Retrobulbar Block: Medication and Volume
The most commonly used medication for retrobulbar block is a mixture of lidocaine 2% and bupivacaine 0.5% (typically in a 1:1 ratio), with a total volume of 3-5 mL, supplemented with hyaluronidase.
Standard Medication Composition
The typical retrobulbar anesthetic solution consists of:
- Lidocaine 2% combined with bupivacaine 0.5% in equal proportions 1, 2
- Hyaluronidase (75 international units diluted in 5 mL of local anesthetic mixture) is routinely added to improve motor blockade and prevent vitreous bulging 1, 3
- The lidocaine-bupivacaine combination leverages the rapid onset of lidocaine with the prolonged duration of bupivacaine 1
Volume Recommendations
The standard volume for retrobulbar injection is 3-5 mL:
- 3 mL is the most frequently cited volume for standard retrobulbar blocks using the Unsöld or Atkinson technique 2
- A study using 3.5 mL of the lidocaine-bupivacaine mixture (3 mL of 0.5% bupivacaine mixed with 2 mL of 2% lidocaine) demonstrated effective anesthesia 3
- 5 mL was used in comparative studies of retrobulbar versus peribulbar techniques 1
- Smaller volumes (1.5 mL) have been reported for specific procedures like trabeculectomy 4
Rationale for Hyaluronidase Addition
Hyaluronidase significantly improves motor blockade:
- Produces consistently better motor blockade at 10,15, and 20 minutes post-injection (P < 0.001) 3
- Highly effective for preventing intraoperative vitreous bulging 1
- The absence of hyaluronidase in the block is a recognized risk factor for secondary strabismus and persistent diplopia 5, 6
Alternative Formulations
While less common, other local anesthetic options include:
- Ropivacaine 0.2% has been studied but showed inadequate analgesia frequencies (24% failure rate) for certain procedures 7
- Lidocaine 1% alone also demonstrated suboptimal analgesia (32% inadequacy) in some applications 7
- Mepivacaine and etidocaine are alternatives with intermediate and long duration respectively, though less commonly used than the lidocaine-bupivacaine combination 1
Clinical Pearls
- Epinephrine addition prolongs action of short and intermediate-duration local anesthetics and reduces hemorrhage incidence 1
- The 50:50 mixture of 0.75% bupivacaine and 2% lidocaine with epinephrine and hyaluronidase achieved 88% success rate for adequate akinesia 2
- Retrobulbar blocks require smaller volumes than peribulbar techniques (which typically use 6-10 mL) to achieve comparable anesthesia 1, 4