RETROBULBAR PRESSURES MEASURED DURING SURGICAL DECOMPRESSION OF THE ORBIT

Aims/background - In Graves' ophthalmopathy the increase in volume of intraocular muscles and fat will cause elevated intraorbital pressure. In order to investigate the pressure levels involved, intraorbital pr essure, or retrobulbar pressure (REP) was measured continuously in orb its of patients with Graves' ophthalmopathy during surgical decompress ion. Methods - Retrobulbar pressure was measured before and during sur gical decompression using an intraorbitally applied pressure transduce r. Results - In eight patients with dysthyroid optic neuropathy (DON) RBPs between 17 and 40 mm Hg were recorded. At the end of the surgical procedure the mean REP was reduced from 28.7 mm Hg to 18.7 mm Hg, the decrease ranging from 8 to 12 mm Hg, which showed a high correlation with the starting pressures (p<0.001). In two cases without DON, press ures were 11 and 9 mm Hg. Forces exerted by spatula manipulation usual ly resulted in a REP level of more than 70 mm Hg. Conclusions - This s tudy shows that RBPs are markedly elevated in Graves' ophthalmopathy a nd that surgical decompression can result in a significant reduction i n the intraorbital pressure. Optic nerve dysfunction in Graves' ophtha lmopathy may not be caused exclusively by the direct pressure of swoll en extraocular muscles upon the optic nerve, but also by a raised REP. It is hypothesised that the damage inflicted upon the optic nerve can be caused in consequence by REP induced incarceration of the nerve, c ompressed by surrounding periosteal lined orbital fat bulging posterio rly into the entrance of the optic canal.