Common A scan (Amplitude scan) Cases in OSCE
(like all imagings, ultrasound scans should supplement clinical findings and 
 not be used alone in clinical diagnosis or management)

1. Most cases of A scan appearing in the OSCE are related to the biometry calculation.
    Common questions being:
    • the structures represented by each peaks (see diagram below)
a = cornea spike 
b = anterior lens spike 
c = posterior lens spike 
d = retinal spike 
e = orbital spike
    • the effect of silicone oil on the axial length (the value will be longer than expected as ultrasound travels slower in the silicone oil than vitreous)
    • the accuracy of the axial length(s)  ie, if the ultrasound scan is accurate as demonstrated by one retinal peak rather than several peaks; if there is significant differences between the two eyes. Note: the Guideline on Cataract Operation from the Royal College of Ophthalmologists finds that :
        - 96% of axial lengths fall within the range 21.0 to 25.5 mm and for 60% 
           this is between 22.5 and 24.5 mm
        - in the absence of pathology that might affect eye size (eg, unilateral 
          refractive error, coloboma or staphyloma), most individuals have similar 
          axial lengths in each eye
    2. The following scans may appear in the future examination as the examiners look for new 
        questions. Using the principles of ultrasound can you diagnose or give a differential diagnosis
        for the following scans ? (click on the scans for questions and answers)

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