Biometry: long and short eyes

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Transcript Biometry: long and short eyes

Biometry: Long & Short eyes
Alireza Peyman, MD
Isfahan University of Medical Sciences
Normal Range:
• Axial lengthBiggest 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
MYOPIA
• 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.
IOL POWER CALCULATION
FORMULA
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