Sir,

Mizuo phenomenon has been rarely reported in juvenile X-linked retinoschisis (XLR).1 We report a case of XLR with peripheral Mizuo phenomenon2 which presented as isolated foveal schisis.

Case report

A 16-year-old male presented with diminution of vision OS since 1 year. He had no history of nyctalopia. Best-corrected visual acuity was 6/9 OD and 6/24 OS. Colour vision was normal. Fundus evaluation showed dull foveal reflex and a tapetal reflex in the periphery. After 5 h of dark adaptation, the tapetal reflex disappeared (Mizuo phenomenon) (Figure 1), which reappeared within a minute of light exposure. Full-field electroretinogram (Metrovision, France) showed reduction of ‘b’-wave amplitude of combined maximal scotopic response and reduced cone ON–OFF responses (Figure 2). Optical Coherence Tomography (OCT) showed intraretinal foveal schisis (Figure 3).

Figure 1
figure 1

Fundus photograph of the left eye. (a) Tapetal reflex in the temporal periphery in the left eye. (b) Disappearance of the reflex following prolonged dark adaptation.

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Figure 2
figure 2

Full field electroretinography recordings. (a) Forty percentage reduction in ‘b’ wave amplitude of combined maximal scotopic response was noted in both eyes. (b) Reduced cone ON and OFF bipolar response in both eyes.

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Figure 3
figure 3

OCT from both eyes. (a) OCT from the right eye showing foveal schitic changes with macular atrophy. (b) OCT from the left eye shows intraretinal bridging columns in between hyporeflective spaces classical of foveal schisis.

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Comment

Muller cells regulate extracellular K+ ion concentration in the retina.3 In XIS, Muller cell dysfunction occurs secondary to the splitting of the nerve fibre layer. This causes defective inward transportation of K+ ions from the inner and outer plexiform layers. The excess of extracellular K+ ions causes Mizuo phenomenon.1 But the fact that most patients with XLR do not demonstrate tapetal reflex is intriguing. The nonuniform distribution of K+ channels across muller cell in retina4 and differential affection of muller cell segment in XLR are possible explanations. This suggests that if adequate numbers of K+ channels are intact, the reflex is absent. As greater numbers of K+ channels are affected, Mizuo phenomenon becomes demonstrable wherein excessive K+ ions are cleared during dark adaptation. When most of the K+ channels are affected, the tapetal reflex becomes constant, as there is persistent K+ ion backlog. Previous reports of increased tapetal reflex with time1 suggest progressive K+ channel damage in XLR. Tapetal reflex disappeared following vitrectomy and posterior hyaloid peeling5 in XLR. The disruption of K+ channels at muller cell foot plate during the surgical procedure could have increased the inward conductance of K+ ions trapped within dysfunctional muller cells causing disappearance of the reflex. The selective reduction of ‘b’ wave of combined maximal ERG and foveal schisis on OCT confirmed diagnosis of XLR in the absence of peripheral retinoschisis and presence of Mizuo phenomenon.