare.
• Induced ametropia (refractive error).
• There may be monocular diplopia (double image) or polyopia (multiple images) or blurred vision if the IOL is decentered or tilted.

Pseudophakia is the condition being produced by implantation of an intraocular lens in the eye.

IOL:

• Design: Currently available IOLs are either biconvex, plano- convex or meniscus. Majority of implanted lenses are biconvex. In addition to minimising the effects of tilt, decentration and spherical aberration, a convex posterior surface of a biconvex lens may reduce the migration of lens epithelial cells which produces opacification of the posterior capsule. Positive meniscus lens is rarely used because when it is tilted or decentered, the induced astigmatism and power change is dramatic. A 10˚- 15˚ tilt can induce enough regular and irregular astigmatism to make the spectacle correction intolerable.
• Edge design: Reflections, flashes and shimmering peripheral lights are related to the edge design. Most lenses had rounded edges to avoid reflected images.
• Optical transmission: After removal of crystalline lens, light with wavelengths of 300- 400 nm reaches to the retina. Ultraviolet light filtration is done in almost all IOLs to reduce blue light hazard to the eye.
• Material: IOLs are mainly made up of PMMA, silicone or acrylic. Silicone and acrylic lenses are foldable so that they can be implanted through small incisions (2.2 to 3.5 mm in length). Higher the refractive index, flatter is the curvatures of lens. Due to flatter curvature, acrylic lens is the thinnest of all.
• Special lenses: There are three special types of IOL viz. Multifocal, toric and aspheric.

-       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

• Net corneal power.
• Axial length.
• IOL power.
• Effective lens position.
• Desired refraction.
• Vertex distance.

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.

• Calculation method: For this, three parameters must be known viz. Net corneal power, refraction status before keratorefractive procedure, and the stabilised refraction after the keratorefractive procedure. This method usually is most accurate because the preoperative net corneal power values and refraction usually are accurate to ± 0.25 D.
• Trial hard contact lens method: The trial hard contact lens method requires a plano hard contact lens with a known base curve and a patient whose cataract does not prevent refraction to about ± 0.50 D. The spheroequivalent refraction is calculated by normal refraction. The refraction is then repeated with hard contact lens. If the spheroequivalent refraction does not change with contact lens, then the power value for cornea of the patient must be same as that of the base curve of the plano contact lens in place.
• Corneal topography: Corneal topography measure more than 5000 points over the entire cornea and more than 1000 points within the central 3 mm of cornea. This allows a very accurate determination of the anterior surface of the cornea. However, it does not provide any information about the posterior surface of the cornea. Most IOL calculations use a net index of refraction (1.3333) and the anterior radius of the cornea to calculate the net power of the cornea. Topography provides an excellent overview of central and peripheral corneal shape. Corneal topography does not provide accurate corneal power following keratorefractive surgery (radial keratotomy, photorefractive keratectomy, or laser assisted in-situ keratomileusis) with optical zones of 3 mm or less. In radial keratotomy with larger optical zones, the topography becomes more reliable. Calculation method and hard contact lens trial method are always more reliable.
• Automated keratometry: Automated keratometers usually are more accurate than manual keratometers for corneas of small optical zone of about 3 mm for radial keratotomy. Automated keratometry measurements following photorefractive keratectomy or laser assisted in-situ keratomileusis, yield better values of the front radius of the cornea. Like corneal topography, measurements are still not accurate.
• Manual keratometry: Manual keratometers provide the least accurate measure of central corneal power following keratorefractive surgery, because the area that they measure usually is larger than 3.2 mm in diameter. Thus, measurements in this area are extremely unreliable for radial keratotomy corneas that have optical zones less than or equal to 4mm.The manual keratometer has similar problem with photorefractive keratectomy or laser assisted in-situ keratomileusis as with topography or automated keratometers and, thus, is less accurate.

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

• Expected lens position in millimetres (distance from corneal vertex to principal plane of IOL).
• IOL power in dioptres.
• Net corneal power in dioptres.
• Preoperative refraction in dioptres.
• Desired postoperative refraction in dioptres.
• Vertex distance in millimetres of refraction.

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

• Emmetropia: Emmetropia or normal refractive state is produced when the power of IOL matches with the refractive state of the eye. This is the most sought after ideal situation. Patients need correction only for reading or near vision.
• Consecutive myopia: This is produced when the implanted IOL overcorrects the refraction of the eye. Such patients require correction with glasses for correction of myopia for distance vision and may or may not require glasses for reading or near vision, depending upon the degree of myopia.
• Consecutive hypermetropia: This condition is produced when the IOL implanted has power less than the refractive state of the eye. Such patients require correction with convex glasses for distance and an additional plus correction for reading or near vision.
• Astigmatism: Surgically induced astigmatism of varying degree may also be present in pseudophakia.

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

• Assessment of visual acuity.
• Tonometry to measure intraocular pressure.
• Corneal examination for any oedema.
• Gonioscopy to assess angle of the anterior chamber.
• Anterior chamber for cells, flare or presence of vitreous.
• Iris for any iridectomy or iritis.
• Position and stability of IOL.
• Integrity and clarity of posterior capsule.
• Ophthalmoscopy to rule out any cystoid macular oedema.

Signs:

• Visible surgical wound in the cornea or sclera near the limbus.
• Anterior chamber is of normal depth or only slightly deeper than normal.
• Iridodonesis (tremulousness) of iris is usually not there.
• There are four Purkinje images. Third and fourth Purkinje images from the IOL are very bright.
• Pupil appears blackish in colour and shining reflexes are observed when light is flashed in pupillary area.
• Presence of an IOL implant, which may be located in anterior chamber, iris plane, capsular bag or ciliary sulcus with or without suture fixation to the iris or sclera.
• Peripheral iridectomy (may or may not be present).
• Fundus examination on ophthalmoscopy shows normal size of the optic disc.
• Visual acuity and refractive state varies, depending upon the power of implanted IOL.
• Possible findings may include decentered IOL, iris capture of IOL, increased intraocular pressure (IOP), hyphaema, iridocyclitis, corneal oedema, vitreous in anterior chamber or cystoid macular oedema.

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

• Accommodating lenses: These are designed to dynamically move in the antero- posterior visual axis in response to ciliary body contraction and relaxation.
• Multifocal static lenses: These are designed to create multiple focal points for distance and near vision.

Prognosis:

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

• Decentered IOL.
• Iris capture of IOL.
• Increased intraocular pressure (IOP).
• Hyphaema (blood in anterior chamber).
• Iridocyclitis.
• Corneal oedema
• Vitreous in anterior chamber.
• Cystoid macular oedema.