Retrobulbar block - volume to use

Calculation of local anaesthetic volume required for retrobulbar block in dogs

Enucleation is a routine salvage procedure performed in dogs presenting with various types of ocular disease. Guidelines for calculating the safest, clinically useful volume of local anaesthetic for performing retrobulbar (RB) injection have always been available. A 2021 study by Greco et al. investigated a method to calculate the retrobulbar cone volume and therefore the injectate volume of local anaesthetic required to perform this technique.

Watch our video describing the retrobulbar block here

What were the study aims?

To develop a formula for calculating the volume of the retrobulbar (RB) cone in dogs with different skull morphologies. The authors hypothesised that both bodyweight and skull morphology would affect the RB cone volume in dogs of different breeds.

How was the study performed?

Ethical committee approval for the retrospective study protocol was gained. All canine head CT studies performed between January 2009 and December 2017 were retrospectively evaluated. The RB cone volume was calculated following measurements performed by an experienced radiologist according to an appropriate formula. In addition, the sex, body weight (kg), skull morphology and size of dog were recorded. Skulls were classified as dolichocephalic, mesocephalic and brachycephalic and body size as small, medium or large.

Statistical analysis of the data was performed and multivariate linear regression was used to derive the best equation for calculating RB cone volume.

What did the study find?

213 skull CT scans were evaluated and 25 were excluded, with a final number of 188 CT scans and 376 eyes.

The cone volume was significantly larger in large dogs compared with medium dogs and small dogs. Entire male dogs had a larger RB cone volume than entire females. Mesocephalic dogs had a smaller cone volume than brachycephalic and dolichocephalic dogs and a positive correlation was found between body weight and RB cone volume.

Following the multivariate analysis the strongest predictor was shown to be body weight and skull morphology and these were used in the final model to develop the formula.

The formula developed to calculate RB cone volume:

·      0.69238 + (0.06177 x BW in kg) for dogs with a mesocephalic skull

·      0.69238 + (0.06177 x BW in kg) + 0.57034 for dogs with a brachycephalic skull

·      0.69238 + (0.06177 x BW in kg) + 0.37414 for dogs with a dolichocephalic skull

What were the limitations of the study?

The study was retrospective and had a small number of dolichocephalic dogs in the dataset. The authors suggest the study should be repeated and multicentre collaboration used to further evaluate their findings. This study did not evaluate the clinical effectiveness of the retrobulbar block using the volume calculated with the formula.

How can we apply this in clinical practice?

The formula developed allows the clinician to calculate a volume of local anaesthetic to use for retrobulbar injection to provide analgesia for enucleation. This volume can then be used alongside calculation of a safe volume/dose of local anaesthetic to guide the clinician to an appropriate volume to inject. Further data will be needed to assess clinical effectiveness, but the recognition of differences in RB cone volume dependent on body weight and skull morphology will allow the clinician to improve decision making when selecting a volume of local anaesthetic prior to RB injection.

Where can I find more information?

Take a look at the videos on the Zero Pain Philosophy website showing the technique to perform a RB injection of local anaesthetic using the inferior-temporal technique and start provided superior analgesia for your canine patients when enucleation is needed.

The peribulbar block is an alternative to the retrobulbar block and you can find a video here

Greco A, Costanza D, Senatore A, Bruzzese D, Micieli F, Chiavaccini L, Di Giancamillo M, Della Valle G, Vesce G, Brunetti A, Meomartino L. A computed tomography-based method for the assessment of canine retrobulbar cone volume for ophthalmic anaesthesia. Vet Anaesth Analg. 2021 Sep;48(5):759-766. doi: 10.1016/j.vaa.2021.03.015.

This post was written by Carl Bradbrook.

Matt & Carl established Zero Pain Philosophy to provide educational resources & telemedicine to veterinary professionals globally, enabling optimal management of pain.

Matt Gurney is an RCVS & European Specialist in Veterinary Anaesthesia & Analgesia and works at Eastcott Referrals. Matt is Past President of the European College of Veterinary Anaesthesia & Analgesia.

Carl Bradbrook is an RCVS & European Specialist in Veterinary Anaesthesia & Analgesia and is Past President of the Association of Veterinary Anaesthetists. Carl works at Anderson Moores Veterinary Specialists in the UK.

The intended audience for this pain update is veterinary professionals. This pain update is based on clinical experience and independent opinion.