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Ophthalmic Pearls: Comprehensive
How to Prescribe a Prism for Combined Vertical and Horizontal Strabismus
By J. Scott Kortvelesy, MD
Edited by Ingrid U. Scott, MD, MPH, and Sharon Fekrat, MD
 
 

Combined vertical and horizontal strabismus is not uncommon. Etiologies include orbital diseases such as Graves orbitopathy, trauma, decompensated phorias, intracranial ischemic or neoplastic processes, and prior scleral buckling or complex eye muscle procedures.

Managing these patients can be quite challenging. Treatment options typically include occlusion, orthoptics, prisms and/or strabismus surgery. Disadvantages of occlusion include loss of binocularity and cosmetic concerns. Orthoptics are usually ineffective in these patients. Small angles of strabismus can be corrected with glasses containing ground-in vertical and horizontal prism. For larger angles of strabismus, bilateral Fresnel prisms may correct the vertical component of the deviation on one side and the horizontal component on the other side, but this technique leaves patients quite bothered by the bilaterally degraded vision (caused by the lines in the Fresnel prisms). Strabismus surgery, while highly effective, typically must be deferred until six to 12 months after the onset of diplopia.

Methods to treat combined strabismus with a unilateral oblique Fresnel prism have been described previously.1 Briefly summarized, these published methods start with quantifying the vertical and horizontal components of the strabismus. The Pythagorean theorem is then applied to come up with a diagonal/oblique equivalent. The referenced articles provide tables that specify the resultant power and angle at which to prescribe the equivalent oblique prism.2,3 The optician is provided with a prescription stating the strength of the prism and the angle at which to apply it. An instrument called a pantograph, along with a positioning device called a compass rose, is used to orient the prism on the spectacles.

Described herein is a technique to prescribe a unilateral oblique prism without the need for a reference table or special equipment.

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The Procedure

After the horizontal and vertical components of the strabismus have been measured in primary gaze, the clinician makes an estimate regarding the equivalent power of the oblique prism, as outlined below. The prism options are limited by the commercially available powers of Fresnel prisms (in increments of 1 from 1 to 10, a power of 12, and in increments of five from 15 to 40).

Estimating prism power. A good way to arrive at a starting prism power is to take the higher power prism measurement and add one-half of the lower power measurement. For example, take a hypothetical patient with 20 prism diopters (PD) of esotropia and 10 PD of left hypertropia. The higher power measurement (20) is added to half the lower power measurement (10 ÷ 2 = 5). Therefore, a 25-PD handheld prism is selected.

Testing the prism. This prism is then placed before the nondominant eye. Assuming the left eye is nondominant, the 25-PD handheld prism would be placed with the base out and slightly down in front of the left eye to correct the combined esotropia and left hypertropia. This prism is rotated slowly clockwise or counter-clockwise until the patient notes that the two images are fused. If the patient is unable to fuse, another prism is selected (start with a prism one increment higher or lower—in this case either a 30-PD or 20-PD prism), and the process is repeated until fusion occurs.

Prescribing the prism. A wax pencil is then used to draw a line directly on the outside of the spectacles, using the base of the handheld prism as a guide. The clinician writes a prescription that includes the prism power and a statement regarding the orientation of the prism. For the above example, the prescription would read: “25-PD Fresnel prism base out and down as marked.” An optician applies the specified power Fresnel prism to the inside of the glasses using the wax pencil line as a guide to the exact orientation of the prism. The wax pencil mark is then erased.

CASE ONE: A 75-year-old woman with Graves disease complained of diplopia. Motility testing showed 25 PD of left hypertropia and 10 PD of esotropia. A 30-PD handheld prism was chosen (25 + half of 10), then held base down and out in front of the nondominant left eye. The prism was rotated slightly clockwise and counter-clockwise until fusion occurred. The outside of the glasses were marked with a wax pencil along the base of the prism. An optician applied a 30-PD Fresnel prism with the base oriented as marked. The prism relieved the diplopia in primary gaze.

CASE TWO: A 52-year-old woman developed incomitant strabismus with binocular oblique diplopia following an embolization procedure for a dural arteriovenous fistula. Motility testing showed 10 PD of exotropia and 8 PD of left hypotropia in primary gaze. A 15-PD handheld prism was chosen (10 + half of 8, rounded off to the closest available Fresnel prism power), held base in and up in front of the left eye, then rotated in a manner similar to that described above until fusion occurred. The glasses were marked with a wax pencil. The patient took the marked glasses to an optician for application of a 15-PD Fresnel prism, base in and up to the left spectacle lens. The prism corrected the diplopia in primary gaze.

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Discussion

Patients with oblique diplopia are often very frustrated with their condition, and they desire relief of their symptoms while they wait the customary six to 12 months for strabismus surgery. The method described above has been used successfully by the author in well over 200 patients during the past 18 years, and patient satisfaction is generally quite high. In fact, a few patients have even declined subsequent eye muscle surgery.

This procedure may pose minor problems or even be unsuitable for some in this patient population, however. Patients with diminished fusional vergence amplitudes may require a little more time to adjust to the prism, and failures may occur in the occasional patient with long-standing strabismus that has severely disrupted binocularity and fusion. Patients with incomitant strabismus need to be informed that they may still have diplopia in eccentric gaze. Torsional strabismus is not corrected by this technique.

The specific etiology of the oblique strabismus is generally irrelevant to the efficacy of the technique, which works well for small as well as large (up to 40 PD) angles of strabismus. After the ophthalmologist has gained a little experience, the prism power and angle determination can be made in a couple of minutes, making it accessible to even the busiest clinicians. And fitting the prism to the nondominant eye will greatly enhance patient satisfaction. To further improve patient satisfaction, it is important to work with an optician familiar with the fitting process. The first time the technique is used, a brief phone call or visit from the ophthalmologist to explain the procedure to the optician can smooth out the experience for the patient.

For the clinician, the principal advantages over previously described techniques are speed, simplicity and accuracy while obviating the need to consult a reference table in order to prescribe the correct power and orientation of the prism. The advantage for the optician is that there is no need to have any special equipment to apply the prism accurately. This simple method can help alleviate diplopia symptoms in patients with oblique strabismus.
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1 Moore, S. and L. Stockbridge. Amer Orthoptic J 1972;22:14–21.
2 Reinecke, R. D. et al. Arch Ophthalmol 1977;95:1255–1257.
3 Reinecke, R. D. et al. Arch Ophthalmol 2001;119:458–459.
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Dr. Kortvelesy is an assistant clinical professor of surgery at the University of Hawaii. He practices ophthalmology at Straub Clinic, a multispecialty group located in Honolulu. He specializes in neuro-ophthalmology, orbital and oculoplastic surgery.

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