Coping With Astigmatism Using RGP Multifocals

BY DAVID W. HANSEN, O.D.
DEC. 1997

Astigmatism must be the most dreaded diagnosed visual problem. Patients report negatively that they have it, practitioners warn patients that it adversely affects contact lens fitting, and when coupled with presbyopia, it limits the options for bifocal contact lens selection. But astigmatism can be a contact lens practitioner's friend if approached systematically. It's important to determine the type of astigmatism to properly design the diagnostic lenses and thus reduce chair time.

CORNEAL VS. REFRACTIVE

Differentiating between corneal toricity and refractive astigmatism helps you select the appropriate multifocal. You can use the following examples as your guide:

• Example 1: When refractive astigmatism and corneal astigmatism are equal.

  Distance Refraction: -2.00-1.00 x 180 OD; -2.00-1.00 x 175 OS .

  Central K's: 42.50/43.50@90 OD; 42.75/43.87@85 OS.

  Expected residual astigmatism: Plano OD; 0.12D OS.

  Choice of RGP multifocal: simultaneous (aspheric) or translating.

  
  

• Example 2: When refractive astigmatism is greater than corneal astigmatism.

  Distance Refraction: -2.00-1.00 x 180 OD; -2.00-1.00 x 180 OS.

  Central K's: 42.50D sph OD; 43.00D sph OS.

  Expected residual astigmatism: 1.00D OD; 1.00D OS.

  Choice of RGP multifocal: translating with front toric design.

  
  

• Example 3: When refractive astigmatism is less than corneal astigmatism.

  Distance Refraction: -2.00D sph OD; -2.00D sph OS.

  Central K's: 42.50/44.50@90 OD; 43.00/45.00@90 OS.

  Expected residual astigmatism: 2.00D OD; 2.00D OS.

  Choice of RGP multifocal: translating with front toric design.

RGP MULTIFOCAL SELECTION

Translating with front toric -- Translating diagnostic lenses usually have a spherical base curve, but you can determine the residual astigmatism using the overrefraction. Remember to watch for rotation of the lens with the biomicroscope. Tell your laboratory the amount and direction of rotation so it can properly compensate for the astigmatism axis.

Most translating RGP bifocals must compensate for lateral movement (usually 10 degrees) by the superior lid. The FluoroPerm ST bifocal, because of the low specific gravity material in the encapsulated segment, usually needs no horizontal rotation adjustment.

Aspheric simultaneous -- Aspheric RGP multifocals are the most forgiving design selection, and they mask astigmatism more often than other multifocal designs. For optimum visual acuity, it's essential to keep the aspheric multifocal centered in front of the pupillary axis. For with-the-rule astigmatism, these lenses usually provide easy translation vertically without excessive lag. For patients with against-the-rule astigmatism, a steeper fit is usually required (I suggest 0.50D more).

Each aspheric multifocal design has a different eccentricity value and is fit with a different philosophy for achieving appropriate alignment, centering and near prescription. Know the manufacturer's suggested base curve selection criteria for each design and incorporate the astigmatism into the equation. Most base curve selections are steepened with added corneal toricity.

When fitting an aspheric contact lens with a fitting philosophy of 2.00D steeper than K, you'll probably need to modify the base curve to accommodate the corneal astigmatism. Calculate the starting base curve selection by splitting the K's (1.00D in Ex. 4) and adding 2.00D for the philosophy:

• Example 4:

  Refraction: -2.00-2.00 x 180 = +1.50 Add OD; -2.00-2.00 x 180 = +1.50 Add OS.

  Central K's: 42.50/44.50@90 OD; 42.75/44.75@90 OS.

  Initial base curve selection: 45.50D = 7.42 mm OD; 45.75D = 7.38 mm OS.

Simultaneous and translating -- Most simultaneous and translating bifocal designs require a lens-to-cornea relationship that's similar to single-vision spherical base curve designs, and astigmatism is calculated the same way. When you expect residual astigmatism, a toric prism ballasted translating design will provide maximum visual acuity. A bitoric design may be necessary when the corneal toricity is over 2.50D to provide a better lens-to-cornea relationship. CLS

Dr. Hansen, a cornea and contact lens diplomate and fellow of the American Academy of Optometry, is in private practice in Des Moines, Iowa.