Sub-Tenon Atracurium Injection in Rabbit Eyes: A Histopathologic Study

Hashemi N, Shomali S, Heyrani-Moghaddam H, Amini S, Izadi S

Journal of Ophthalmic and Vision Research, 2009

An experimental study in 39 rabbits evaluating the histopathologic safety of sub-Tenon atracurium injection, finding that while congestion is a transient complication, liquefaction necrosis is a significant late complication that raises concerns about the safety of atracurium as an adjuvant to sub-Tenon anesthesia.

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This experimental study evaluates the early and late histopathologic changes following posterior sub-Tenon injection of atracurium in rabbit eyes. The study was published in the Journal of Ophthalmic and Vision Research in 2009. It provides important safety data on the use of non-depolarizing neuromuscular blocking agents as adjuvants to local anesthesia in ophthalmic surgery.

Key Findings

Congestion was the most common early finding (at 1 hour), seen more frequently in the control group, and was transient -- disappearing by one week
Liquefaction necrosis was significantly increased at one week in the atracurium group (40% vs. 10.5% at 1 hour), suggesting it is a delayed complication of atracurium
No statistically significant difference was found between atracurium and normal saline groups when time of biopsy was not considered (11 vs. 10 cases of necrosis, P = 0.5)
Fibrosis rates were comparable between atracurium (10 cases) and normal saline (4 cases) groups (P = 0.14)
All necrosis was of the liquefaction type and was found near scleral tissue, likely originating from Tenon's or conjunctival tissue
Safety of atracurium as a sub-Tenon adjuvant is questionable based on the trend toward late necrosis in this animal model

Background

Local anesthesia offers significant advantages over general anesthesia for ocular surgery, including the potential for ambulatory surgery, rapid recovery, and lower incidence of serious complications. Sub-Tenon anesthesia is an alternative to retrobulbar and peribulbar methods that avoids many of their complications. By using blunt dissection into the posterior sub-Tenon space, local anesthetics can be injected directly and spread along the extraocular muscles, diffusing into the retrobulbar space.

Non-depolarizing neuromuscular blocking agents (ND-NMBAs) such as atracurium and vecuronium have been used as adjuvants to local anesthesia to hasten the onset of akinesia and optimize surgical conditions. Atracurium has good neuromuscular blocking potential at physiologic pH and temperature and is mostly degraded in the plasma, with a half-life of approximately 20 minutes. While prior studies had shown that adding low-dose atracurium to peribulbar injections hastened block onset and provided excellent akinesia, the safety of sub-Tenon atracurium injection had not been thoroughly investigated at the histopathologic level.

Methods

This experimental study was performed in accordance with the ARVO (Association for Research in Vision and Ophthalmology) statement for use of animals in ophthalmic and vision research.

Study Design

39 healthy white New Zealand rabbits of either sex, weighing 2-3 kg, aged 2-3 months
Left eyes (study group): Sub-Tenon injection of 0.05-0.08 mg/kg atracurium diluted in 0.5 mL normal saline
Right eyes (control group): Sub-Tenon injection of 0.5 mL normal saline alone
Group 1 (19 rabbits): Bilateral enucleation 1 hour after injection (early changes)
Group 2 (20 rabbits): Bilateral enucleation 1 week after injection (late changes)

Surgical Technique

All injections performed under general anesthesia (intramuscular ketamine hydrochloride 50 mg/kg)
A 2 mm conjunctival incision made 4 mm superotemporal to the limbus using Westcott scissors
Tenon's fascia dissected and a 22-gauge IV catheter inserted into the posterior sub-Tenon space
Injections performed followed by gentle digital massage for 2 minutes
All procedures performed by a single surgeon

Histopathologic Analysis

Enucleated eyes fixed in 10% formaldehyde in phosphate buffer saline (pH 7.4) for 24 hours
Embedded in paraffin and cut into 5 micrometer sections
Stained with hematoxylin and eosin (H&E) and trichrome
Evaluated by a single masked pathologist using light microscopy
Statistical analysis: Chi-square test, Fisher's exact test, and adjusted residuals (significance level P < 0.05)

Results

Early Changes (1 Hour Post-Injection)

Finding	Normal Saline	Atracurium
Congestion	42.1% (8/19)	31.6% (6/19)
Liquefaction necrosis	21.1% (4/19)	10.5% (2/19)
Hemorrhage	10.5% (2/19)	5.3% (1/19)
Fibrosis	5.3% (1/19)	26.3% (5/19)
Normal	15.8% (3/19)	15.8% (3/19)

Late Changes (1 Week Post-Injection)

Finding	Normal Saline	Atracurium
Congestion	0% (0/20)	0% (0/20)
Liquefaction necrosis	35.0% (7/20)	40.0% (8/20)
Hemorrhage	10.0% (2/20)	0% (0/20)
Fibrosis	15.0% (3/20)	25.0% (5/20)
Normal	40.0% (8/20)	35.0% (7/20)

Key Statistical Findings

Differences between early and late complications were statistically significant in both groups (P = 0.023 for N/S group; P = 0.029 for atracurium group, Fisher's exact test)
Tissue congestion was the most significant time-dependent complication (adjusted residual = 3.3 for N/S, 2.7 for atracurium)
Without considering biopsy time, no statistically significant difference between N/S and atracurium for necrosis (P = 0.5) or fibrosis (P = 0.14)
Liquefaction necrosis in the atracurium group showed a significant time-dependent increase (adjusted residual = 2.1 for late findings)

Clinical Significance

This study provides several important findings for ophthalmic surgeons considering the use of atracurium as an adjuvant to sub-Tenon anesthesia:

Congestion is transient: The injection-related congestion seen at 1 hour resolves completely by one week in both groups, regardless of atracurium use
Necrosis is a concerning late effect: The significant increase in liquefaction necrosis at one week in the atracurium group (from 10.5% to 40.0%) suggests that atracurium may induce tissue necrosis as a delayed complication
Mechanism of necrosis: Atracurium may affect endothelial cells and progressively decrease neovascularization at the injection site, leading to poor healing and necrosis. Prior studies have shown that atracurium and cisatracurium inhibit human cell proliferation in vitro, possibly via reactive acrylate metabolites
Questionable safety: The use of atracurium as an adjuvant to sub-Tenon anesthesia appears to be of questionable safety based on this animal model

Safety Concern: Although atracurium has been shown to hasten the onset of akinesia in peribulbar blocks, this histopathologic study raises concerns about its safety when injected into the sub-Tenon space. The trend toward increased liquefaction necrosis at one week suggests tissue toxicity. Clinicians should weigh the benefit of faster akinesia onset against the potential risk of tissue necrosis, particularly for sub-Tenon delivery routes.

Pharmacologic Context: Atracurium does not affect analgesia -- its value as an adjuvant lies in its effect on motor nerves, inducing akinesia in extraocular muscles and orbicularis oculi to optimize the surgical field. Importantly, atracurium and pancuronium are known not to increase intraocular pressure during general anesthesia. The dose used in this study (0.05-0.08 mg/kg) was deliberately kept low, as higher doses carry the risk of apnea and death.

Citation

Hashemi N, Shomali S, Heyrani-Moghaddam H, Amini S, Izadi S. Sub-Tenon atracurium injection in rabbit eyes; a histopathologic study. J Ophthalmic Vis Res. 2009;4(1):35-39.

References

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