Hirschberg and Krimsky tests: measuring strabismus with the corneal light reflex
Quick answer
The Hirschberg test estimates strabismus from where a penlight's reflection sits on each cornea: a reflex displaced temporally means esotropia, nasally means exotropia, downward means hypertropia and upward means hypotropia. Each millimeter of decentration is traditionally counted as about 7 degrees, or 15 prism dioptres, although photographic studies suggest closer to 21 prism dioptres per millimeter. The Krimsky test measures the angle more precisely by placing prisms in front of an eye until the corneal reflexes are symmetric in both eyes.
Open the simulatorHow to perform the Hirschberg test
- Hold a penlight about 33 cm in front of the patient, level with your own eyes, and ask them to look at the light.
- Observe the corneal reflex in each eye from directly behind the light.
- Use the fixing eye as the reference: its reflex is normally slightly nasal to the pupil center.
- Estimate how far, and in which direction, the reflex in the other eye is decentered compared with the fixing eye.
Reading the reflex
| Reflex in the deviating eye | Deviation |
|---|---|
| Temporal to the pupil center | Esotropia (eye turned in) |
| Nasal to the pupil center | Exotropia (eye turned out) |
| Below the pupil center | Hypertropia (eye turned up) |
| Above the pupil center | Hypotropia (eye turned down) |
Converting decentration into an angle
| Reflex at | Approximate angle (traditional scale) |
|---|---|
| Pupil margin (about 2 mm from center) | About 15° (about 30Δ) |
| Mid-iris (about 4 mm) | About 30° (about 60Δ) |
| Limbus (about 6 mm) | About 45° (about 90Δ or more) |
These values use the classic ratio of about 7° (15Δ) per millimeter. Photographic measurements (Brodie, 1987) found a larger ratio of about 21Δ per millimeter, so the traditional scale can underestimate larger deviations. Treat the Hirschberg result as an estimate and confirm it with the Krimsky or prism cover test.
Angle kappa: the pseudo-strabismus trap
The corneal reflex normally sits slightly nasal to the pupil center because the fovea lies temporal to the eye's optical axis (a positive angle kappa). A large positive angle kappa — for example when the macula is dragged temporally after retinopathy of prematurity — moves the reflex nasally in both eyes and mimics exotropia. A negative angle kappa mimics esotropia. In pseudo-strabismus the cover test shows no refixation movement, because the eyes are actually aligned.
How to perform the Krimsky test
- Ask the patient to fixate a penlight at about 33 cm, as for the Hirschberg test.
- Place a prism in front of the deviating eye — or in front of the fixing eye if the deviating eye sees poorly or cannot be observed well. Textbooks differ on which placement they call "classic" and which "modified".
- Point the prism apex toward the deviation: base-out for esotropia, base-in for exotropia. For vertical deviations the base depends on the eye: base-down over a hypertropic eye, base-up over the fellow eye.
- Increase the prism power until the corneal reflexes look symmetric in the two eyes. The prism power is the deviation in prism dioptres.
When to use Hirschberg and Krimsky instead of the cover test
Corneal reflex tests do not need each eye to take up fixation in turn, so they work for infants, uncooperative patients and eyes with poor vision, where the cover test fails. When a cover test is possible, the prism cover test is more accurate and also detects phorias.
Practice it on the virtual patient
- Open the Hirschberg grading cases — esotropia 15°, 30° and 45° — and note where the reflex sits in each.
- Compare with pseudo-exotropia from a positive angle kappa and confirm that the cover test shows no movement.
- Open the Krimsky prism console, place prisms and increase the power until the reflexes match.
- In the settings, switch the Hirschberg scale between 15Δ/mm and 21Δ/mm and see how the estimate changes.
Frequently asked questions
How many prism dioptres is 1 mm on the Hirschberg test?
Traditionally, 1 mm of corneal reflex decentration is counted as about 7 degrees, or 15 prism dioptres. Photographic studies suggest a higher ratio of about 21 prism dioptres per millimeter, so the Hirschberg test is best treated as an estimate.
Which way does the prism go for esotropia?
Base-out. The prism apex points in the direction the eye is turned, so an esotropia (eye turned in) is measured with base-out prism and an exotropia with base-in prism.
What is the difference between the Hirschberg and Krimsky tests?
Both use the corneal light reflex. The Hirschberg test estimates the deviation from how far the reflex is decentered; the Krimsky test measures it by adding prisms until the reflexes are symmetric.
Can angle kappa be mistaken for strabismus?
Yes. A large angle kappa shifts the corneal reflex in both eyes and can mimic exotropia (positive kappa) or esotropia (negative kappa). A cover test with no refixation movement confirms pseudo-strabismus.
Related guides
- The cover test: cover–uncover, alternate cover and prism cover test Tell a tropia from a phoria and measure the angle with prisms, step by step.
- Ocular motility and the H-test: ductions, versions and the nine positions of gaze Trace the H, know which muscle each gaze position tests and interpret limited movements.
- Extraocular muscles: anatomy, actions and nerve supply Origins, insertions, actions and nerve supply of the six eye muscles — and why actions change with gaze.