Aphakia

Aphakia is a condition in which crystalline lens of the eye is not present in its normal position of pupillary area. Either there is no crystalline lens inside the eye or the lens is subluxated or luxated. This induces a refractive state which is not a refractive error in true sense. An emmetropic (normal refraction) eye or one having low grade ametropia (refractive error) becomes extremely hypermetropic after cataract surgery. A previously emmetropic eye requires a correction of about 10 to 11 dioptres (D) in spectacle when worn in the usual position.

Replacement of the crystalline lens with a spectacle lens causes the image on patient’s retina to be roughly 25% larger than the image formed with the crystalline lens. The exact magnification is determined by the power of the aphakic spectacles. There is about 2 % of magnification for each dioptre of power in the spectacles.

Aphakia comes from two Greek words (a means none and phacos means lens). It refers to both an anatomical and an optical condition.

Von Helmholtz (1855) worked on the optics of aphakia. Benito Daza De Valdes (1623) suggested that aphakia may be corrected with spectacles.

References:

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

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

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

Agarwal Sunita, Agarwal Athiya, Apple David J, Buratto Lucio, Aliό Jorge L, Pandey Suresh K, Agarwal Amar. Textbook of Ophthalmology Vol 1. Jaypee Brothers Medical Publishers (P) Ltd 2002. P 170- 173.

Basak Samar K. Atlas of Clinical Ophthalmology Second Edition. Jaypee Brothers Medical Publishers (P) Ltd. 2013. P 269.

Yanoff Myron, Duker Jay S. Ophthalmology Third Edition. Mosby Elsevier 2009. P 413- 420.

Bennett Edward S, Weissman Barry A. Clinical Contact Lens Practice. Lppincott Williams & Wilkins 2005. P 595- 604.

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

Friedman Neil J, Kaiser Peter K. Essentials of Ophthalmology First Edition. Saunders Elsevier 2007. P 228.

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

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4074478

http://www.nature.com/eye/journal/v2/n1/full/eye198812a.html

http://www.ijo.in/article.asp?issn=0301-4738;year=1989;volume=37;issue=2;spage=54;epage=57;aulast=Alpar

https://www.ncbi.nlm.nih.gov/pubmed/3044126

Helmholtz von H. Uber die Akommodation des Auges. Graefe Arch Ophthalmol. 1855; 1: 1- 89.

Helmholtz von H. Handbuch der Physiologishen Optik Third Edition Vol 1, Menasha, Wisconsin: The Optical Society of America, 1909.

Aphakia causes symptoms such as

• Diminution of uncorrected vision.
• Inability to focus (due to loss of accommodation caused by aphakia).

Causes of aphakia are

• Surgical aphakia: Surgical aphakia is the commonest type and is produced after removal of the crystalline lens.
• Congenital absence of lens: There may be congenital absence of lens. It is a rare condition.
• Absorption of lens matter: Rarely, absorption of lens matter may be seen after trauma in children.
• Traumatic extrusion of lens: Rarely, there may be traumatic extrusion of lens from the eye leading to aphakia.
• Posterior dislocation of lens: Posterior dislocation of lens in to the vitreous may cause optical aphakia.
• Heritable disorders associated with dislocated lenses: These may be

-       Marfan’s syndrome.

-       Weil-Marchesani syndrome.

-       Homocystinuria.

-       Sulphite oxidase deficiency.

• Heritable disorders reported with subluxated lenses: These include

-       Aniridia.

-       Ehlers- Danlos syndrome.

-       Craniofacial dysostosis.

-       Alport’s syndrome.

-       Megalocornea.

• Ocular diseases which may lead to subluxated lenses: It includes

-       Hypermature cataract.

-       Buphthalmos.

-       Exfoliation syndrome of lens.

-       Intraocular tumours.

Optics of aphakia:

• Optical changes: Optical changes seen after removal of crystalline lens are

-       The eye is highly hypermetropic with total power of about 44 D (power of eye with crystalline lens is about 60 D).

-       The anterior and posterior focal points are about 23 mm and 31 mm, respectively.

-       The two principal points are close to each other near anterior surface of cornea.

• Image formation: The size of image varies in spectacle- corrected aphakia and contact lens- corrected aphakia.

-       Spectacle- corrected aphakia: There is magnification of about 25 % with aphakic spectacle correction.

-       Contact lens- corrected aphakia: There is less magnification (6-8%) as compared to spectacle correction.

• Visual acuity: Visual acuity in spectacle- corrected aphakia seems better due to large size of the image. The recorded vision is theoretically better than the actual visual acuity.
• Binocular vision: Aniseikonia (unequal size of retinal image) less than 5 %, is compatible with binocular vision. Aniseikonia is harmful to the development of normal binocular function.
• Diplopia (double vision): Adult patients who have normal vision in un-operated eye find it difficult to attain binocular single vision and may suffer with diplopia.
• Suppression amblyopia: Monocular aphakic children usually develop suppression amblyopia. It may lead to deviation of the operated eye.
• Accommodation: Absence of crystalline lens leads to total loss of accommodation. Therefore, a patient needs bifocal, progressive or separate pair of glasses.

Diagnosis depends upon the symptoms and signs of aphakia.

Signs of aphakia:

• Retinoscopy: Retinoscopy shows high hypermetropia.
• Purkinje’s image: There are only two Purkinje’s images coming from the cornea. Images from the lens surface are missing.
• Scar at limbus: Surgical aphakia may show scar at the limbus.
• Anterior chamber: Anterior chamber is deep due to absence of lens from the eye.
• Pupil: The pupil appears jet black in the absence of lens.
• Iridodonesis: Iridodonesis or tremulousness of iris is present due to lack of support being provided by the lens.
• Fundus: Examination shows hypermetropic fundus and the optic disc appears small.

Management should be carried out under medical supervision.

Medical optical therapy:

Optical therapy of aphakia comprises of prescribing appropriate convex lenses. The convex lenses may be prescribed in the form of

• Spectacles:  Usually about + 10 D of correction with appropriate cylindrical power for surgically induced astigmatism is prescribed to correct aphakia in previously emmetropic eye. Exact power of glasses required differs in individual cases. Further addition of plus power is required for presbyopia (near vision) to compensate for loss of accommodation. Aphakic spectacles may have glasses in various forms such as aspherical, lenticular or full field lenses. Spectacles causes

-       Magnification of image: The size of the image is magnified by about 25 %. Spectacles produce diplopia in cases of unilateral aphakia. The larger image of an object is interpreted to be closer to the eye. It requires efforts to coordinate between eyes and hand due to magnification. It produces difficulty in climbing stairs as well.

-       Prismatic aberrations: It produces roving ring scotoma, usually described as Jack-in-the-box phenomenon. The prismatic effect is produced by the peripheral part of the correcting lens. It produces a ring scotoma. When patient sees an object and turns the eye towards it, the scotoma may shift inwards to occlude the object. On shifting the eye from the object, the scotoma shifts again and the object becomes visible.

-       Spherical aberrations: When the objects are viewed through the periphery, the objects look enlarged, proximal, and elongated in radial direction. Pin-cushion effect produces distortion and it is the property of all plus lenses proportional to their diopteric power. This distortion makes a square look like a pincushion- the corners of the square have a stretched out appearance, and the sides are pushed in. The world becomes parabolic which continuously change its shape with eye movement. The objects appear to move faster. Driving a motor vehicle thus becomes very difficult.

-       Field of vision: Monocular as well as binocular field of vision is markedly reduced. The actual field of view through the spectacles is reduced by 25 %, which makes it impossible to see that much of peripheral field.

-       Coloured vision: There may be coloured hue in the vision. It occurs due to absence of natural filter of a crystalline lens. Removal of the crystalline lens increases blue and ultraviolet transmission of light.

-       Thick glasses: The glasses are very thick and heavy. Bifocal glasses are especially difficult to adjust with and may require separate pair.

• Contact lenses: Correction of aphakia at the corneal plane requires use of contact lenses. As the position at which the optical correction is made moves closer to retina, the necessary diopteric power increases but the subsequent magnification decreases. A patient who needs 12.5 D in aphakic spectacles would need about 14.7 D in a soft rigid contact lens. At the corneal plane the magnification is 6- 8%. Since the value is near the limit of aniseikonia, most unilateral aphakia patients can have binocular vision. Contact lenses have certain advantages over the glasses such as

-       Provides wider and better field of vision.

-       There is less magnification of the image since the contact lenses are close to the cornea.

-       More suited for uniocular aphakia.

-       It eliminates spherical aberration and prismatic effect.

-       It is cosmetically more acceptable.

However, there may be lack of dexterity in older patients and intolerance owing to a foreign body sensation.

Surgical therapy:

Surgical therapy includes

• Intraocular lens (IOL) implantation: IOL implantation is the best method of correcting aphakia. Calculation of power of an IOL depends upon keratometry (measured with keratometer) of the cornea and axial length of the eyeball (measured with ultrasonography). A patient who needs 12.5 D in apahkic spectacles would need about 21 D of an IOL in posterior chamber of eye. The average magnification due to an IOL in posterior chamber is about 1.5%, compared with the original crystalline lens. Almost everyone can achieve binocular vision with one eye pseudophakia and other phakic. 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.

• Refractive corneal surgery: Corneal onlays, such as epikeratophakia and inlays, such as keratophakia are not used commonly in clinical settings. The optical effects are no different from those of contact lens, but onlays and inlays have the advantage that the patient does not need maintenance. Excellent success of contact lenses and IOLs mean that surgical techniques to correct aphakia at the cornea are not reasonable clinical alternatives. The procedures include

-       Keratophakia: A lenticule obtained from donor cornea is placed in between the lamellar layers of the cornea of the patient.

-       Epikeraophakia: A lenticule obtained from donor cornea is stitched over the surface of cornea after removing the epithelium. Epikeraophakia has the advantage that it may be done in patients where, because of age or ocular condition, a secondary or even primary IOL is not feasible. Since the eyeball is not entered, many complications can be avoided. Also, this procedure may be repeated.

-       Hypermetropic Laser assisted in-situ keratomileusis (LASIK).

Prognosis:

Prognosis for aphakia is generally good, unless there are complications like corneal oedema, cystoid macular oedema or secondary glaucoma.

However, aphakia increases the risk of retinal detachment, especially in high myopes and if the posterior capsule is ruptured.