Retinal dialyses near the ora are large, crescent-shaped retinal defects immediately behind the ora serrata. It is assumed that they practically always occur without posterior vitreous detachment and spread within the vitreous base (intrabasal retinal dialysis). However, there are evidently exceptions to this rule: there may be posterior vitreous detachment with the posterior hyaloid membrane which encroach on the anterior margin of the retinal defect; the defect then spreads along the posterior margin of the vitreous base and is hence a juxtabasal retinal dialysis. While the ophthalmoscopic features of intrabasal and juxtabasal retinal dialysis near the ora serrata are always identical, there are distinct biomicroscopic differences between the two forms.