Newer Imaging Technology in Glaucoma
Advanced Eye Centre,
Postgraduate Institute of Medical Education and Research, Chandigarh
Definition:
Clinical examination of the disc has been the basis of disc and nerve fibre layer evaluation for ages but it is marred by its subjectivity and non-reproducibility, in the diagnosis and detection of glaucoma. Though visual field changes give concrete and reproducible evidence of glaucomatous changes, it becomes manifest only after considerable damage has occurred to the retinal ganglion cells(RGCs) and the nerve fibre layer(NFL).There are normally 1.2 - 2.4 million nerve fibres and corresponding number of ganglion cells in the retina. Kerrigan-Baumann and Quigley et al 1 documented that a loss of 35.7% of the RGCs was required for the manifestation of corrected pattern standard deviation(CPSD) <0.5% in the visual fields and a loss of 5dB in the sensitivity was associated with 25% loss in the RGCs. This was because even when some RGCs were dead, surrounding RGCs subserving the same area signal the presence of the target. This stage of glaucoma undetected by conventional Standard white-on-white automated perimetry is known as Pre-perimetric glaucoma.
Thus arose the need to devise methods and technology to detect the presence of pre-perimetric glaucoma and be able to control the disease before it does considerable damage.
The pre-perimetric tools known to us till date are: Frequency Doubling Perimetry (FDP), Heidelberg Retina Tomogram (HRT) and Retinal Nerve Fibre Analysers such as Optical Coherence Tomogram and GDx. We will be discussing each of these by turn.
Fig1:Cross-sectional view of lateral geniculate body
Frequency Doubling Perimetry (FDP):
There are two types of RGCs, M type (10% of total RGCs) and P type. The M type cells detect low contrast and high temporal frequency (motion) and project to the Magnocellular layer of Lateral geniculate body. The P cells are responsible for high contrast and low temporal frequency (or static) and project to the Parvocellular layer of Lateral geniculate body.
The larger diameter M cells are the first to die in glaucoma and they form the basis of FDT, which detects this damage whereas standard automated perimetry cannot pick it up.2A low spatial frequency sinusoidal grating with alternating wide light and dark bars is the stimulus, which undergoes high temporal frequency counter phase flicker i.e. the black bands reverse to become white and the white bands reverse to become black in rapid sequence. The grating, thus, appears to have twice as many light/dark bars i.e. its spatial frequency appears doubled.
Figure2: Frequency doubling of the target
It is these vulnerable M cells which are thought to transmit signals related to the above target, thereby identifying earlier retinal neuron loss due to glaucoma. FDT is not affected by external room illumination or variations in pupil size. It offers screening (C20) and threshold (N20, N30) programs. On the centre of the screen a black dot is seen for the patient to fixate, and whenever the grating (5 degrees square) is seen in the field (17 locations), the button is pressed.
Figure3: The screen of the FDT test
Based upon the contrast required to perceive gratings, a threshold is estimated at the locations tested.
Screening program (C20):
Abnormal screening is considered when either of following is seen
a) Any defect in the central 5 locations
b) Two mild or moderate defects in the outer 12 locations
c) One severe defect in the outer 12 locations
d) Screening test time > 90 seconds per eye
Figure4: Print out of a normal C20 programme
Threshold program (N20 and N30):
This program uses more contrast levels to search for the patient's threshold at each of the tested location. N30- horizontal area extended to include an extra portion of the nasal visual field, resulting in a total 30 degree horizontal field.
Figure5: Print out of a N30 program
