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A 34-year-old man with visual complaints and a tapetal-like reflex
Digital Journal of Ophthalmology 2008
Volume 14, Number 14
July 28, 2008
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Joao C.M.L. Ribeiro | Massachusetts Eye and Ear Infirmary Cynthia S. Chiu | Massachusetts Eye and Ear Infirmary Christine Ament | Massachusetts Eye and Ear Infirmary John I. Loewenstein | Massachusetts Eye and Ear Infirmary
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| Diagnosis and Discussion | This case illustrates a particular phenomenon observed on fundus examination called the Mizuo-Nakamura phenomenon. This phenomenon is characterized by a change in the color of the fundus from red in the dark-adapted state to golden immediately or shortly after the onset of the light.(6,7) The color of the fundus reflex in the light adapted state has also been described as golden-yellow, gray-white, and yellow-white. This reflex can appear either homogeneous or in streaks in the fundus. The retinal vessels appear to be protruding in contrast to the radiant background. Dark adaptation leads to disappearance of the unusual fundus coloration. The phenomenon was first described in connection with Oguchi disease, an autosomal-recessive form of stationary night blindness occurring mainly in Japan.(4) In Oguchi disease, prolonged dark adaptation, ranging from 2 to 12 hours or more, is typically required for the change in fundus color to take place.
The origin of this phenomenon is unknown. Excessive extracellular potassium in the retina has been hypothesized, but not proven as the pathophysiological basis of the Mizuo-Nakamura phenomenon.(5) A case report of retinal detachment seen in a patient with Oguchi disease reported that the detached retina lost its golden reflex, but partially regained the reflex 7 months after segmental buckling surgery. The area of the golden sheen had enlarged during the following 14 months. This result suggested that a connection between the retinal pigment epithelium and sensory retina must be necessary for the abnormal fundus reflex to occur.(8) In another patient with Oguchi disease, diffuse, fine, white particles, which do not exist in normal subjects, were clearly demonstrated in the light-adapted retina with helium-neon laser. These particles were not seen by scanning laser ophthalmoscopy with the use of argon and infrared lasers, suggesting that they appear to be located in the outer retina, retina pigment epithelium, or both.(9) The time course of disappearance and appearance of the white particles correlated with the change in fundus color, suggesting that these particles contribute to the abnormal fundus coloration and Mizuo-Nakamura phenomenon. The authors proposed that these particles may be products of an abnormal metabolic pathway that operates during light adaptation.
In the present case, after excluding other possible causes of loss of consciousness, the episode of syncope was considered most likely related to a situational syncope due to a neurally-mediated reflex that occurs during urination or immediately thereafter.(10)
The poor vision in the right eye was probably related to the history of trauma during childhood, which could explain the traumatic optic neuropathy seen in the right eye during the fundus examination. The hypertelorism and shallow orbits findings were unrelated to the diagnosis. In addition, the ERG was performed by standardized methods designed to test separately the cone system with the photopic and flicker responses and the rod system with the scotopic responses. According to our findings, the ERG suggests a cone dystrophy. A cone-rod degeneration is a less likely possibility. In Oguchi's disease, the classic ERG findings show non-recordable scotopic responses after 30 minutes of dark adaptation and normal photopic and 30Hz flicker responses.(11)
The Mizuo-Nakamura phenomenon has been reported in X-linked cone dystrophy.(1) It has not been reported in cone-rod degeneration. This suggests that cone dystrophy is the likely diagnosis.
The typical visual symptoms and exam findings in cone dystrophy are photophobia, loss of visual acuity, loss of color, and central vision abnormalities. There is exclusive cone system involvement found on the ERG. Nevertheless, in some progressive forms, the rod system may be affected. The fundus examination is characterized by central macular pigment epithelial atrophy, although early cases may appear normal. In some cases, only a faint granular appearance may be present in the macula, while in others a bull’s eye pattern of the macula is evident. Visual acuity often declines to 20/200 by the end of the third decade.(12) | |
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