We cannot conclude that 25G TSV has a higher rate of postoperative endophthalmitis compared to the 20G PPV. Future retrospective or prospective trials need to take into consideration multiple factors.

SUMMARY Vitreous samples were obtained from 41 eyes undergoing vitrectomy, and radioimmunoassays were performed to measure concentrations of the prostaglandins PGE2, PGF2a, prostacyclin, and thromboxane. Presumably physiological levels (approximately 100 picograms/ml) were found in vitreous from eyes undergoing cataract extraction. Eyes with vitreous haemorrhage, retinal detachment, or cystoid macular oedema had similarly low levels. Vitreous prostaglandins were mildly elevated in trauma and endophthalmitis and markedly elevated in aphakic bullous keratopathy. The role prostaglandins may play in cystoid macular oedema is reviewed.Prostaglandins are important mediators of inflammation, and the synthesis of these vasoactive prostaglandin compounds (PGE2, PGF2a, prostacyclin [PGI2]) by ocular tissues has been demonstrated. Low levels of these prostaglandins are found in normal human aqueous. A variety of conditions in which anterior chamber inflammation is present (keratitis, paracentesis, panretinal photocoagulation, etc.) have higher aqueous levels.' Elevated aqueous levels have also been found in association with aphakic cystoid macular oedema. Several investigators believe that prostaglandins synthesised in the anterior segment (as in iris and ciliary body) diffuse through the vitreous and then act on retinal capillaries to produce macular oedema.56 Miyake, Yannuzzi, Klein, and others have reported clinical trials in which a topical or oral prostaglandin inhibitor, indomethacin, given preoperatively and in the early postoperative period was associated with a lower incidence of fluorescein angiographic aphakic cystoid macular oedema.7-" Miyake also noted concurrent reduction in aqueous prostaglandin levels and clinical improvement in the cystoid macular oedema. Furthermore, Miyake et al. have reported 12 elevated aqueous prostaglandins in eyes which developed cystoid macular oedema after intracapsular cataract extraction with vitreous loss and vitreous incarceration to the wound. Pars plana vitrectomy Correspondence to M A Thomas, MD, Bascom Palmer Eye Institute,