In the 4-diopter prism base-out test to detect microtropia (<10 PD), which response confirms the diagnosis of microtropia with identity?
- A No movement occurs in either eye when the prism is placed over the microtropic eye; the fellow eye moves when the prism is placed over it ✓
- B The eye moves to refixate when the prism is placed; both eyes show refixation movements
- C Rapid alternating movements in both eyes
- D The microtropic eye shows the Hirschberg reflex displaced beyond 15 degrees
Explanation
In microtropia with identity (small-angle esotropia with central suppression scotoma), placing a 4PD BO prism over the microtropic eye causes NO refixation movement because the prism displaces the image onto the suppression scotoma — the eye doesn't need to refixate. However, placing the prism over the sound (fixing) eye causes a version movement as that eye adjusts. This asymmetric response — absence of movement over microtropic eye, movement over fellow eye — is diagnostic. Microtropia is associated with amblyopia despite small angle, as the suppression scotoma prevents normal bifoveal development.
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.