Prisms in Ophthalmology

1. Definition and Basic Optics

1 prism dioptre (Δ) = the power required to deviate a ray of light by 1 cm at a distance of 1 metre

• 1 mm of corneal light reflex displacement ≈ 7° of ocular deviation (Hirschberg estimate)
• 1° of ocular deviation ≈ 2 prism dioptres (Δ)

2. Types of Prisms Used Clinically

3. Prism Orientation Rules

• Light is bent toward the base of the prism
• The image is displaced toward the apex
• The eye must rotate toward the base to re-fixate - so the eye moves in the direction of the base
• Convention: prism orientation is named by its base position:
  • Base-out (BO) - base is temporal; induces convergence
  • Base-in (BI) - base is nasal; induces divergence
  • Base-up (BU) / Base-down (BD) - for vertical deviations
• To correct a deviation: place the prism with its base in the direction of the deviation (apex toward fixation)
  • Right esotropia → base-out prism before the right eye
  • Right hypertropia → base-down prism before the right eye

4. Diagnostic Uses

4a. Hirschberg Test (Corneal Reflex Estimation)

• Each 1 mm of decentration ≈ 7° ≈ 14Δ
• Reflex at pupil margin (~2 mm from centre) ≈ ~15°
• Reflex at limbus ≈ ~45°

4b. Krimsky Test

4c. Prism Cover Test (Gold Standard for Measuring Deviation)

1. The alternate cover test is first performed to establish direction and approximate extent of deviation
2. Prisms of increasing strength are placed in front of one eye with the base opposite the direction of deviation (e.g. base-out for esotropia, base-down for right hypertropia)
3. The alternate cover test is performed continuously as stronger prisms are introduced (typically via a prism bar)
4. The re-fixation movement gradually decreases as prism power approaches the deviation magnitude
5. End-point: no movement is seen; the prism strength at which movement reverses and then neutralizes gives the angle of deviation

4d. 20Δ Base-Out Prism Test (Fusion Test in Infants)

1. A 20Δ base-out prism is placed in front of one eye (say, the right eye)
2. This displaces the retinal image temporally, causing diplopia
3. The right eye adducts (to re-fixate), and by Hering's law, the left eye abducts
4. The left eye then makes a fusional re-adduction convergence movement
5. On removing the prism, both eyes re-fixate correctly

4e. 4Δ Base-Out Prism Test (Microtropia / Central Suppression Scotoma)

• The 4Δ prism placed base-out causes both eyes to shift away from prism base, then the fellow eye makes a fusional re-fixation movement

• When placed over the microtropic left eye: the image falls within the CSS - no movement of either eye is observed
• When placed over the right (fixing) eye: the right eye adducts; the left eye moves by Hering's law, but the second image again falls within the CSS - no re-fixation saccade is seen

4f. Prism and Alternate Cover Test (PACT) - Wills Eye Manual

4g. Fusional Amplitude Measurement

• An increasingly strong prism is placed before one eye
• The eye abducts (base-in prism) or adducts (base-out prism) to maintain bifoveal fixation
• When the prism exceeds the fusional amplitude, diplopia is reported or one eye drifts - this marks the limit of vergence ability

5. Therapeutic / Corrective Uses

5a. Prismatic Correction in Spectacles (Permanent Prisms)

• Used for small, stable residual deviations after strabismus surgery (or as an alternative to surgery)
• Ground into spectacle lenses - suitable for deviations up to approximately 10-15Δ (larger prisms are heavy and produce visual distortion)
• Used especially in lateral rectus palsy: mild residual deviation may be treated with a prism incorporated into spectacles
• Also in skew deviation: prismatic correction in spectacles, along with botulinum injections and vertical muscle surgery in persistent cases

5b. Fresnel Press-On Prisms

• Thin, lightweight Fresnel prisms can be pressed onto existing spectacle lenses
• Allow much larger prism powers without the weight/thickness of ground-in prisms
• Used for temporary correction while awaiting surgery or spontaneous recovery (e.g. in microvascular CN VI palsy)
• Also useful to assess tolerance before committing to permanent spectacle prisms
• From Kanski's: "Observation with monocular occlusion or prismatic (e.g. temporary Fresnel stick-on) correction of diplopia is appropriate in idiopathic and presumed microvascular lesions"

5c. Postoperative Diplopia Test (Pre-surgical Screening)

• Corrective prisms are placed in front of one eye (usually the deviating eye)
• The patient fixates a straight-ahead target with both eyes open
• Prisms are slowly increased to significantly overcorrect the angle
• The patient reports whether diplopia occurs at any point
• If suppression persists throughout: little risk of postoperative diplopia
• If diplopia occurs (e.g. from 30Δ in a consecutive exotropia of 35Δ): indicates high risk; botulinum toxin test should be considered

6. Prisms in Vergence / AC:A Ratio Assessment

• Normal: 3-5Δ per dioptre
• Abnormalities of the AC:A ratio are central to the aetiology of several types of strabismus (e.g. high AC:A esotropia)

7. Summary Table: Prism Tests and Their Purposes

8. Clinical Pearls

• "Base opposite the direction of deviation": to measure esotropia use base-out, for exotropia use base-in, for right hypertropia use base-down before the right eye
• Prism bar vs. loose prisms: prism bars are preferred for the prism cover test because they allow rapid sequential changes; loose prisms are used in trial frames
• Large deviations: ground-in prisms are impractical above ~15Δ due to weight and optical distortion; Fresnel prisms or surgery are preferred
• Botulinum toxin + prismatic correction: botulinum to ipsilateral medial rectus can sometimes facilitate prismatic correction in large deviations (e.g. VI nerve palsy) by temporarily reducing the angle
• Prism dioptres vs degrees: remember 1° ≈ 2Δ; the Hirschberg reflex estimate gives 1 mm ≈ 7° ≈ 14Δ

Prisms in Ophthalmology

Definition

Prism Dioptre (Δ) — Unit of Prism Power

1 prism dioptre (Δ) = power that deviates a ray of light 1 cm at 1 metre distance

• 1 mm corneal reflex displacement = 7° of deviation
• 1° = 2 prism dioptres (Δ)
• 1 mm = ~14Δ

Fundamental Rule

• Esotropia → Base-out (BO)
• Exotropia → Base-in (BI)
• Right hypertropia → Base-down before right eye

Types of Prisms

Diagnostic Uses

1. Hirschberg Test

• Penlight held 30 cm from patient; corneal reflex position is estimated
• 1 mm decentration = 7° = ~14Δ
• Reflex at pupil margin (~2 mm) = ~15°
• Reflex at limbus = ~45°
• Quick, objective; useful in children/uncooperative patients
• No prism is used — this is estimation only

2. Krimsky Test

• Prisms placed in front of the fixating eye until corneal light reflexes are symmetrical in both eyes
• More accurate than the prism reflection test (prism over deviating eye)
• Reduces parallax error
• Used when cover test is not possible (poor cooperation)

3. Prism Cover Test (Gold Standard)

1. Perform the alternate cover test first — establishes direction and approximate angle
2. Place prism of increasing strength in front of one eye with base opposite to direction of deviation
3. Continue alternate cover testing while increasing prism strength (using a prism bar)
4. Re-fixation movement gradually diminishes as prism approaches the true angle
5. End-point = no movement; can increase further until reversal occurs, then reduce to neutral

4. 20Δ Base-Out Prism Test (Fusion test in infants)

• 20Δ base-out placed before one eye
• Displaces image temporally → diplopia stimulus
• Expected response (fusion present):
  1. Both eyes shift away from prism base (Hering's law)
  2. Fellow eye makes fusional re-adduction convergence movement

5. 4Δ Base-Out Prism Test (Microtropia / Central Suppression Scotoma)

6. Fusional Amplitude Measurement

7. Postoperative Diplopia Test (Pre-surgical screening)

• Corrective prisms placed before deviating eye
• Angle is progressively overcorrected
• Patient reports if diplopia occurs

Therapeutic Uses

1. Fresnel Press-On Prisms

• Pressed onto existing spectacle lens
• Allow large powers without lens weight/thickness
• Used for temporary correction while awaiting surgery or spontaneous recovery (e.g. microvascular VI nerve palsy, thyroid eye disease)
• Also used to assess tolerance before permanent prism prescription

2. Permanent Prism in Spectacles

• Ground into spectacle lenses
• Practical for deviations up to ~10–15Δ (beyond this: heavy, distorting)
• Indicated for small, stable residual deviations after surgery
• Also used in: lateral rectus palsy (residual deviation), skew deviation, decompensated phorias

3. Prism + Botulinum Toxin (in CN VI Palsy)

• Botulinum injected into ipsilateral medial rectus to reduce the angle
• Facilitates prismatic correction of large deviation while awaiting recovery

AC:A Ratio and Prisms

• Expressed as prism dioptres per dioptre (Δ/D)
• Normal: 3–5Δ/D
• High AC:A → convergence excess esotropia
• Low AC:A → convergence insufficiency

Summary: Prism Tests at a Glance

High-Yield Points for Exams

1. Prism dioptre = 1 cm deviation at 1 m
2. Image moves to apex; eye moves to base
3. Base opposite to deviation to correct or measure
4. Prism cover test = gold standard for measuring strabismus angle
5. 4Δ test — no re-fixation saccade = CSS = microtropia
6. 20Δ test — absent fusional response before 6 months = abnormal fusion
7. Fresnel prisms = temporary, large deviation; ground-in = permanent, small deviation
8. Postop diplopia test is mandatory before strabismus surgery in patients >7–8 years
9. 1° = 2Δ; 1 mm Hirschberg = 7° = ~14Δ
10. Normal AC:A ratio = 3–5Δ/D