10.2 Hirschberg, Krimsky, Brückner & Fusional Vergence Amplitudes
Key Takeaways
- On the Hirschberg test, each millimetre of corneal light reflex displacement corresponds to approximately 7 degrees or about 15 prism dioptres.
- A reflex at the pupil margin is roughly 15 degrees, at mid-iris roughly 30 degrees, and at the limbus roughly 45 degrees of deviation.
- The Krimsky test places prism over the fixating eye until the corneal reflex in the deviated eye is centred.
- Normal fusional amplitudes at near are roughly 35 prism dioptres convergence and 15 prism dioptres divergence, with vertical amplitudes of only 2 to 3.
- Vertical fusional amplitudes are tiny, which is why small vertical prism errors in spectacles are so poorly tolerated.
When reflex tests are used
The prism and alternate cover test is the reference method for measuring strabismus and is the task named on the COMT Performance Test. It requires the patient to fixate accurately with either eye. When that is impossible — an infant, a patient with dense amblyopia or eccentric fixation, a patient with very poor vision in one eye, or an uncooperative adult — corneal reflex methods take over.
Hirschberg test
Shine a penlight at about 33 to 40 cm along the patient's visual axis and observe where the corneal light reflex falls in each eye. In a straight eye the reflex sits slightly nasal to the pupil centre (because the visual axis passes nasal to the optical axis, giving a positive angle kappa of a few degrees).
Conversion. Each 1 mm of reflex displacement corresponds to roughly 7 degrees, which is approximately 15 prism dioptres.
| Reflex position in the deviated eye | Approximate deviation |
|---|---|
| Slightly nasal to pupil centre | Normal (positive angle kappa) |
| Pupil margin (roughly 2 mm) | ~15 degrees, ~30Δ |
| Mid-iris (roughly 4 mm) | ~30 degrees, ~60Δ |
| Limbus (roughly 6 mm) | ~45 degrees, ~90Δ |
A reflex displaced temporally in the deviated eye means the eye is turned in (esotropia); displaced nasally means the eye is turned out (exotropia); displaced downward means the eye is up (hypertropia).
Angle kappa is the confounder. A large positive angle kappa (reflex displaced nasally in both eyes) mimics exotropia; a negative angle kappa mimics esotropia. This is why a child with retinopathy of prematurity and dragged maculae can appear exotropic on Hirschberg while the cover test shows straight eyes. Always compare the two eyes and, where possible, confirm with a cover test.
Krimsky test
The Krimsky test converts the Hirschberg estimate into a measured value.
- Shine the penlight as for Hirschberg and identify which eye is fixating.
- Place increasing prism over the fixating eye, apex toward the deviation.
- Increase the prism until the corneal reflex in the deviated eye is centred and symmetric with the fixating eye.
- Record the prism power and direction.
Some protocols place the prism over the deviated eye, and both variants are described in the literature; the convention used must be recorded. Placing prism over the fixating eye is the more common description and produces a version movement that recentres the deviated eye by Hering's law.
Krimsky is less accurate than the prism and alternate cover test because it relies on judging reflex centration rather than observing a refixation movement. It is used only when cover testing is impossible, and the record should state which method was used.
Brückner test
With a direct ophthalmoscope held about 50 to 100 cm away, view both red reflexes simultaneously in a dim room. Compare them.
| Finding | Suggests |
|---|---|
| Both reflexes equal in brightness and colour | Normal |
| One reflex brighter or lighter | That eye is deviated, ametropic or has an obstruction; a strabismic eye typically gives the brighter reflex |
| White reflex (leucocoria) | Urgent — retinoblastoma, cataract, Coats disease, retinal detachment, persistent fetal vasculature |
| Dark spots or shadows in the reflex | Media opacity |
The Brückner test is a rapid screening tool, especially in infants. Any leucocoria is a same-day referral and must never be filed as a photographic artefact.
Fusional vergence amplitudes
A phoria is a latent deviation held straight by fusional vergence. Whether it causes symptoms depends on whether the patient's fusional reserves are large enough to compensate for it comfortably. Measuring those reserves is therefore the second half of a phoria assessment.
Method. With the patient fixating an accommodative target (a line of letters one or two lines above threshold) at distance or at 33 cm, introduce increasing prism in front of one eye using a prism bar, and record three endpoints:
- Blur — the point at which the target first blurs because accommodation is being dragged along. There is no blur point in divergence testing at distance.
- Break — the point at which fusion fails and the patient reports diplopia.
- Recovery — the point at which fusion is regained as prism is reduced.
Record as blur / break / recovery, for example "BO at near 20/32/24."
Prism direction:
- Base-out prism forces the eyes to converge and measures positive fusional vergence (convergence reserve).
- Base-in prism forces the eyes to diverge and measures negative fusional vergence (divergence reserve).
- Base-up and base-down measure vertical fusional amplitudes.
Approximate normal values:
| Vergence | Distance (6 m) | Near (33 cm) |
|---|---|---|
| Convergence (base out) break | ~15–20Δ | ~30–35Δ |
| Divergence (base in) break | ~6–8Δ | ~12–15Δ |
| Vertical (base up/down) break | ~2–3Δ | ~2–3Δ |
The critical clinical number is the vertical amplitude of only 2 to 3 prism dioptres. Horizontal fusional ranges are generous, but vertical ranges are tiny — which is why a mis-centred spectacle lens producing 1.5Δ of vertical imbalance causes intolerable asthenopia while the same amount of horizontal error passes unnoticed. It is also why slab-off correction exists.
Sheard's criterion offers a practical compensation test: the fusional reserve opposing the phoria should be at least twice the phoria. A patient with 10Δ of exophoria at near needs at least 20Δ of base-out break to be comfortable. Falling short of that predicts symptoms and supports either vergence therapy or relieving prism.
Percival's criterion is an alternative that asks whether the demand line sits in the middle third of the zone of clear single binocular vision.
Practical points: always use an accommodative target, not a light, so accommodation is controlled; increase prism smoothly and slowly; and recognise that repeated testing fatigues the reserves, so the first measurement is the most representative.
On the Hirschberg test the corneal light reflex in the left eye sits at the pupil margin, displaced temporally. What is the approximate deviation?
What are approximate normal vertical fusional amplitudes?
During the Brückner test one red reflex appears white. What is the appropriate action?
A patient has 12 prism dioptres of exophoria at near and a base-out break of 14 prism dioptres. What does Sheard's criterion predict?
A child with retinopathy of prematurity and dragged maculae appears exotropic on the Hirschberg test but the cover test shows no movement. What explains this?