June 2009

Teen With Ocular Albinism Successfully Switches From Spectacles to Contact Lenses

History and Clinical Exam

HS was a 14 year old girl diagnosed with oculocutaneous albinism who expressed interest in contact lens correction because her spectacles were thick and heavy. She denied any light sensitivity. Her best-corrected visual acuities were 20/100 OD and 20/160 OS, with a refraction of:

OD +10.00 –5.50 x 003
OS +8.50 –5.50 x 180

Keratometry findings were:

OD 41.37 @ 011/47.50 @ 101
OS 41.00 @ 175/ 48.00 @ 085

In addition to congenital nystagmus, cover test revealed a 15-prism diopter left exotropia at distance and a nearly equal right exotropia at near. Obvious oculocutaneous albinism was observed (see photo), but otherwise all ocular structures appeared healthy.

Assessment and Plan

Due to HS’s high degree of astigmatism, I recommended toric GP lenses. This approach offered the potential for more consistent, clear vision than what could be expected with a soft toric lens. In addition, superior oxygen transmissibility provided by a GP lens minimized the risk of corneal complications associated with hypoxia. Because of the remarkable absence of photophobia, it was not necessary to incorporate an occlusive feature in the lenses.

The patient’s upper eyelid rested tangent to the superior limbus, so I decided on an interpalpebral fitting approach and chose an intermediate diameter of 8.8 mm for each eye. I selected base curves to provide a near-alignment fitting relationship in the horizontal meridian and a slightly flatter fitting relationship vertically. I felt this approach would enhance vertical lens movement with the blink. I chose a base curve 0.25D flatter than K for the horizontal meridian and 1.00D flatter than K for the vertical meridian, resulting in a 0.75D flatter fit in the vertical meridian than in the horizontal meridian.

The following base curves were derived:

OD 41.12 D (8.21 mm) horizontal, 46.50D (7.26 mm) vertical
OS 40.75 D (8.28 mm) horizontal, 47.00D (7.18 mm) vertical.

Accounting for vertex changes and tear lens effects, the resulting powers were:

	Spectacle plane	Corneal plane	Tear lens	Final Power
OD	+9.25	+10.75	–0.25	+11.00 horizontal
	+3.50	+3.50	–1.00	+4.50 vertical
OS	+8.50	+9.50	–0.25	+ 9.75 horizontal
	+3.25	+3.25	–1.00	+4.25 vertical

A toric base curve will create about one and a half times more toric power than what exists in the toricity of the base curve. This is because of the higher index of lens materials compared to the index of the keratometer (n=1.3375), whose index is used in diopter-to-mm base curve conversion charts. For Boston EO material (n=1.429), 1.3 times is a more precise conversion factor.

For the right lens, a calculated 5.38D difference in base curve could be expected to create 5.38 x 1.3 = 6.99D of toric power. The desired toric power was 11.00 –4.50 = 6.50D. Because this is within a half diopter, I ordered a toric base/spherical front design (back surface toric). Many labs assess a lower fee for a back surface toric design compared to a bitoric because a toric surface needs to be generated on only one surface.

For the left lens, a calculated 6.25D difference in base curve could be expected to create 6.25 x 1.3 = 8.12D of toric power. The desired toric power was 9.75 –4.25 = 5.50D. It was, therefore, necessary to order a toric back/toric front design (bitoric).

Dispensing Visit and Long-term Outcome

Lenses were ordered and dispensed, providing HS with visual acuity comparable to what she achieved with her spectacles. I prescribed a pair of +4.00 readers for her to wear over her contact lenses for additional magnification when performing detailed near tasks.

Due to their thickness, both contact lenses positioned inferiorly on the cornea, but moved well with the blink. No apparent corneal compromise was observed with slit lamp examination. Nonetheless, I reordered lenses in a minus carrier design to try to promote lid attachment. These lenses also dropped and were not as comfortable as the previous lenses, so I returned the previous lenses to the patient. This design has continued to serve HS well for more than 8 years.

By Thomas G. Quinn, OD, MS, FAAO
Athens, Ohio

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