Ocular Parameters & Keratometry

Question 1

State 5 key ocular parameters and their clinical relevance

Answer 1

Axial length (IOL implantation/cataract surgery, myopia development research)
ACD (phakic IOL surgery/AACG)
Cornea/Lens Thickness (refractive/cataract surgery)
Corneal Curvature (CL fitting/disease diagnosis)
Retina Thickness (assessing macular disease/myopia development research)

Question 2

Describe the pros/cons of invasive vs non-invasive measurement methods

Answer 2

Invasive (In-Vitro): precise but eyes typically abnormal/deceased + post-mortem-induced tissue change

Non-invasive (X-rays/Ultrasound): more px friendly, faster results, more realistic values but susceptible to artefacts e.g. blinking/moving during measurement

Question 3

How does an X-ray measure axial length?

Answer 3

Px reports when achromatic phosphene ring just disappears
no tissue prep needed, no post-mortem distortion but increased exosure to ionising radiation can damage DNA, measurements subjective and px need to be dark adapted

Question 4

How is an Ultrasound used for ocular parameters?

Answer 4

generator uses high/very high f soundwaves to measure AL, corneal thickness, ant.segment parameters
topical anaesthetic before device coupled to eye with a saline-filled eyecup to hold lids open

Question 5

Explain how ultrasound uses the principle of reflectance?

Answer 5

amount of US transmittance/reflectance at a curved boundary depends on acoustic impedance values (Z1, Z2) either side of the boundary

Question 6

Describe the clinical applications of US Scans (A/B)

Answer 6

A: measures ocular parameters ~ predicting required IOL power, positional/dimensional changes of lens with age/accomm., CCT prior to LASIK
B: RetDet, Intra-vitreous haemorrhage, intraocular FBs, tumours, vitreous abnormalities

both require topical anaesthesia/coupling medium (Saline) and resolution is not perfect