Clinical application

ERG response to heterochromatic flicker and pattern contrast reversal stimulation

Results in humans (Porciatti et al., 1987, 1989; Ghirlanda et al., 1991; Falsini et al., 1991, 1995; Porciatti and Falsini, 1993) showed that the 2F component is selectively altered (i.e. with normal 1F component) in diseases affecting primarily the inner retina, such as glaucoma, optic neuritis, or optic nerve compression (Porciatti and Falsini, 1993), and appeared to be highly sensitive to retinal vascular disorders, such as diabetic retinopathy (Ghirlanda et al., 1991) and branch occlusion of the central retinal artery or vein (Porciatti et al., 1989; Falsini et al., 1995). Different clinical studies on cohorts of patients with hypertension (OHT) or glaucoma (Porciatti et al., 1987; Falsini et al., 1991; Colotto et al., 1995) have demonstrated that the P-ERG, a retinal potential requiring, for its generation, the functional integrity of the inner retina, is altered in a substantial proportion of OHT eyes and in the great majority of eyes with early glaucoma. Diagnostic accuracy of this test in detecting glaucomatous damage to the optic nerve is relatively higher than that observed for VEPs and psychophysical tests, such as conventional automated perimetry, blue-on-yellow perimetry, and contrast sensitivity. One possible reason is that the P-ERG, unlike other electrophysiological and psychophysical tests, can provide a direct and objective probe of inner retinal function, and does not reflect possible compensatory processes of peripheral neural damage occurring at higher levels along the visual pathways. It has been also demonstrated that the P-ERG can be of value in the clinical follow-up of glaucoma (Falsini et al., 1992). In a 24 months follow-up study on OHT and glaucoma patients under topical beta-blocker treatment (Falsini et al., 1992), progressive P-ERG amplitude losses were associated with poor intraocular pressure (IOP) control. By contrast, a recovery or stabilization of the response amplitude was found in patients with satisfactory IOP control. No visual field changes were observed in patients during the follow-up. These data suggest the possible use of the P-ERG to estimate the

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‘‘target’’ IOP in an individual patient undergoing medical treatment. In agreement with this suggestion, a predictive value of the P-ERG response for the development of early visual field defects has been found in a 24-month follow-up of treated OHT patients (Colotto et al., 1995), confirming previous results obtained by others (Pfeiffer et al., 1993). More recently, subclinical functional damage in OHT patients, as determined by the P-ERG, was compared with early anatomical abnormalities of the optic disc, found in the same patients by using confocal scanning laser ophthalmoscopy (Salgarello et al., 1999). All patients had normal conventional white-on-white perimetry and optic disc appearance at biomicroscopic examination. P-ERG amplitude was found to be significantly correlated with the shape of the optic disc cup, with increasing losses as the cup shape approached abnormal values. These data provided evidence that, already in the OHT stage, a parallel involvement of both function and morphology may occur, and stressed the value of combined structural and functional analysis in detection of early glaucomatous damage.

RFonh in OHT and glaucoma patients

In a previous work, from which some of the following text is borrowed with permission from

the publisher, RFonh to luminance (l-fl) and chromatic (c-fl) flicker was evaluated in OHT and

early glaucoma (EOAG) patients (Riva et al., 2004b). These stimuli generate neural activity dominated by M- and P-cellular system, respectively. OHT, POAG patients, and normal controls were selected based on their ability to maintain adequate head stability, excellent target fixation, a pupil dilatation W6 mm or more and no cataract. Fifteen early EOAG, 27 OHT, and 12 age-matched control subjects were examined. RFonh was determined in response to a 15 Hz green luminance flicker (301 field). Two to three responses were collected at different temporal sites of the optic disk. The highest RFonh value was used.

As compared to controls, OHT and EOAG patients showed a decrease ( po0.01) in both mean

baseline Fonh and RFonh ( po0.001). In individual patients, both Fonh were positively