Aim To investigate optic nerve function using the pattern-reversed visual evoked cortical potentials (VECP) before and after bony orbital decompression in dysthyroid optic neuropathy (DON) due to Graves' disease.

Methods A total of 30 eyes of 15 patients (n=14 female) were observed over 30±13 months after bony three-wall orbital decompression. We examined visual acuity (VA), VECP P100 amplitudes and latencies, as well as proptosis using Hertel's exophthalmometry.

Results Mean logarithm of the minimum angle of resolution (logMAR) VA increased, statistically significantly, by 2.4 lines during 30±13 months (from 0.38±0.25 before surgery to 0.14±0.1 at the end of observation, p=0.0001). All eyes maintained or improved vision by at least one line. Mean postoperative reduction of proptosis was 6.4±3 mm. While VECP P100 amplitudes improved significantly, P100 latencies remained abnormal in 18 eyes (60%) during follow-up of 10±7 months. Nine eyes (30%) with previous latency defects improved in at least one check test, five of which normalised completely. Worsening was evident in seven eyes (23%), and three previously normal eyes developed new pathological latencies. P100 latencies in 14 eyes (47%) remained unchanged.

Conclusion After decompression surgery, DON remission was observed in all patients regarding vision and VECP amplitudes. New or persistent P100 latency defects were seen in 60% of eyes after surgery. DON is considered to be caused by compressive ischaemic damage, which further underlines the importance of early decompression surgery.