Myopia

Myopia or short-sightedness is that type of refractive error where parallel rays of light coming from infinity are focused in front of the light sensitive layer of retina with eyes in position of rest.

The term ‘myopia’ was introduced due to the habit of myopic people of half closing the eyelids while looking at distant objects, so that they may have the advantages of a stenopaeic slit (a black disc punctured with a fine slit or hole).

Refractive power of the eye is determined predominantly by variables like power of the cornea, power of the 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. Astigmatism change relatively little with age.

 

References

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Khurana AK. Comprehensive Ophthalmology Sixth Edition. Jaypee Brothers Medical Publishers (P) Ltd. 2015. P 17- 18.

Khurana AK. Theory and Practice of Optics and Refraction Second Edition. Reed Elsevier India Private Limited 2008. P 71- 79.

Nema HV, Nema Nitin. Textbook of Ophthalmology Sixth Edition. Jaypee Brothers Medical Publishers (P) Ltd. 2012. P 30- 32.

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Bope Edward T, Kellerman Rick D. Conn’s Current Therapy 2016. Elsevier 2016. P 352- 358. 

Chuah Dr Gerard. A Patient's Guide to Myopia and Myopia Treatment. Effective Asset Management Pte. Ltd. 2004.

Denniston Alastair K O, Murray Philip I. Oxford Handbook of Ophthalmology. Third Edition.Oxford University Press 2014. P 548- 549.

 TokoroTakashi. Atlas of Posterior Fundus Changes in Pathologic Myopia. Springer- Verlag Tokyo 1998.

Grosvenor Theodore. Primary Care Optometry Fifth Edition. Butterworth Heinemann Elsevier 2007. P 41.

Hoyt Creig S, Taylor David. Pediatric Ophthalmology and Strabismus Fourth Edition. Elsevier Saunders 2013. P 31- 35.

Kanski Jack J, Bowling Brad. Clinical Ophthalmology- A Systematic Approach Seventh Edition. Elsevier Saunders 2011.

Lee WR. Ophthalmic Histopathology. Springer- Verlag London 1993. P 191.

Rosenfield Mark, Logan Nicola, Edwards Keith. Optometry- Science, Techniques and Clinical management Second Edition. Butterworth Heinemann Elsevier 2009. P 159- 171.

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Wright Kenneth W, Spiegel Peter H, Thompson Lisa S. Handbook of Pediatric Retinal disease. Springer 2006. P 387- 409.

https://nei.nih.gov/health/errors/myopia

http://www.aoa.org/patients-and-public/eye-and-vision-problems/glossary-of-eye-and-vision-conditions/myopia?sso=y

http://www.nhs.uk/conditions/short-sightedness/Pages/Introduction.aspx   

Congenital myopia: Child may present with

  • Anisometropia.
  • Amblyopia.
  • Congenital convergent squint.
  • Difficulty in distant vision.
  • Holding things very close to eyes for viewing.

Simple myopia

  • Poor vision for distance.
  • Asthenopia or eyestrain.
  • Change in psychological attitudes leading to more indoor activities.

Pathological myopia

  • Defective vision due to high refractive error.
  • Muscae volitantes or floating black spots due to vitreous degeneration.
  • Night blindness in high myopia due to vitreo-retinal degeneration.

 

Aetiologically, myopia may be of following types

  • Axial myopia: This is due to increased antero-posterior length of the eyeball. This is the commonest type.
  • Curvature myopia: This produced by the increased curvature of the cornea, lens or both.
  • Positional myopia: This is produced by the anterior placement of crystalline lens of the eye.
  • Index myopia: Index myopia is produced due to increase in the refractive index of crystalline lens due to nuclear sclerosis.
  • Myopia due to excessive accommodation: This occurs in patients with spasm of accommodation.

Optics

Image of distant object consists of circles of diffusion formed by the divergent beam of light, since the parallel rays of light coming from infinity are focused in front of the retinal layer.

Far point of the myopic eye is a finite point in front of the retina. Due to this, near object situated at the far point is focussed on retina without an effort of accommodation.

Nodal point lies further away from retina in myopia. Thus, the image formed is appreciably larger than an emmetropic eye and in spectacle corrected eye.

In uncorrected myopia, accommodation is not developed normally, since they need not accommodate to see the near objects clearly. This results in convergence insufficiency, exophoria and early presbyopia.

Diagnosis depends upon clinical features and retinoscopy.

Clinical types of myopia

Congenital myopia: Congenital myopia is present since birth but is usually diagnosed by the age of about 2- 3 years. Mostly, it is unilateral and manifests as anisometropia. Rarely, it may be bilateral. It is usually produced due to increase in axial length of the eyeball. Refractive error is usually around 8- 10 D. It may be associated with

  • Prematurity.
  • Marfan’s syndrome.
  • Homocystinuria.
  • Congenital convergent strabismus (squint).
  • Congenital anomalies of eyes such as aniridia, cataract, microphthalmos (small eyeballs), megalocornea (large cornea), and congenital separation of retina.

Congenital myopia may be associated with amblyopia and congenital convergent squint. In bilateral myopia, child show some noticeable difficulty in seeing distant objects and tend to hold things very close to the eyes.

 

Simple or developmental myopia: This is the commonest type and is also known as physiological or school myopia. It is not associated with any other disease of the eye. This results from biological variation in the development of eye. It may be autosomal dominant or autosomal recessive condition. Axial type may signify just a physiological variation in the length of the eyeball. Curvature type is considered to be due to underdevelopment of the eyeball.

Most patients are born hypermetropic, but later become myopic with age. It usually begins around 7 to 10years of age and increases during the growing years. Besides visual symptoms, child may have signs such as

  • Large and prominent eyeballs.
  • Slightly deeper anterior chamber.
  • Slightly dilated pupils.
  • Fundus is usually normal. Rarely, there may be myopic crescent.

 

Pathological or degenerative myopia: Pathological or degenerative myopia is a rapidly progressive refractive error resulting in high myopia during early adult life. It is usually associated with degenerative changes in the eye. It results from rapid axial growth of the eyeball. It may be linked with heredity or general growth process.

Pathological myopia results from rapid axial growth of the eyeball which does not fall within normal biological variations of development. Beside symptoms, patient show signs such as

  • Prominent large eyes due to elongation of posterior pole of eye.
  • Larger cornea.
  • Deep anterior chamber.
  • Slightly larger pupils with sluggish reaction to light.
  • Large and pale optic disc with temporal myopic crescent. It may be peri-papillary (around optic disc) in some cases.
  • Super-traction crescent (retina being pulled over the disc margin) may be present on nasal side of optic disc.
  • Degeneration of choroid and retina.
  • Foster-Fuch’s spots, as dark red circular patch due to sub-retinal neovascularisation and choroidal haemorrhage, may be present.
  • Associated lattice degeneration and/or snail track lesion may be present in the periphery of retina.
  • Retinal tear, haemorrhage or retinal detachment may occur.
  • Posterior staphyloma due to scleral ectasia at the posterior pole.
  • Vitreous degeneration.
  • Posterior vitreous detachment seen as Weiss’ reflex.
  • Contraction of visual fields and ring scotoma may be present.
  • Electroretinography may reveal sub-normal electroretinogram due to chorio-retinal atrophy.

 

Acquired myopia: Acquired myopia may be produced due to

  • Index myopia: It may be produced due to change in refractive index e.g. nuclear sclerosis of the crystalline lens.
  • Curvature myopia: Increase in curvature of cornea or lens produces curvature myopia.
  • Positional myopia: It may be produced by anterior subluxation of the crystalline lens.
  • Consecutive myopia: It may be produced due to surgical overcorrection of hypermetropia or in pseudophakia with overcorrecting intraocular lens implants.
  • Pseudo-myopia or artificial myopia: This may be produced due to excessive accommodation and spasm of accommodation. Full hypermetropic correction in children may also lead to this.
  • Drug induced myopia: Certain drugs such as pilocarpine, steroids or sulphonamides may also produce myopia.

 

Management should be carried out under medical supervision.

Medical therapy

Visual hygiene: This is important to avoid asthenopia. Proper posture and adequate illumination is required during close work.

Optical therapy:

Appropriate glasses/contact lenses may be used to provide clear retinal image.

Minimum correcting power providing maximum vision is given.

Children: Myopia up to 6 D in children, especially younger than 8 years, should be fully corrected and are advised to wear glasses constantly. It helps in developing normal accommodation- convergence reflex. Myopia should never be over-corrected.

Adults: Patients younger than 30 years, usually accept full correction. Patients over 30 years of age usually do not accept full correction, and are prescribed with glasses keeping power slightly under-corrected.

High myopia: As little correction as compatible with good vision is prescribed. Under-correction also avoids minification of the image.

Optical correction of eye includes

  • Spectacle correction: Spectacle correction may be given, but it produces minification of object size and distortion of peripheral image in cases with high myopia.
  • Contact lenses: These are particularly useful in cases of high myopia. These avoid peripheral minification in size as well as peripheral distortion of image.
  • Low vision and visual aids: Visual aids are indicated in patients with progressive myopia with retinal degenerative changes where useful vision may not be obtained with spectacles or contact lenses.

 

Surgical therapy includes:

  • Laser in situ keratomileusis (LASIK): Keratomileusis refers to carving of cornea. Excimer laser ablation is applied directly to the corneal stroma after reflecting a lamellar or partial- thickness corneal flap. The flap is returned to its original position, once ablation is complete. Recovery of vision usually takes few days and some patients even take few weeks.
  • Wavefront- guided LASIK: Wave front- guided LASIK or custom LASIK is a new wave front technology to program ablation pattern more precisely than the conventional LASIK.
  • IntraLASIK (IntraLase): IntraLASIK uses femtosecond laser, which is more accurate than a blade and provides a huge margin in terms of precision and safety. Femtosecond laser creates a corneal flap at a pre-programmed depth and position.
  • EpiLASIK: EpiLASIK is designed to create a thin flap in the epithelium. It is an excellent alternative for patients with thin and steep or flat cornea. The layer is preserved and later replaced following reshaping of cornea using excimer laser.
  • Photorefractive keratectomy (PRK): PRK also uses excimer laser to reshape cornea, but this procedure does not require lamellar flap. The laser is applied directly to the anterior stromal surface after removal of surface epithelium. A soft contact lens is prescribed after laser treatment. The corneal epithelium typically heals in about 4 to 7 days. Improvement of vision may take few weeks to several months.
  • Laser sub-epithelial keratomileusis (LASEK): It involves cleaving the epithelial sheet at the basement membrane with dilute alcohol, applying laser as in conventional PRK, and repositioning the epithelium afterward.
  • Radial keratotomy (RK): In RK, spoke- like cuts radiating from central cornea at about 90% corneal depth were given to balloon the peripheral cornea. Although RK surgery was effective, the results were often unpredictable and led to overcorrection, with progressive hypermetropia. RK surgery was soon replaced by laser vision correction with the introduction of PRK.
  • Refractive lens exchange (RLE): Refractive lens exchange (RLE) is a surgical procedure that may correct refractive error by removing and replacing the clear crystalline lens of the eye with intraocular lenses (IOLs). RLE is an option for high refractive errors, where laser vision correction is not an option.
  • Phakic intraocular lens (phakic IOL) implant: Phakic intraocular lens (phakic IOL) may be implanted in front of and attached to the iris or is placed just behind the iris. Phakic IOL is an alternative to laser vision correction with keratorefractive surgery.

 

Contraindications to surgery:

  • Corneal or anterior segment diseases such as keratitis, conjunctivitis or corneal ulcers.

 

Prognosis

The prognosis is good in simple myopia. The refractive error usually stabilises by the age of 21 years.

In pathological myopia, visual prognosis is always guarded. There is possibility of progressive visual loss due to degeneration. The retina is stretched and thin, which may lead to retinal detachment.

Complications of myopia include

  • Complicated cataract: Apparently due to abnormality in metabolism of lens.
  • Haemorrhage: It is related to degenerative changes in vitreous and retina.
  • Retinal tears: Breaks in retina occurs which may lead to retinal detachment.
  • Vitreous haemorrhage: It usually accompanies retinal tears. Choroidal haemorrhage may also leak into the vitreous.
  • Choroidal haemorrhage and thrombosis: These may lead to severe visual loss and are quite common.

 

Prevention may be needed for

  • Pathological myopia: Patients with pathological myopia may be advised to avoid marriage amongst them self to avoid similar disease in their children. 

There is no restraint on marriage and procreation between patients with simple myopia.

  • PUBLISHED DATE : Feb 21, 2017
  • PUBLISHED BY : DEEPAK CHANDRA
  • CREATED / VALIDATED BY : Dr. S. C. Gupta
  • LAST UPDATED ON : Feb 21, 2017

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