To the best of our knowledge, this is the first case report of unilateral peripheral cone dysfunction syndrome in which SS-OCT showed pathological changes in the GCL and IPL. SS-OCT is a significant improvement over conventional SD-OCT due to the optimized long-wavelength scanning light (1,050 nm) that facilitates better penetration of the deeper ocular layers, which facilitates the ability to obtain high-quality images from the vitreous to the choroid. Further, a macular thickness map and normal database values for the retinal thickness are available in this model. Thickness maps of the retinal nerve fiber layer (RNFL); GCL and IPL; and RNFL, GCL, and IPL also are available.
When patients report visual field defects without abnormal fundus changes and visual loss, the differential diagnosis includes diseases such as malingering, amblyopia, occult macular dystrophy (Miyake’s disease), retinitis pigmentosa sine pigmento, congenital stationary night blindness (CSNB), cancer-associated retinopathy (CAR), acute zonal occult retinopathy (AZOOR), early cone dystrophy, unilateral peripheral cone dystrophy, and unilateral cone dysfunction syndrome. In cases of malingering and amblyopia, all ERG responses are normal and these diagnoses were excluded in the current case. Miyake’s disease is not characterized by an abnormality in the ff-ERG, but the multifocal ERG (mf-ERG) shows an abnormal macular response, and OCT shows attenuation of the ellipsoid zone (EZ) and IZ in the fovea. Therefore, we excluded Miyake’s disease as a diagnosis in the current case based on the ERG and OCT findings. In retinitis pigmentosa sine pigmento, CSNB, and CAR, the rod response is decreased, and these diseses also were excluded from the differential diagnosis. Patients with AZOOR usually complain of sudden visual field loss accompanied by photopsia. This disease develops in young women and is often unilateral. OCT images show that the EZ and IZ corresponding to the visual field defect are impaired. The ERG shows that all reactions are normal to subnormal. The cone responses tend to be impaired compared with the rod response. FAF using scanning laser ophthalmoscope shows hyperfluorescence around the optic nerve disc . In the current case, AZOOR was suspected strongly based on the medical history but was excluded because photopsia was absent, the IZ was impaired selectively in the SS-OCT images, the FAF was normal, and the cone response was impaired significantly in the ff-ERG. In the localized impairment s indicated by the visual field, the cone response should be unaffected. However, in the current case, the cone response markedly decreased, which reflected widespread cone dysfunction. Peripheral cone dystrophy has been reported to occur in families [1, 9]. Although the current case seemed to be a genetic disease such as a macular dystrophy, the family history could not be obtained and the changes were unilateral, so this particular case would not be a case of hereditary macular dystrophy. The possibility of drug-induced cone dysfunction syndrome such as that caused by chloroquine  and dioxin  has been reported. However, there was no history of long-term medication use in the current case, which eliminated drug-induced cone dysfunction syndrome. In early cone dystrophy, as in the current case, the cone response is decreased or lost in the ff-ERG. The foveal bulge is absent on OCT images. However, in the current case the foveal bulge was maintained despite loss of the peripheral IZ. Therefore, the findings suggested that this case differed from typical cone dystrophy. Five cases of unilateral cone dystrophy have been reported [12-15], but the current case is not in the same category because of the residual foveal bulge. Previous studies have reported the electrophysiologic and clinicopathological findings in peripheral cone dystrophy [1-7]. The ff-ERG of peripheral cone dystrophy shows attenuation or disappearance of the cone response similar to typical cone dystrophy [1, 4-7]. Further, peripheral cone dystrophy was characterized by a normal response at the focal macular ERG and a central response in the mf-ERG . Using FAF, Vaphiades and Doyle reported hyperfluorescence in cases of unilateral peripheral cone dystrophy . However, in the current case it was normal as reported by Yamada et al . Mochizuki et al.  reported thinning of the outer nuclear layer (ONL), EZ, and IZ using SD-OCT in peripheral cone dysfunction. In the current case, the ONL thickness was not measured, but the disappearance of the IZ in a region other than the fovea occurred only in the right eye. Furthermore, in the current case 3D, macular SS-OCT analysis showed thinning of the GCL and IPL that corresponded to the area of the IZ defect with foveal sparing only in the right eye. SS-OCT findings corresponded well to the changes in the ERG and visual field abnormality. Based on this analysis, we speculated that the primary lesion in peripheral cone dysfunction syndrome is not in the cone photoreceptor cells but in the horizontal cells and/or amacrine cells, in that cone photoreceptor cells in the foveola are connected to the retinal ganglion cells in a one-to-one correspondence without connection to the horizontal cells or amacrine cells because the GCL and IPL are not present in the foveal area. The clinicopathological changes in the ganglion cells and cone photoreceptor cells might be secondary changes due to pathology in the horizontal cells and/or amacrine cells in peripheral cone dystrophy. However, the oscillatory potentials (OP) remain in the ERG recording (Fig. 4a). Therefore, the amacrine cells might not be the lesion in the current case because the amacrine cells are the source of the OP. Thus, we considered that the main lesions in peripheral cone dystrophy are the horizontal cells. Sieving previously measured the long-flash cone response and suggested that the bipolar cells and/or horizontal cells were the primary lesions , which supports our speculation about the pathology in the current case.
As Vaphiades and Doyle  pointed out, peripheral cone dystrophy is often misdiagnosed. Cases in which patients report a lateral focal visual field abnormality with normal fundus findings and good vision often can be misdiagnosed or left untreated. However, the pathological mechanism of peripheral cone dystrophy can be analyzed by a combination of electrophysiologic examinations and recently developed SS-OCT as in the current case. However, the exact pathogenesis of peripheral cone dystrophy remains uncertain. In addition, no cases of peripheral cone dysfunction syndrome have been followed over the long term; thus, the natural history of this disease remains unknown. We believe that we should follow this case over the long term.