Oguchi's disease & Mizuo phenomenon

Oguchi's disease, first described by Chuta Oguchi in 1907. is a rare autosomal recessive trait characterized by congenital stationary night blindness and an unique morphological and functional abnormality of the retina. Patients have nonprogressive night blindness since young childhood with normal day vision, but they often claim improvement of light sensitivities when they remain long in the dark environment; dark-adaptation study demonstrates that highly elevated rod thresholds decrease several hours later and eventually result in a recovery to the normal or nearly normal level. The eyegrounds have an appearance of diffuse or patchy, silver-gray or golden-yellow metallic sheen and the retinal vessels stand out in relief against the radiant background. A prolonged dark adaptation of three hours ormore, leads to disappearance of the unusual discoloration the normal reddish appearance, called Mizuo-Nakamura phenomenon. 



Oguchi's disease is also unique in the electroretinographic responses in the light- and dark-adapted condition. Recent identification of the arrestin gene mutation in patients with Oguchi's disease may account for the characteristic fundus and 
functional abnormality.

The fundus oculi presents a most peculiar appearance. The posterior pole and in many cases the whole of fundus, instead of having a normal orange-red colour, presents a curious shining greyish-pink background, on which the retinal vessels stand out sharply. The vessels appear dark with little distinction between the arteries and the veins. The finer divisions of the blood vessels can be easily followed to their finest ramifications. At places a dark shadow or a bright white line may be seen alongside the blood vessel.

The most interesting feature of this disease in well marked cases is the reversal of all the features described above, if the patient sits in the dark for about one hour. This is called Mizuo's phenomenon. The retina looks nor­mal and the night-blindness disappears. After the fundus has resumed the normal colour, exposure to room light of 40-50 foot candles soon leads to the appearance of fundus dis­colouration. It begins with a spotty distribu­tion after about 15 minutes' exposure and within one hour the colour transformation is complete.

In some cases the discolouration of the fundus is mild and Mizuo's phenomenon is slight or absent, but night-blindness is definitely complained of.

Mizuo–Nakamura phenomenon. (A) A golden sheen is seen in the mid-peripheral fundus of both eyes before dark adaptation. (B) The golden sheen is extinguished after 30–45 min of dark adaptation.

Microscopical examination :Two peculiar fea­tures have been noticed

1. The cones are unusually numerous and are present to the practical exclusion of the rods in a large area. Many of the cones are abnormally long and their nuclei are ectopic,i.e. they are outside the outer limiting mem­brane. The bipolar layer is thicker than nor­mal and the ganglion cell layer is eight to ten cell deep.

2. There is an anamolous layer of tissue between the cones and the pigment epithelium. It is not a true cellular layer, but rather appears as a degenerated syncytial structure containing many pigment granules.

Classification

Oguchi's disease has been further classified into various types

Type I : Typical cases with marked fund us discolouration, Mizuo's phenomenon and re­covery of dark adaptation.

Type II: 

(a) slight fundus discolouration, partial Mizuo's phenomenon and without recovery of dark adaptation.

(b) slight fundus discolouration without Mizuo's phenomenon and without recovery of dark adaptation.

Heredity

It is universally agreed that the condition is transmitted as an autosomal recessive without sex discrimination. Consanguinity also plays an important part.