On the face of it, there must be an essential relationship between subjective measurements by perimetry and certain objective measurements, such as optical coherence tomography (OCT) measurements of the thickness of the retinal nerve fiber layer (RNFL).
10 11 Logically, the degree of reduced visual sensitivity in an area of the visual field must be proportional to the amount of loss of ganglion cells in the corresponding area of the retina.
12 13 14 15 Similarly, the number of axons entering the optic nerve from an area of the retina must be equal to the number of RGCs and therefore also proportional to the degree of reduced visual sensitivity. However, recent clinical studies have suggested less straightforward interpretations of mechanistic relationships between standard automated perimetry (SAP) and OCT. For example, OCT has been reported to be a highly reproducible measurement with very high sensitivity and specificity in discriminating between control and patient populations.
16 17 18 19 20 21 Without exception, RNFL thickness measurements that are outside the 95% normal confidence limits identify patients with glaucoma who have moderate glaucomatous field defects
17 and, thus, in stages where there is an established relationship between visual and neural losses, both procedures detect the neuropathy.
14 15 Other studies, however, have suggested a “disconnect” between the structural and functional measurements.
22 23 24 In a longitudinal study of persons with suspected glaucoma and patients with known disease, a larger percentage of patients were classified as having progressive disease by OCT than by SAP measurements, and it was suggested that structural changes may precede functional changes in early stages of the disease (preperimetric glaucoma), or lag behind functional changes in the late stages, because of a curvilinear relationship between functional and structural defects caused by glaucoma.
24 In support of preperimetric neural loss, several other investigations have found evidence of an earlier detection of neuropathy by measurement of the nerve fiber layer compared with perimetry.
25 26 27 28 In addition, although RNFL thickness parameters have relatively high sensitivity for identifying glaucoma, there may be less agreement between perimetry and nerve fiber layer instruments for classifying eyes with glaucoma, which suggests that different techniques may identify different characteristics of glaucomatous damage.
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