Wednesday, December 29, 2010

Hypermetropia

Hypermetropia (hyperopia) or long-sightedness is therefractive state of the eye wherein parallel rays oflight coming from infinity are focused behind theretina with accommodation being at rest (Fig. 3.22).Thus, the posterior focal point is behind the retina,which therefore receives a blurred image.

EtiologyHypermetropia may be axial, curvatural, index,positional and due to absence of lens.1. Axial hypermetropia is by far the commonestform. In this condition the total refractive powerof eye is normal but there is an axial shorteningof eyeball. About 1–mm shortening of the anteroposteriordiameter of the eye results in 3 dioptresof hypermetropia.2. Curvatural hypermetropia is the condition inwhich the curvature of cornea, lens or both isflatter than the normal resulting in a decrease inthe refractive power of eye. About 1 mm increasein radius of curvature results in 6 dioptres ofhypermetropia.3. Index hypermetropia occurs due to decrease inrefractive index of the lens in old age. It may alsooccur in diabetics under treatment.4. Positional hypermetropia results from posteriorlyplaced crystalline lens.5. Absence of crystalline lens either congenitally oracquired (following surgical removal or posteriordislocation) leads to aphakia — a condition ofhigh hypermetropia.Clinical typesThere are three clinical types of hypermetropia:1. Simple or developmental hypermetropia is thecommonest form. It results from normal biologicalvariations in the development of eyeball. It includesaxial and curvatural hypermetropia.2. Pathological hypermetropia results due to eithercongenital or acquired conditions of the eyeball whichare outside the normal biological variations of thedevelopment. It includes :Index hypermetropia (due to acquired corticalsclerosis),Positional hypermetropia (due to posteriorsubluxation of lens),Aphakia (congenital or acquired absence of lens)andConsecutive hypermetropia (due to surgicallyover-corrected myopia).3. Functional hypermetropia results from paralysisof accommodation as seen in patients with third nerveparalysis and internal ophthalmoplegia.Nomenclature (components of hypermetropia)Nomenclature for various components of thehypermetropia is as follows:Total hypermetropia is the total amount of refractiveerror, which is estimated after complete cycloplegiawith atropine. It consists of latent and manifesthypermetropia.1. Latent hypermetropia implies the amount ofhypermetropia (about 1D) which is normallycorrected by the inherent tone of ciliary muscle.The degree of latent hypermetropia is high inchildren and gradually decreases with age. Thelatent hypermetropia is disclosed when refractionis carried after abolishing the tone with atropine.2. Manifest hypermetropia is the remaining portionof total hypermetropia, which is not corrected bythe ciliary tone. It consists of two components,the facultative and the absolute hypermetropia.i. Facultative hypermetropia constitutes thatpart which can be corrected by the patient'saccommodative effort.ii. Absolute hypermetropia is the residual partof manifest hypermetropia which cannot becorrected by the patient's accommodativeefforts.Thus, briefly:Total hypermetropia = latent + manifest (facultative +absolute).Clinical pictureSymptomsIn patients with hypermetropia the symptoms varydepending upon the age of patient and the degree ofrefractive error. These can be grouped as under:1. Asymptomatic. A small amount of refractive errorin young patients is usually corrected by mildaccommodative effort without producing anysymptom.2. Asthenopic symptoms. At times the hypermetropiais fully corrected (thus vision is normal) but due

to sustained accommodative efforts patientdevelops asthenopic sysmtoms. These include:tiredness of eyes, frontal or fronto-temporalheadache, watering and mild photophobia. Theseasthenopic symptoms are especially associatedwith near work and increase towards evening.3. Defective vision with asthenopic symptoms.When the amount of hypermetropia is such thatit is not fully corrected by the voluntaryaccommodative efforts, then the patients complainof defective vision which is more for near thandistance and is associated with asthenopicsymptoms due to sustained accommodativeefforts.4. Defective vision only. When the amount ofhypermetropia is very high, the patients usuallydo not accommodate (especially adults) and thereoccurs marked defective vision for near anddistance.Signs1. Size of eyeball may appear small as a whole.2. Cornea may be slightly smaller than the normal.3. Anterior chamber is comparatively shallow.4. Fundus examination reveals a small optic discwhich may look more vascular with ill-definedmargins and even may simulate papillitis (thoughthere is no swelling of the disc, and so it is calledpseudopapillitis). The retina as a whole may shinedue to greater brilliance of light reflections (shotsilk appearance).5. A-scan ultrasonography (biometry) may reveal ashort antero-posterior length of the eyeball.ComplicationsIf hypermetropia is not corrected for a long time thefollowing complications may occur:1. Recurrent styes, blepharitis or chalazia mayoccur, probably due to infection introduced byrepeated rubbing of the eyes, which is oftendone to get relief from fatigue and tiredness.2. Accommodative convergent squint may developin children (usually by the age of 2-3 years) dueto excessive use of accommodation.3. Amblyopia may develop in some cases. It may beanisometropic (in unilateral hypermetropia),strabismic (in children developing accommodativesquint) or ametropic (seen in children withuncorrected bilateral high hypermetropia).4. Predisposition to develop primary narrow angleglaucoma. The eye in hypermetropes is smallwith a comparatively shallow anterior chamber.Due to regular increase in the size of the lenswith increasing age, these eyes become prone toan attack of narrow angle glaucoma. This pointshould be kept in mind while instilling mydriaticsin elderly hypermetropes.TreatmentA. Optical treatment. Basic principle of treatment isto prescribe convex (plus) lenses, so that the lightrays are brought to focus on the retina (Fig. 3.23).Fundamental rules for prescribing glasses inhypermetropia include:

1. Total amount of hypermetropia should always bediscovered by performing refraction undercomplete cycloplegia.2. The spherical correction given should becomfortably acceptable to the patient. However,the astigmatism should be fully corrected.3. Gradually increase the spherical correction at 6months interval till the patient accepts manifesthypermetropia.4. In the presence of accommodative convergentsquint, full correction should be given at the firstsitting.5. If there is associated amblyopia, full correctionwith occlusion therapy should be started.Modes of prescription of convex lenses1. Spectacles are most comfortable, safe andeasy method of correcting hypermetropia.2. Contact lenses are indicated in unilateralhypermetropia (anisometropia). For cosmeticreasons, contact lenses should be prescribedonce the prescription has stabilised, otherwise,they may have to be changed many a times.

Surgical Treatment:

Refractive surgery for hyperopiaIn general, refractive surgery for hyperopia is not aseffective or reliable as for myopia. However, followingprocedures are used:1. Holmium laser thermoplasty has been used forlow degree of hyperopia. In this technique, laser spotsare applied in a ring at the periphery to produce centralsteepening. Regression effect and inducedastigmatism are the main problems.2. Hyperopic PRK using excimer laser has also beentried. Regression effect and prolonged epithelialhealing are the main problems encountered.3. Hyperopic LASIK is effective in correctinghypermetropia upto +4D.4. Conductive keratoplasty (CK) is nonablative andnonincisional procedure in which cornea is steepenedby collagen shrinkage through the radiofrequencyenergy applied through a fine tip inserted into theperipheral corneal stroma in a ring pattern. Thistechnique is effective for correcting hyperopia of upto3D.