Ocular Parameters & Keratometry Flashcards
State 5 key ocular parameters and their clinical relevance
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)
Describe the pros/cons of invasive vs non-invasive measurement methods
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
How does an X-ray measure axial length?
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
How is an Ultrasound used for ocular parameters?
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
Explain how ultrasound uses the principle of reflectance?
amount of US transmittance/reflectance at a curved boundary depends on acoustic impedance values (Z1, Z2) either side of the boundary
Describe the clinical applications of US Scans (A/B)
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
Explain how a partial/low coherence interferometer works with a modern example
US A-Scan along 1 axis: records/analyses the number/intensity/(ir)regularity of reflected interference fringes from the eye’s individual optical surfaces/structures
HaagSTreit LenStar LS 900 measures AL, ACD and crystalline lens thickness to 0.01mm resolution, retinal/central corneal thickness to 8um resolution
Explain the 2 types of OCT
Standard time-domain: reference/scanning mirrors along x/y capture multiple A-scans along different axes for detailed 2D images
Spectral/Fourier-domain: Same set-up as standard but reference mirror is fixed, a spectrometer splits reflected IR radiation by ‘grating’ into narrow bands each with differing wavelength. Computer detects/analyses reflected spectral interference patterns to derive spatial depth info (very detailed image).
Fast-Fourier transforms (FFT) derives A-scan data
Define keratometry with some clinical uses
measuring radius of curvature of anterior cornea to determine eye refractive power, CL fitting, detecting/monitoring corneal disease
What theoretical conclusions does keratometry provide regarding image height?
minified, virtual, upright behind the cornea
How does a compound microscope work?
series of converging lenses, objective system formed inverted image (h’o) of the actual mire image (h’) with a magnifying eyepiece system for better clinical viewing.
While we don’t directly view h’, we assume the axial positions of h’/h’observer are conjugate to each other
How do we counter eye movements during measurements and what might they be?
Drifts, saccades, micro-nystagmus
‘image doubling system’: bi-prism between objective/eyepiece systems measures degree of separation between 2 doubled h’o images
Explain variable image doubling with a fixed mire (Bausch&Lomb)
bi-prism splits objective image and degree of separation between 2 images is altered by varying the bi-prism position from the objective lens - allows H/V K readings together
Explain fixed image doubling with a variable mire (JavalSchiotz)
vary the object mire’s size until separation of the split images reach a predetermined size so cannot do H/V K readings together
Describe some limitations of keratometry
assumes lower cornea n (1.3375) to compensate for post. power (-5.88D)
can’t capture central/peripheral corneal reflections, only evaluates small ‘annular’ region of ant. surface which is assumed perfectly spherical (ITS NOT)
How can reflected Placido-Ring images be used to indicate corneal curvature?
multiple-ringed image reflected off ant. surface build up topographical map of corneal curvature variations. Images captured via photos and specialised computer algorithms evaluate spaces between each reflected mire (closer rings show steeper curve near centre and vice versa for periphery)
