biometry: long & short eyes

Biometry: Long & Short eyes

Alireza Peyman, MDIsfahan University of Medical Sciences

Normal Range:

• Axial lengthBiggest source of error in IOL power calculations.

• 96% of axial lengths fall within the range 21.0 to 25.5 mm

• 60% of AL is between 22.5 and 24.5 mm

• Ultrasounds are calibrated with average velocities for normal length eyes. These velocities are incorrect for short eyes, causing significant measurement errors.

• Accurate measurement of axial length in hyperopic eyes is especially important since any error is greatly magnified in proportion to the length of the eye.

• Immersion or LASER biometry can provide superior results in these cases.

• In a hyperopic eye with axial length of 20 millimeters each mm error in measurement cause 3.75 D post operative refractive error.

• Difference in axial length of more than 0.3mm requires confirmation

• Most individuals have similar axial lengths in each eye except:

– unilateral refractive error

– coloboma

– Staphyloma

• In the axial myopia, the presence of a staphyloma should be considered, especially if there is difficulty obtaining a distinct retinal spike during A-scan.

• an immersion A/Bscan approach to axial length measurement in the setting of a posterior staphyloma

• nearly all eyes with pathologic myopia have some form of posterior staphyloma.

• For highly myopic eyes (axial > 28mm) a B-scan should be carried out to determine the presence or otherwise of staphylomata

Long eye, foveal deep

Very long eye, posterior pole staphyloma

Buphthalmic globe: very long eye (37·5 mm): deep anterior chamber.

Megalocornea: average length eye (23·1 mm): deep anterior chamber (5·2 mm).

• The Ascan vector is adjusted to pass through the middle of the cornea as well as the middle of the anterior and posterior lens echoes to assure that the vector will intersect the retina in the region of the fovea.

• IOL Master permits accurate measurements when posterior staphylomata are present. Since the patient fixates along the direction of the measuring beam.

Calculation formula:

• The SRK/T and Holladay formulae worked best overall.

• SRK I and II were found to be least accurate

• No statistical difference was found between SRK/T, Hoffer Q and the Holladay formulae

Axial Length Formula<22 mm Hoffer Q or SRK/T

22 – 24.5 mm SRK/T or Holloday, Haigis

> 24.6 SRK/T

When using IOL Master do not forget

• For AL < 22 do immersion A scan

• For AL > 30 do A and B scan

Silicone oil filled vitreous cavity: eye measures 49 mm on B-scan, but actual axial length is 34·3 mm.

Thank you for your attention