THE ELECTRORETINOGRAM DURING ORBITAL COMPRESSION FOLLOWING INTRAORBITAL REGIONAL BLOCK FOR CATARACT-SURGERY

The electroretinogram (ERG) is a transient biopotential that reflects the electrical response of the distal retina to photostimulation. Dist urbances in retinal circulation produce characteristic abnormalities i n the ERG wave form. The objective of this study was to investigate th e changes in the ERG produced by combined retrobulbar and peribulbar i njections of a large volume (8 ml) of local anaesthetic, followed by o cular compression. Electroretinogram recordings were obtained from ski n electrodes placed on the infero orbital ridge in response to strobos copic flash stimulation in 34 adult patients undergoing cataract surge ry: (a) prior to regional anaesthesia (baseline condition); (b) within one minute after regional anaesthesia of the orbit (block condition); (c) after ten minutes of orbital compression with a Honan's device at 30 mmHg. (compression condition); (d) and five minutes after removal of orbital compression (recovery condition). The ERG implicit times of both a- and b-wave increased (P < 0.001) after anaesthetic block. The amplitude of the a- and b-waves also decreased (P < 0.001) immediatel y following anaesthetic block and continued to decrease following appl ication of the compression device (P < 0.01). Following removal of ocu lar compression the amplitude of the b-wave increased (P < 0.01). Only the a-wave implicit time (P < 0.005) decreased with release of ocular compression. These findings are compatible with the ERG changes of tr ansient retinal ischaemia produced by ocular compression.