Refractive error

Manifest Refraction. Topically applied cycloplegics may affect the manifest (subjective) refractive error. When indicated, cycloplegic refraction may be performed after the initial manifest refraction or as the initial refractive procedure in children (see Chapter 21). 

Ocular Effects. Drug-induced transient myopia has been reported with several sulfonamides. Acetazolamide, an imsubstituted heterocyclic sulfonamide, has also been associated with myopic shifts in refractive error. Shallowing of the anterior chamber is the only variable documented to change in eyes exhibiting this increase in myopia after sulfonamide therapy. Myopia probably results from ciliary body edema that produces a forward displacement of the lens-iris diaphragm. The myopia subsides on reduction or discontinuation of acetazolamide therapy (see Chapter 35). 

It is wise to perform a cycloplegic examination in infants, toddlers, and preschoolers because these children often have variable fixation with accommodative fluctuations. Clearly, as an objective method for determining refractive error in infents and young children, cycloplegic techniques are superior to those that are noncycloplegic. Not only is cycloplegic retinoscopy of infants and young children more accurate, it is also more easily performed because the examination does not depend on the patient s fixation distance. 

Cycloplegic examination is recommended for patients who are mentally impaired and for patients who are unresponsive or inconsistent in their responses to subjective refraction. Indeed, this may be the only way the clinician can determine the degree of refractive error, if any. In a... 

Visual acuity not corrected to a predicted level Patients whose symptoms seem unrelated to the nature or degree of the manifest refractive error... 

Cycloplegic refraction is also indicated for patients with active accommodative systems whose best corrected visual acuity in each eye is less than 20/20 and for whom there is no apparent reason for the decreased vision. It allows the clinician to determine whether uncorrected refractive error is responsible for the reduced acuity. This data may be particularly helpful in young patients with uncorrected antimetropia, latent hyperopia, or hyperopic anisometropia. 

Amblyopic patients tend to have inaccurate responses during subjective refraction, thus necessitating cycloplegic evaluation. Cycloplegic retinoscopy reveals the true refractive error from which the clinician can base the patient s refractive correction. 

What are the patient s imaided visual acuities at distance and near If visual acuity is reduced secondary to uncorrected refractive error, a spectacle prescription is indicated. 

Is the refractive error outside of normal limits for the patient s age If so, reflactive correction is likely indicated. 

Is the refractive error potentially amblyogenic If the answer is yes, then prescribing a refractive correction is a must. 

If the patient is a child, is the child having any school difficulties An uncorrected refractive error may contribute to academic problems. 

Posterior scleritis is associated with anterior scleritis 60% of the time. Monitoring for change in visual acuity is important in scleritis because it may indicate new or progressive posterior involvement. Serial refractions can reveal scleritis-induced refractive error changes and scleral depression can identify and localize an area of posterior scleritis by eUciting intense pain when applied to the involved site. 

Three basic types of cataract extraction are available intracapsular cataract extraction, exttacapsular cataract extraction with nuclear expression, and extracapsular cataract extraction with phacoemulsification-aspiration. Intraocular lens (lOL) implantation typically is performed at the time of cataract extraction to help correct postoperative refractive error. Extracapsular cataract extraction with phacoemulsification-aspiration is the predominant surgery today, whereas the relative numbers of planned extracapsular cataract extraction, and particularly intracapsular cataract extraction, procedures have diminished. 

Repka MX,Wellish K,Wisnicki FIJ, et al. Changes in the refractive error of 94 spectacle treated patients with acquired accommodative esotropia. Binocular Vision 1989 4 15-21. 

Contact lenses can produce changes in corneal shape and/or corneal thickness substantive enough to cause lOP measurement error. This may be particularly true of patients who are prescribed orthokeratology for the management of refractive error. In addition, there is evidence that many soft lens wearers may develop corneal edema during the day. Low levels of contact lens-related edema (<5%) may produce a stiffening of the corneal tissue with a corresponding measured increase in lOP When edema levels increase beyond 6% to 10% (which is less common in contact lens wear), the cornea becomes substantially softer with subsequent lower measured lOP... 

Direct Ophthalmoscopy. The direct ophthalmoscope is perhaps an imdemtilized instmment in the assessment of glaucoma. It can provide information regarding pupil function, an estimation of the anterior chamber angle depth, spherical refractive error of the patient, presence of media opacity, and a magnified view of the optic nerve... 

Because carbonic anhydrase inhibitors are sulfonamide derivatives, the mechanism for carbonic anhydrase inhibitor-induced myopia is expected to be similar to that associated with sulfonamides. Indeed, it has been speculated that myopia resulting from acetazolamide use is due to a hypersensitivity reaction that leads to ciliary body edema. The instillation of cycloplegics has little influence on the refractive error, which suggests that the mechanism is unrelated to ciliary spasm. 

Figure 35-7 Mechanism of drug-induced myopia. (A) Image on retina in normal eye. (B) Drug-induced ciliary body edema causes relaxation of zonules, which in turn causes thickening of crystalline lens and myopic shift of refractive error.

Allergy is managed by withdrawal of the drug and supportive therapies (consider steroids). Reduce or discontinue the drug in consultation with the prescribing physician. Positive dechaUenge wiU occur if the refractive error change subsides within several days or weeks. 

Daubs JG, Crick RP (1981) Effect of refractive error orr the risk of ocular hypertension and open angle glaucoma. Trarrs Ophthalmol Soc UK 101 121-126. 

The mydriatic and cycloplegic activities of anticholinergics in humans are listed in Table 3.18. Atropine is recommended in situations requiring complete and prolonged relaxation of the sphincter of iris and the ciliary muscle. Mydriatics, like cyclopentolate, eucatropine, and homatropine bromide, with a shorter duration of action, are usually preferred for measuring refractive errors because of the relative rapidity with which their cycloplegic effects are terminated. 

Surgery to correct refractive errors for the improvement of visual acuity generally aims at changing the corneal curvature. Corneal tissue is mainly constituted of a network of natural collagen polymer with the void volume filled with a special type of aqueous saline solution. A wide variety of laser and nonlaser procedures, such as laser-assisted in situ keratomileusis (LA-SIK), laser-cut channels for intracorneal ring segment (ICRS) implantation, femtosecond lamellar keratoplasty (FLK), intrastromal vision correction, and corneal transplantation have been developed for refractive surgical correction [83, 84]. They include removal of corneal tissue from the surface, removal of corneal tissue from the interior or stroma, and alteration of the corneal mechanical properties to produce a refractive effect. The most popular by far are procedures based on excimer laser ablation of the corneal surface and/or stroma [85]. 

Eye Examination of retina and optic disc. In the treatment of iridocyclitis, choroditis and keratitis as well as for accurate measurement of refractive errors... 

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