Etiology
Combined ( vertical and horizontal) strabismus is common.
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.
Management
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. 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.
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.
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).
Prism power estimation
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 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 STUDY 1
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 STUDY 2
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.
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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