Myopic Refraction

Myopia presents with blurry distance vision but generally gives good near vision. Myopia may be low ranging between –0.25D to –3.00D, it may be medium which is usually between –3.00D to –6.00D and high myopia which is anywhere above –6.00D.

Fig: Uncorrected Myopia

• Myopia is rare at birth

• Manifests after 4th year of life

• Progression is relatively constant until the time of puberty. At around this age power changes rapidly

• Usually myopia is arrested when full maturity is reached.

Fig: Corrected Myopia

Myopia is a one symptom refraction problem, i.e. blurring of distance vision. Correction of the myopia should eliminate this symptom, but a prescription for glasses may, produce other symptoms which may be equally or more disturbing. Some of the common difficulties in prescribing for the myopic patient are:

1. Failure to recognize accommodative stress masquerading as low myopia.

2. Confusion as to when, or whether, to prescribe for the unilateral myopic patient.

3. Overcorrecting the myopic refractive error and failure to appreciate the symptoms that result from overcorrection.

4. Difficulties in gaining acceptance of bifocals by the myopic presbyopic patient.

Although myopia would appear to be the least troublesome of all refractive problems, maximizing of visual potential with comfort and safety needs refracting skills, experience, and clinical judgment.

Issues to be considered:

To create a truly clear retinal image of a distant object, the full extent of myopia must be corrected. However, several issues need to be considered.

• Room length

• Room lights

• Pseudomyopia: Over stimulation of accommodative response

• Progressive myopia : Requires frequent changes

• Prescribing full correction is based on clinical decision.

If the length of the examination room is significantly shorter than standard distance which may result in slight under correction but this is typically no more than 0.25D. to 0.50D. This discrepancy may be irrelevant. However, it depends on the individual’s tolerance for blur and his or her visual requirements.

Another factor is night myopia, which results in reduction of contrast induced by low illumination. This causes the patient’s focus to drift towards a resting level of accommodation which is not at infinity, thus inducing some degree of residual myopia. Night myopia may be symptomatic, and patients may report

blurred vision or halos around lights at night. Symptomatic night myopia requires correction, such as wearing night time driving glasses. Night myopia is primarily produced by ocular accommodative response under very low illumination level.

If a significantly myopic patient has never worn a correction before, prescribing the full correction may result in significant asthenopia. The patient may feel dizzy and uncomfortable and end up rejecting the glasses. In such cases, especially in older patients, partial correction of the myopia will dramatically improve vision, and permit easier adaptation. It is a matter of clinical judgment whether or not to prescribe the full prescription to start with or, alternatively, to prescribe only a partial correction of the myopia.

Younger patients who are engaged more in near tasks sometimes report pseudomyopia which results in blurring of distance vision brought about by spasm of the ciliary muscles.

Patient appears to have myopia due to an inappropriate accommodative response. The diagnosis is done by cycloplegic refraction using a strong cycloplegics like Atropine or Homatropine eyedrops.

Progressivemyopia requires relatively frequent alterations in the prescription. Refractive development can be ascertained through a history, previous patient record and referral information. They need more frequent consultations to monitor refractive changes.

Fogging the lens with eyes with plus lens is not very critical in case of high myopia as the insufficient spherical minus lens in front of uncorrected eye, is enough to blur the distant vision and does not allow the patient to bring the focus onto the retina by accommodation. However, in very low myopia, fogging with plus lens essentially makes the patient’s eye myopic in both the prinicipal meridians. A test chart is then presented and plus is reduced or the minus is increased. Unfogging is done until least minus brings the desired acuity.