Sir,

Case report

A 22-year-old man was referred with decreased visual acuity of 4 years duration and bilateral ‘positive’ scotomata. Past medical and ocular history was unremarkable. Visual acuity (VA) was 0.30 logMAR in the right eye (OD) and 0.60 in the left (OS). Slit lamp and fundus examination were normal. Automated perimetry (Humphrey 24–2) showed small bilateral central scotomata. Electrophysiologic tests, including full field and pattern electroretinography (PERG), were normal, but modified dark adaptometry was consistent with enhanced rod–cone interaction (E-RCI) (Figure 1). After 6 years, he was complaining of poor central vision and constant flickering, especially when going from a dark to an illuminated environment and VA had dropped to 0.79 OD and 0.83 OS. Fundus exam remained normal. PERG elicited significantly reduced activity from both eyes, and a multifocal ERG (mfERG) showed central macular dysfunction. Full-field ERGs remained normal showing the dysfunction to be confined to the maculae. Three dimensional Optical Coherence Tomography (3D-OCT) showed bilateral disruption of the photoreceptor inner segment/outer segment (IS/OS) layer (Figure 2).

Figure 1
figure 1figure 1

Electrophysiology and psychophysics. (a) ERGs and PERGs. Full field ERGs were normal in both 2004 and 2010, and only the later data are shown. PERG shows significant deterioration with a standard field size between 2004 and 2010. Large field PERG data, only available for the recent visit, suggest relative sparing of paramacular function. Data from a representative normal subject are shown in the lower row. The paramacular sparing is confirmed by the mfERG (b) where there is marked loss of the responses to central hexagons, but preservation of the responses to peripheral hexagons (field size=55 degrees). (c) Dark adaptometry: dark adaptation (DA) shows enhanced rod–cone interaction. The dashed line shows the conventional DA response from a normal subject; the solid line shows the threshold measurements to a red 15 Hz flicker (cone mediated). Note the normal elevation of the cone threshold by 0.7–1.2 log units as rod sensitivity increases. The hollow squares show the rod data from the patient; the filled squares are the cone-mediated responses showing abnormal threshold elevation (note that the responses shown as 0.0 log units include (undetected) responses where the threshold response exceeded the range of the instrument). The two blocks of data at 40 min are those to a 1 Hz 200 ms flash using blue (rod) and red (cone) stimuli for final threshold measurement.

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Figure 2
figure 2

The 3D-OCT showing bilateral disruption of the photoreceptor inner segment/outer segment layer and intact outer limiting membrane.

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Comment

E-RCI, first described by Arden1, 2 in 1985, is a rarely reported disorder in which the normal elevation of dark adapted cone thresholds by dark adapting rods is greatly enhanced. A modified dark adaptometer is needed to make the diagnosis, as conventional Goldman-Weekers dark adaptometer cannot adequately differentiate rod and cone thresholds and cannot evaluate dark adapted cone sensitivity/thresholds. In this case, modified dark adaptometry1 demonstrated enhanced elevation of cone thresholds by dark adapting rods and established the diagnosis of E-RCI. The condition is usually benign and previous reports have not shown progression.3, 4, 5 The continuing reduction in VA in the present case and the markedly reduced PERG and mfERG suggested progressive macular dysfunction, which was confirmed by 3D-OCT demonstration of bilateral disruption of the photoreceptor IS/OS layer. To our knowledge, this is the first case of progressive macular dysfunction in association with E-RCI. It demonstrates the value of electrophysiology, psychophysics, and OCT in revealing functional and structural abnormalities despite the presence of a normal fundus exam.