|Year : 2017 | Volume
| Issue : 2 | Page : 79-85
Prisms in clinical practice
Department of Ophthalmology, Sree Gokulam Medical College and Research Foundation, Thiruvananthapuram, Kerala, India
|Date of Web Publication||10-Aug-2017|
Sree Gokulam Medical College and Research Foundation, Thiruvananthapuram, Kerala
Source of Support: None, Conflict of Interest: None
Prisms are used in diagnosis and Therapy of Ophthalmic disorders. Prismotherapy provides symptomatic relief not only in many kinds of squints, but also in non strabismic conditions like convergence insufficiency, nystagmus, heminopia etc. One of the most important therapeutic skill we have acquire in this direction, is the localisation of such clinical needs and the timely use of the wonderful therapeutic functions of prisms for the relief of such conditions. Thin Fresnel prisms are now more used in clinical practice.
Keywords: Fresnel prism, hemianopic spectacles, prism base, prism diopter, vertical diplopia
|How to cite this article:|
Antony J. Prisms in clinical practice. Kerala J Ophthalmol 2017;29:79-85
| Introduction|| |
Only few ophthalmologists prescribe prisms now. Is it due to lack of awareness among novice? or other reasons? We don't know! However, it a fact that prisms are really useful and become the only way of salvation in many ophthalmic clinical situations, if their secrets are understood, judiciously used, and properly prescribed. This article is an attempt to revive interest in this direction.
| How Prisms Function|| |
Prism is a transparent, solid, triangular refracting medium with a base and apex. Its apical angle determines the power of prism. A prism of one prism diopter power (Δ ) produces an apparent displacement of one centimeter to an object situated one meter away. Light entering the prism will deviate toward its base. However, image appears shifted to the apex, and the eye examined or treated tend to deviate toward its apex. This is how prisms function and manipulate the special location of image clinically [Figure 1].
| Response of Eye to Prism|| |
If a prism is placed before one eye, with its base directed outward, the light rays from object will be deflected outward (to the base) and fall on the retina outer to fovea. Since the light still falls on the macula of the other eye, double vision would be produced. Consequently, the eye with prism base directed outward will deviate inward so that the deflected light falls on the macula once again and binocular vision is made possible.
The maximum effort which can be put in this way (the capacity to maintain fusion) is measured by the strongest prism with which diplopia is not produced. Prismatic power of even 30– 60 Δ can be overcome normally by convergence and 10– 15 Δ divergence, and 2– 4 Δ by circumvergence. The measurements of this artificially produced diplopia (vergence power) are important in diagnosis and treatment of muscular in balance.
Even though the light deviates toward the base of prism and eye deviates to the apex of prism, in practice, we denote the prisms in terms of the direction of their base - in, out, up, or down.
- Thus, to bring an eye inward, keep the base of prism directed outward in front of the eye
- To make an eye deviate outward, use base-in (BI) prism
- To bring down an eye, base-up prism is needed
- To take up an eye, base-down prism is used.
A base-out prism is an adducting prism, and a BI prism is an abducting prism [Figure 2].
| Detection of Prism|| |
To detect the presence of a prism, hold it over an object (eg: cross line), the portion of line under the prism will appear broken and displaced towards its apex [Figure 3].
| Types of Prisms|| |
Prisms are available as loose prism in glass trial set and prism set, and also as prism bars and Fresnel prism [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9].
- Loose prisms in the glass trial set from ½ Δ to 12 Δ and in prism set from 5 Δ to 60 Δ powers
- Prism bars (horizontal and vertical) from 1 Δ to 40 Δ powers
- Fresnel prism - thin prisms arranged in a plastic sheet, in powers from 1 Δ to 40 Δ .
| Clinical Uses of Prisms|| |
- Diagnostic indications
- Prisms form part of many ophthalmic equipment such as gonioscopes, keratometers, applanation tonometers, and ophthalmoscopes
- For assessment (measurement) of squint
- Therapeutic indications [Figure 13], [Figure 14], [Figure 15], [Figure 16], [Figure 17], [Figure 18]
- Treatment of phorias
- Treatment of tropias
- Convergence insufficiency
- Divergence insufficiency
- Visual field defects (hemianopias)
- In the management of glaucoma and retinitis pigmentosa (RP)
- Prisms in treatment in bedridden patients
- Prisms in paralytic squint (diplopia)
- Prisms in contact lens (CL) correction.
| Measurement of Squint|| |
Apart from other methods of measurement of squint, the angle of deviation can be measured by placing prisms of increasing strength in front of fixing the eye with their base inward in exodeviation and base outward in esodeviations until the corneal reflection of the squinting eye becomes centered. The strength of prism necessary to achieve this is the amount of squint (Krimsky test) [Figure 10]. In modified Krimsky test, prisms are placed in front of the deviating eye. However, there is some difficulty in seeing the corneal reflex.
| Prisms in Treatment of Phorias|| |
Prisms are considered in the treatment of phorias only after correcting any precipitating causes such as general weakness, convergence insufficiency, and refractive errors.
For giving exercise to weak muscles, prisms with their base toward the direction of deviation (adverse prism) are tried [Figure 12].
When the above treatments fail and also in cases of phoria developing in presbyopes, elderly persons, and poorly motivated patients, prism treatment is undertaken. Prism trial is performed using prisms in trial set, and appropriate power can be prescribed. Prisms with their base against the direction of deviation (relieving prism), are given [Figure 11].
Following guidelines will help in the proper prism prescription:
- Prisms with their base opposite to deviation are given. BI prism are given to correct exodeviations, base-out prism for esodeviations, base-up prism for hypodeviations, and base-down prisms for hyperdeviations
- The range of prism powers prescribed in the treatment of phoria is usually from 0.5 Δ to 10 Δ . For example: In exodeviation of 10 Δ , 5 Δ BI prism in the right eye (RE) and 5 Δ BI in the left eye (LE) are given
- Both vertical and horizontal prisms can be prescribed
- A vertical prismatic correction of 10 Δ is the maximum that can be tolerated. For example: In a vertical deviation of 10 Δ (right hyper deviation), 5 Δ base-down in RE and 5 Δ base-up prism in LE are preferred
- Prism up to 6 Δ can be given in one eye and half of it in the other eye
- If the prism power required is high, the total power can be divided between two eyes
- Prisms can be given as glass prisms or Fresnel prisms in spectacles.
| Prism Therapy in Tropias|| |
- Prisms are used less commonly in the treatment of tropias than phorias and only in cases where deviation is more than 20° and does not get corrected with other methods, and also as an alternative to surgical correction for cosmetic improvement. Principles of prescription are as the same those for phorias
- Prism adaptation – prisms are also given in spectacles, 1 month before surgery to obtain presurgical fusion of images thereby improving the outcome of surgery. By knowing how much prism will make the eyes work together, we can judge how much surgical adjustments of eye muscle is necessary.
| Convergence Insufficiency|| |
Prisms are used both in measurements of convergence (base-out prisms) and for treatment (BI prism) of convergence insufficiency; when the condition is not getting relieved by correcting the causes, refractive errors, and also by orthoptic exercises, BI prisms are prescribed for near work. Smaller powers below 5 Δ can be attained by decentering lenses also.
| Divergence Insufficiency|| |
When divergence insufficiency is benign (without any neurological signs), it can be treated with base-out prisms in spectacles or Fresnel prisms.
| Prisms Treatment for Nystagmus|| |
Base-out prisms can stimulate fusional convergence which will increase the amplitude of nystagmus, resulting in improvement in the visual acuity. Prisms also correct abnormal head turn and chin elevation in congenital nystagmus. Prisms with their base kept opposite to null zone, where the amplitude or magnitude of nystagmus is minimum (preferred direction of gaze), allow the eyes to rotate into position without large head turn. Prisms with base opposite to the preferred direction of gaze are given.
For example: In a case of nystagmus, if nystagmus is minimum in the right gaze (RE-abducted and left-adducted position – null zone), eyes are in dextroversion and patient turns face toward left (face turn) to find the null point of nystagmus.
If we can give BI prism in RE and base-out prism in LE (apex of both prism directed to right) will shift the image to the right or null zone, and this can enable the patient to keep face straight without turning face to the left.
Compensatory chin elevation caused by null zone in dorsum version can be corrected by base-up prism.
| Prisms in Treatment of Hemianopias|| |
Patients with hemianopias especially homonymous hemianopia and vision in the only one eye can be helped by giving prism with its base oriented toward the blind area. This will push the eye to the seeing area providing a large view of field which was normally occupying the blind area. Prism covers only that portion of spectacle corresponding the blind area. Hemianopic spectacles, incorporated with prism of about 8 Δ , with its base directed to the blind side will serve the purpose. In a patient with only seeing LE and left homonymous hemianopia can be provided with a base-out prism to see object on his left field. Prism covers the temporal portion of the left lens.
| Prisms in Glaucoma and Retinitis Pigmentosa|| |
Advanced glaucoma and RP patients with tubular vision have to restrict the eye movement to a limited range. Prisms help to displace peripheral blind area of visual field toward the straight ahead position to the seeing area of visual field. Small segment of Fresnel prism 5 mm from the center of pupil with its BI the same direction of restricted visual field will help the patient.
| Prisms as an Aid to the Bedridden Patient|| |
Bedridden patients are forced to read or watch television in extreme downgaze. If they are provided with 15– 30 Δ , base-down prism in the form of recumbent spectacle will allow the patient to read comfortably. Prisms in Fresnel form are given temporarily till the patients recover.
| Prisms in Incomitant Strabismus|| |
In paralytic strabismus, prisms are used to deplete diplopia by directing the extra image into suppression area or into retinal periphery where it can be easily ignored. Complete occlusion or segmental occlusion in one position of gaze in single or bifocal form of 10 Δ to 20 Δ is given. Prisms are also used to stimulate the unaffected antagonistic muscle of paretic muscle thereby preventing its secondary contracture.
In myasthenia gravis, multiple sclerosis, and Graves' disease, variable diplopia can be corrected using prisms. They are applied at an oblique angle to correct horizontal and vertical diplopia. Prism requirements vary with disease progression.
Ankylosing spondylitis and other postural deformities with limited head movement, prism applied base up to spectacle can change the image angle [Figure 19].
| Prisms in Contact Lens Correction|| |
In CL practice, prisms are used to stabilize the near vision portion of a segmental bifocal CL and to stabilize a toric CL, using prism ballast [Figure 20].
| Fresnel Prism|| |
Fresnel prisms are now used more in prism practice as they are very thin, have only negligible weight, and more cosmetically acceptable even in high powers (30 Δ ) than conventional prisms. They are made of polyvinyl chloride [Figure 21], [Figure 22], [Figure 23].
Fresnel prisms consist of thin narrow prisms arranged in a plastic sheet. Continuous surface of a conventional prism is replaced by a series of steps. Their design is based on the principle that power of prism depend on the prism angle and not on its thickness (Augestin Fresnel). Hence, the thickness is reduced to 1 mm. They are available in powers from 1 Δ to 40 Δ . Since Fresnel prism is thin and flexible, it can be cut into pieces and applied to the back surface of spectacle. They are preferred in most of the clinical conditions managed with prisms, especially in the treatment of squint and to relieve sudden onsight symptomatic diplopia resulting from trochlear and abducens nerve palsies, thyroid-related orbitopathy, postcataract surgery diplopia, and trauma-induced diplopia.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.
Financial support and sponsorship
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], [Figure 11], [Figure 12], [Figure 13], [Figure 14], [Figure 15], [Figure 16], [Figure 17], [Figure 18], [Figure 19], [Figure 20], [Figure 21], [Figure 22], [Figure 23]