Quantification of relative aVerent pupillary defects induced by posterior sub-Tenon’s, peribulbar, and retrobulbar anaesthetics

Aims—The eVect of local anaesthetics on optic nerve function can be investigated by quantifying the relative aVerent pupillary defect (RAPD). Methods—The study compared the depth of induced RAPD following posterior subTenon’s, retrobulbar, and peribulbar local anaesthetics using crossed polarising filters before cataract surgery (time 1 = 5 minutes), immediately after surgery (time 2 = 42 minutes (av)), and once again on the ward (time 3 = 107 minutes (av)). Results—All patients developed a RAPD. There was no significant diVerence in the depth of RAPD between the groups at any one time period. The peribulbar group had a significantly steeper decay in RAPD from time 1 to time 2 (p = 0.014). This eVect was reduced when the shorter operation time for this group was entered as a cofactor (p = 0.063). By time 3 the RAPDs for all groups had decayed similarly so that no diVerences could be detected. Conclusion—All three anaesthetic methods caused a similar level of disruption to optic nerve conduction immediately following administration and at the time of day case discharge. (Br J Ophthalmol 2001;85:1445–1446) Local anaesthetics delivered within the orbit are known to impair optic nerve conduction. This study measured the degree to which three local anaesthetic techniques attenuated optic nerve conduction by quantifying induced RAPDs by a crossed polarising filter technique. 5 Optic nerve function following peribulbar and retrobulbar anaesthesia has been investigated in terms of visual evoked potentials and visual function. 2 Posterior sub-Tenon’s anaesthesia is a newer technique that has been less extensively investigated. 7 Although RAPDs have been recorded following local anaesthetics 2 they have not been quantified. This study measured the eVect of these three local anaesthetic techniques on RAPD using crossed polarising filters. This method is simple, quick, reproducible and, in contrast with neutral density filters, has a continuous scale of measurement. Materials and methods Sixty consecutive patients undergoing phacoemulsification were randomly allocated to one of the three anaesthetic groups. Subjects with pre-existing RAPDs or other ocular disease were excluded. Each of the three standardised anaesthetic techniques was performed by a single ophthalmologist. A standard anaesthetic cocktail of 750 units of hyaluronidase in 7 ml 2% lignocaine was used. The posterior subTenon’s technique involved delivering 3.5 ml through an inferomedial conjunctivo-episcleral buttonhole via a blunt, curved, sub-Tenon’s cannula passed posteriorly around the globe. The peribulbar group received 7.5 ml via two injections with a 40 mm needle directed parallel to the bony orbit along the inferolateral orbital floor and below the trochlear notch. Retrobulbar injections of 3.5 ml of the mixture were performed using a 38 mm retrobulbar needle directed within the muscle cone. RAPD measurement was performed using an indirect ophthalmoscope light under dim background illumination using a standardised technique. Quantification of the pupil defect was achieved by attenuating the light to the unaVected pupil by rotating a polarising filter against another until the defect was neutralised using a reverse testing technique. RAPD measurement was performed preoperatively, 5 minutes after local anaesthetic administration (time 1), immediately postoperatively (time 2), and finally back on the ward (time 3). Degrees of rotation of the crossed polarising filters were converted to neutral density filter log units. Results were analysed using paired and unpaired Student’s t tests and analysis of variance. Results All patients developed an RAPD by 5 minutes. All anaesthetics were reported by surgeon and patient to have given satisfactory anaesthesia and akinesia. There were no significant intergroup diVerences in the mean RAPD, which lightened with time (Table 1, Fig 1). There was a significant decay in RAPD from time 1 to 2 for all groups (p<0.001), and a significant diVerence in the rates of decay between the three anaesthetic types (p = 0.014) with the peribulbar group lightening fastest. Analysis of covariance revealed the level of decay was related to the amount of time elapsed (p = 0.03). However, when operation time was also included as a covariate, there was no significant relation between anaesthetic type and attenuation of RAPD (p = 0.278). By time 3 the RAPD had decayed so that no significant eVects could be detected with respect to time of measurement, operation time, or anaesthetic type. Br J Ophthalmol 2001;85:1445–1446 1445 Department of Ophthalmology, Royal Victoria Infirmary, Queen Victoria Road, Newcastle upon Tyne NE1 4LP, UK