distortion, whereas young children always adapt to their cylindrical corrections.

The following guidelines may prove helpful in prescribing astigmatic spectacle corrections:

For children, prescribe the full astigmatic correction at the correct axis.

For adults, try the full correction initially. Give the patient a “walking-around” trial with trial frames before prescribing, if appropriate. Inform the patient about the need for adaptation. To reduce distortion, use minus cylinder lenses (most lenses dispensed today are minus cylinder) and minimize vertex distance.

Because spatial distortion from astigmatic spectacles is a binocular phenomenon, occlude 1 eye to verify that spatial distortion is the cause of the patient’s difficulty.

If necessary, reduce distortion by rotating the axis of the cylinder toward 180° or 90° (or toward the old axis) and/or reduce the cylinder power. Adjust the sphere to maintain spherical equivalent, but rely on a final subjective check to obtain the most satisfactory visual result.

If distortion cannot be reduced sufficiently, consider contact lenses or iseikonic corrections.

For a more detailed discussion of the problem of, and solutions for, spectacle correction of astigmatism, see Appendix 3.1 at the end of the chapter.

Prescribing for Children

The correction of ametropia in children presents several special and challenging problems. In adults, the correction of refractive errors has 1 measurable endpoint: the best-corrected visual acuity. Prescribing visual correction for children often has 2 goals: providing a focused retinal image and achieving the optimal balance between accommodation and convergence.

In some young patients, subjective refraction may be impossible or inappropriate, often because of the child’s inability to cooperate with subjective refraction techniques. In addition, the optimal refraction in infants or small children (particularly those with esotropia) requires the paralysis of accommodation with complete cycloplegia. In such cases, objective techniques such as retinoscopy are the best way to determine the refractive correction. Moreover, the presence of strabismus may require modification of the normal prescribing guidelines.

Myopia

There are 2 types of childhood myopia: congenital (usually high) myopia and developmental myopia, which usually manifests itself between the ages of 7 and 10 years. Developmental myopia is less severe and easier to manage because the patients are older and refraction is less difficult. However, both forms of myopia are progressive; frequent refractions (every 6–12 months) and periodic prescription changes are necessary. The following are general guidelines for correction of significant childhood myopia:

Cycloplegic refractions are mandatory. In infants, children with esotropia, and children with very high myopia (>10.00 D), atropine refraction may be necessary if tropicamide or cyclopentolate fails to paralyze accommodation in the office.