A-scan biometry is an ultrasound technique that measures the axial length of the eye using one-dimensional echoes. It has been a traditional method for intraocular lens power calculations.
Technique
A high-frequency ultrasound probe is aligned along the visual axis, either in contact with the anaesthetised cornea or via an immersion shell. Echoes from the cornea, lens, and retina are used to compute distances between structures.
- Useful when dense cataract or corneal opacity prevents optical biometry
- Requires careful alignment to avoid under- or overestimation
- Multiple readings are often averaged for accuracy
- Contact methods may slightly shorten measured length if pressure is applied
Clinical Use
Although increasingly supplemented by optical methods, A-scan remains valuable in complex cases and as a backup technique. The axial length obtained feeds into standard lens power formulae.
- Particularly helpful in eyes with poor fixation or extreme anatomy
- Training and experience improve reliability of measurements
- Immersion techniques reduce corneal compression artefacts
- Results should be cross-checked with clinical impression and refraction
