To compare different screening procedures for hydroxychloroquine sulfate (Plaquenil) toxicity at different stages of damage.
Ten patients were studied using 10-2 automated fields, multifocal electroretinography, spectral domain optical coherence tomography (SD-OCT), and fundus autofluorescence.
All 10 patients used hydroxychloroquine for more than 6 years, and those with severe toxicity had been overdosed. Fundus examination findings were normal except for the patients with severe toxicity. All the patients showed parafoveal field loss, but this was sometimes subtle. Multifocal electroretinography demonstrated parafoveal weakness in the milder cases. The SD-OCT subfield thickness plots showed a ring of parafoveal thinning in all the patients. The SD-OCT cross-sections showed parafoveal loss of the inner segment-outer segment and cone outer segment tip lines at early stages of toxicity, progressing to parafoveal thinning of the outer nuclear layer and eventually to retinal pigment epithelium damage. There was a ring of autofluorescence in most patients.
Overdosage with hydroxychloroquine seemed a significant risk factor for toxicity. Different individuals were more or less sensitive to different tests. Fields can be sensitive but only if read with a low threshold for change. Hydroxychloroquine causes early parafoveal loss of the outer segment lines on SD-OCT, with the first changes often evident in the inferotemporal quadrant. Parafoveal thinning of the outer nuclear layer follows, before retinal pigment epithelium damage is visible. Careful screening with multiple tests can detect toxic damage before prominent loss of the outer nuclear layer.