Strabismus Measurement and Surgical Dose Tables: Quick Reference
Last updated: October 2026
How to measure a deviation quickly and reliably, convert between methods, and find a starting point for surgical planning.
Measurement methods at a glance
| Method | When to use | Pearls |
|---|---|---|
| Prism cover test (alternate prism cover) | The gold standard when both eyes fixate | Measure at near (33 cm) and distance (6 m) with an accommodative target and full correction. Base toward the apex of the deviation: base-out for esotropia, base-in for exotropia. |
| Hirschberg test (corneal light reflex) | Quick estimate in infants and poor fixators | About 1 mm of reflex decentration is about 7°, or roughly 15 PD. Reflex at the pupil margin is about 15° (30 PD); mid-iris about 30° (60 PD); limbus about 45° (90 PD). |
| Krimsky test | One eye does not fixate (poor vision) | Place prisms in front of the fixing eye until the reflexes are symmetrical |
| Modified Krimsky | Infants, uncooperative patients | Prism in front of the deviating eye |
| Maddox rod and Maddox wing | Small heterophorias; cyclodeviation (double Maddox rod) | Patient-reported, so requires good cooperation |
| Hess or Lees screen | Incomitant (paralytic or restrictive) strabismus | The smaller field belongs to the eye with the underacting muscle. Follow up to see recovery. |
1 prism dioptre deviates light 1 cm at 1 m; 1° is about 1.75 PD for small angles. Hirschberg conversions vary in studies (some suggest up to about 20 PD per mm), so confirm with prism cover whenever possible.
Always record
- Deviation at near and distance, with and without glasses, and in the nine positions of gaze for incomitant cases.
- A and V patterns (difference between up-gaze and down-gaze): significant if 10 PD or more for an A pattern, or 15 PD or more for a V pattern.
- Head posture, oblique overaction, DVD, nystagmus, and the 3-step test (Parks-Bielschowsky) for vertical deviations.
- Fusion (Worth 4-dot) and stereoacuity before and after treatment.
Surgical dose tables (starting points)
| Esotropia (PD) | Bilateral MR recession (mm) | Bilateral LR resection (mm) |
|---|---|---|
| 15 | 3.0 | 3.5 |
| 20 | 3.5 | 4.5 |
| 25 | 4.0 | 5.5 |
| 30 | 4.5 | 6.0 |
| 35 | 5.0 | 6.5 |
| 40 | 5.5 | 7.0 |
| 50 | 6.0 | 8.0 |
| 60–70 | 6.5–7.0 | 8.0 |
| Exotropia (PD) | Bilateral LR recession (mm) | MR resection (mm) |
|---|---|---|
| 15 | 4.0 | 3.0 |
| 20 | 5.0 | 4.0 |
| 25 | 6.0 | 5.0 |
| 30 | 7.0 | 6.0 |
| 35 | 7.5 | 6.0 |
| 40 | 8.0 | 6.5 |
| 50 | 9.0 | 7.0 |
| 60 or more | 10.0 | 8.0 (more than one procedure may be needed) |
Adapted from Parks' tables (as modified in standard strabismus texts). For a monocular recess-resect procedure, read the recession and resection from the same row for one eye. These are guidelines only: adjust for your own results, age, axial length, adjustable sutures and the near-distance disparity.
Before planning surgery: give full refractive correction and treat amblyopia first; measure on at least two visits; for exotropia, measure distance after patch-occlusion and with +3.00 D lenses at near to uncover the full deviation; rule out restrictive or neurological causes in incomitant strabismus (forced ductions, imaging).
Related: strabismus surgery basics, nerve palsies, anti-elevation syndrome and the paediatric glasses card.
Key sources
- Classic strabismus surgery: rectus muscle dosage tables (after Parks)
- von Noorden GK, Campos EC. Binocular Vision and Ocular Motility. 6th ed. Mosby; 2002
- AAO BCSC Section 6: Pediatric Ophthalmology and Strabismus (current edition)
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