|POST GRADUATE SECTION
|Year : 2021 | Volume
| Issue : 3 | Page : 152-156
Prisms in ophthalmology
Department of Paediatric Ophthalmology and Strabismus, Vision Eye Centre, New Delhi, India
|Date of Submission||03-Aug-2020|
|Date of Decision||22-Mar-2021|
|Date of Acceptance||07-May-2021|
|Date of Web Publication||27-Sep-2021|
Department of Paediatric Ophthalmology and Strabismus, Vision Eye Centre, 19 Siri Fort Road, New Delhi - 110 049
Source of Support: None, Conflict of Interest: None
Prisms are used commonly in ophthalmic practice. In strabismus, they have a diagnostic and therapeutic role. Various types of prisms used in ophthalmology have been explained in the article. Positions and placement of a prism lens have also been dealt with. The clinical applications have been explained in detail.
Keywords: Nystagmus, prisms, strabismus
|How to cite this article:|
Kapoor S. Prisms in ophthalmology. J Clin Ophthalmol Res 2021;9:152-6
A prism is a lens which deflects light and can correct ophthalmic misalignment. An ophthalmic prism has an apex and a base. Light rays refracted through a prism always bend toward the base (Snell's law). The power of a prism is designated as “prism diopter” which is a measure of the strabismic deviation. This is estimated by the amount that a light ray is deviated in centimeters (cm), while passing 100 cm away from the prism [Figure 1]. The amount of deflection depends on the refractive index of the material and the position in which the prism is held.
| Types of Ophthalmic Prisms|| |
Prisms are used for diagnostic and therapeutic purposes. Diagnostic prisms are made of glass or plastic. Glass prisms have a higher refractive index and therefore deflect light more than plastic prisms.
For therapeutic purposes, there are two kinds of prisms: glass and Fresnel. Glass prisms have the disadvantage of being heavy, cosmetically dissatisfying, and giving rise to disturbing reflections and aberrations. Fresnel prisms are based on the principle that the power of the prism will not depend on the thickness of the prism but on the prism angle. These prisms are thinner, cosmetically acceptable, and have less aberrations. However, they reduce visual acuity and contrast sensitivity.
| Prism Positioning|| |
There are three ways in which prisms are held – position of minimum deviation, frontal position, and Prentice position [Figure 2]. If the angle of incident light equals that of the refracted light, the prism is said to be in the position of minimum deviation. Plastic prisms are to be used in the position of minimum deviation. As it is difficult to estimate this position, plastic prisms can be held in the frontal plane position. Glass prisms should be held perpendicular to the visual axis. This is known as the Prentice position. If a prism bar calibrated for Prentice position is placed in the frontal position (parallel to the frontal plane, against the supraorbital margin), an overestimation of the angle of deviation is obtained [Table 1].
|Figure 2: (a) Prentice position, (b) Position of minimum deviation, (c) Frontal plane position|
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| Image Formation by Prisms|| |
If a prism is placed in front of one eye, with a base-out prism, the light rays are deflected toward the base inducing diplopia. Thus, the eye will move inward to achieve binocular single vision. The normal convergence amplitudes of 20–30 prism diopter (PD) and divergence amplitudes of 10–15 PD do not induce double vision.
| Prism Placement|| |
When deviations exceed the largest amount of prisms available, two horizontal prisms should not be stacked. This is because a deviation larger than the sum of the two prisms is produced. Even if the prisms are split between the two eyes, the measurement is inaccurate. [Table 2] gives the combined effect of different pairs of prisms held in front of the two eyes. Splitting is still preferred over stacking, but surgery should be performed for the angle according to the conversion table to get accurate results and prevent undercorrection. However, a horizontal and vertical prism can be stacked together [Table 2].
| Prismatic Effect of Spherical Lenses|| |
A plus lens is two prism lenses stacked base to base, while a minus lens is two prisms stacked apex to apex [Figure 3]. Therefore, spectacles affect the measured strabismic deviations. A plus lens will decrease the measured deviation [Figure 4], and a minus lens will increase it [Table 3] and [Table 4].
|Figure 4: A patient with esotropia measuring more with - 19 D spectacles|
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| Prentice's Rule|| |
Prentice's rule is named after the optician Charles F. Prentice. It states that the prismatic power of a lens at any point on its surface is equal to the distance from its optical center in centimeters times the power of the lens in diopters. Thus, there is no prismatic power in the center of the lens. It is important that the optical center of a lens is placed directly in front of the pupils to avoid the prismatic effect.
| Range of Available Prisms|| |
For diagnostic purposes, loose prisms, prism bars, trial set prisms, or Fresnel prisms can be used [Figure 5], [Figure 6], [Figure 7], [Figure 8]. Prisms in the trial set range from ½ Δ to 12 Δ. Loose prisms range from 5 Δ to 60 Δ. Prism bars range from 1 Δ to 40 Δ. Fresnel prisms range from 1 Δ to 40 Δ.
| Clinical Applications|| |
- For diagnostic purposes, prisms are used in keratometers, applanation tonometers, gonioscopes, and ophthalmoscopes
- Prisms can be used in treating phorias due to convergence insufficiency and divergence insufficiency. Exophoria is seen in convergence insufficiency and is treated with base-in prisms. Esophoria is seen in divergence insufficiency and is treated with base-out prisms
- Phorias can be treated up to 10 PD. Beyond that, surgical correction is indicated
- In comitant and incomitant tropias, measurement of strabismus is performed using prisms. Primary and secondary deviations are measured with prisms placed in front of the fixing and nonfixing eye, respectively. Since surgical correction is the treatment of choice, a prism adaptation test should be performed to determine the angle of treatment. In incomitant strabismus, prisms are used to correct the head posture and reduce diplopia in primary gaze
- Prisms are denoted in terms of their base [Figure 9]. For an exotropia, a base-in prism, for esotropia base-out prism, for hypertropia base-down (BD) prism, and for hypotropia base-up (BU) prism are used
- Sometimes, patients with anisometropia experience double vision while reading if they look off-center because of the prismatic effect of the different lenses (Prentice's rule). Vertical prisms known as slab-off and reverse slab prisms are used to correct diplopia
- When a prism is prescribed, the amount and direction should be mentioned. A 4 PD hypertropia in the right eye should be prescribed as (OD) 4 PD BD or divided between the two eyes (OD) 2 PD BD and (OS) 2 PD BU
- In a patient with nystagmus, a left face turn would place the eyes in dextroversion. Thus, a base-in prism in front of the right eye and a base-out prism in front of the left eye would place the eyes in null zone [Figure 10]. The measured deviation can be also be corrected surgically by Anderson's procedure or Kestenbaum's procedure
- Prisms can be used in patients with homonymous hemianopia to bring the images from the area of visual field defect to the area where vision is intact. A similar benefit is seen in patients with hemispatial neglect
- Prism adaptation can be done in cases where preoperative fusion of images needs to be obtained to improve the result of surgery. A trial of prisms is given for a month before surgical correction of squint is planned. If the patient fuses the images, surgery for that angle is performed
- In orthopedic chin-down positions (ankylosing spondylitis), BU-yoked prisms help in improving head posture [Figure 10].
|Figure 9: Modified Krimsky's test using base-in prism to measure exotropia|
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10]
[Table 1], [Table 2], [Table 3], [Table 4]