Imaging the RNFL Flashcards
What are 3 methods for imaging the retinal nerve fibre layer?
- Scanning laser ophthalmoscopy (SLO)
- Scanning laser tomography
- Scanning laser polarimetry
What are the indications for scanning laser ophthalmoscopy?
similar to colour fundus photography, but enhanced capabilities for evaluation of patients with medical retina pathology
How do scanning laser ophthalmoscopes work?
- SLO devices employ a confocal (‘pinhole’) aperture, generating single point of laser light at a specific wavelength, scanned across the retina in a raster pattern (series of parallel horizontal lines)
How can higher-contrast images be generated from scanning laser ophthalmoscopy + light scatter reduced?
only a small area of fundus illuminated at any time
What are 2 common instrments used for scanning laser ophthalmoscopy?
- Heidelberg Retina Angiograph-2
- Nidek F-10
What is a disadvantage of scanning laser ophthalmoscopy compared with fundus cameras?
fundus cameras typically have higher temporal and spatial resolution
What are 2 indications for scanning laser tomography?
- to distinguish normal optic disc anatomy from glaucomatous optic neuropathy
- to monitor longitudinal or progressive change of glaucomatous optic neuropathy
How does scanning laser tomography work?
confocal aperture in SLO devices allows acquisition of images at different focal planes within a tissue of interest (i.e. generation of a stack of en face images). 3D reconstruction then allows tomographic (corss sectional) visualisation of the fundus
What is an example of an SLO device that allows for 3D reconstruction of the optic nerve head?
Heidelberg Retina Tomograph-3
What are 3 cautions to bear in mind regarding scanning laser tomography?
- lower resolution than OCT
- media opacities and ability of pt to fiate can affect quality and variability
- measurements influenced by acute changes in IOP and possibly cardiac cycle
How is a false-colour topographic image created from scanning laser tomography?
the stack of confocal images is aligned and their reflectivities summed, generating the image
How is optic nerve head morphology generated from scanning laser tomography (e.g. disc and cup area)?
software calculates detaield measurements from the stacked confocal images producing a topographic images
stereometric parameters compared with normative database + risk of glaucoma assessed using regression model
What type of analysis is used for calculting the risk of glaucoma from the informaton on optic nerve morphology provided by scanning laser topography?
regression model - Moorfields Regression Analysis
How many sections is the image obtained from scanning laser topography split into and how is each section scored?
6: green tick indicates within normal limits, yellow exclamation mark borderline, red cross is outside normal limits
What are 2 indications for scanning laser polarimetry?
- distinguish normal optic disc anatomy from glaucomatous optic neuropathy
- monitor progression of glaucomatous optic neuropathy
What is a key property of the RNFL due to the parallel arrangement of its axons?
birefringent: ray of light entering it is broken into two rays
What are the basic principles of how scanning laser polarimetry works?
polarised light reflected from the RNFL undergoes a phase shift, depending on amount of birefringent material present. Scanning of polarised light across a region centred on the optic nerve head can be used to assess phase shifts and estimate RNFL thickness
What is the only commercially available scanning laser polarimeter?
GDx
How can newer models of scanning laser polarimeters improve the accuracy of results?
compensate for corneal birefringence
How are the images from scanning laser polarimetry processed after acquisition? 3 forms of output
- nerve fibre thickness maps presented in colour-coded spectrum from blue to red
- deviation maps used to show the magnitude and location of RNFL defects using colour coded squares
- TSNIT graphs comparing RNFL thickness to a normative database
What is the nerve fibre indicator (NFI) from scanning polarimetry?
global value based on the entire thickness map, used for discriminating normal vs glaucomatous eyes: normal 1-30, borderline 31-50, abnormal 51-100
What is the benefit of advanced serial analysis with scanning laser polarimetry?
shows trend analysis over time from baseline in optic disc anatomy/optic neuropathy
When may measurements from scanning laser polarimetry be erroneous?
in areas of peripallary atrophy (PPA) or chorioretinal scarring
Do mild to moderate cataracts degrade the results from scanning laser polarimetry?
no
