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Ultrasound Gel-Nerve Contact

An Experimental Animal Histologic Study

El-Dawlatly, Abdelazeem, MD*; Kathiry, Khalid, MBBS*; Al Rikabi, Ammar, MD, FRCPath; Hajjar, Waseem, FRCS; Al Obaid, Omar, FRCSC§; Alzahrani, Tariq, MD*

doi: 10.1213/ANE.0b013e3182240191
Analgesia: Brief Report
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BACKGROUND: Ultrasound (US) regional nerve block requires the use of gel applied over the skin. With subsequent needle insertion, some of the gel may adhere either on the shaft or within the needle lumen and may be carried to the perineural structures or intraneurally. We performed this experimental animal study to investigate the effects of US gel contact on the nerve histologic structure.

METHODS: Nine male beagle dogs were studied. Dogs 1 to 3 were the control group and dogs 4 to 9 were the study group. Bilateral posterior tibial nerves were dissected and exposed for the control group. Nerve specimens were obtained for histologic examination immediately for the first dog, at 24 hours for the second dog, and at 48 hours for the third dog followed by wound closure. For the study group, bilateral posterior tibial nerves were exposed, and 2 mL US gel was applied locally directly on the nerve, followed by wound closure. Nerve specimens were excised at 24 hours from one side and at 48 hours from the other side. Nerve specimens were examined by a neuropathologist for evidence of nerve inflammation.

RESULTS: The control nerve specimens showed no significant pathology. Nerve specimens of the study group at the end of 24 hours of gel-nerve contact showed mild focal perineural inflammatory changes with clusters of polymorph leukocytes. At 48 hours, perineural moderate inflammatory changes with clusters of lymphocytes and macrophages were demonstrated in 2 animals. Long-term neurologic deficit in the form of limping was observed for all dogs.

CONCLUSION: Histologic features after perineural exposure to US gel are rather nonspecific and likely of no clinical significance. However, further studies are needed to determine the effect of US gel injection on intraneural tissues.

Published ahead of print June 16, 2011

From the Departments of *Anesthesia, Pathology, Thoracic Surgery, and §Surgery, College of Medicine, King Saud University, Riyadh, Saudi Arabia.

The authors declare no conflicts of interest.

Reprints will not be available from the authors.

Address correspondence to Abdelazeem El-Dawlatly, MD, Department of Anesthesia, College of Medicine, King Saud University, PO Box 2925, Riyadh 11461, Saudi Arabia. Address e-mail to dawlatly@ksu.edu.sa.

Accepted May 4, 2011

Published ahead of print June 16, 2011

Ultrasound (US) gel is used during regional block techniques. With needle insertion, the gel may adhere to the shaft or within the needle lumen and may be carried to either the surrounding nerve structures or intraneurally. Belavy1 reported that US gel may be injected around and perhaps in the nerves during regional anesthesia procedures. We performed this experimental animal study to investigate the effects of US gel contact on the nerve histologic structures and the adjacent perineural tissues.

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METHODS

After receiving approval from the institutional Animal Laboratory and Experimental Surgery Board, 9 male beagle dogs (weight, 11–16 kg body weight) were included in this double-blind and prospective randomized study. Dogs 1 to 3 were considered the control group and dogs 4 to 9 were the study group. Preoperative 1 g IM cefuroxime was given to all dogs. Anesthesia was induced with IM ketamine and xylazine (2 mg/kg body weight). Bilateral posterior tibial nerves were dissected and exposed for the control group. Nerve specimens were excised for histologic examination for the first dog, at 24 hours for the second dog, and at 48 hours for the third dog followed by wound closure. For the dogs of the study group, bilateral posterior tibial nerves were exposed and 2 mL US gel (Pharmaceutical Innovations, Newark, NJ) was applied directly on the nerve followed by wound closure. This gel amount was chosen because it fills the posterior tibial fossa of the dog to ensure gel-nerve contact. Nerve specimens (5-cm longitudinal segments) were excised 24 hours later from one side and 48 hours later from the other side under general anesthesia. The specimens were stored in bottles containing 10% neutral buffered formalin, the bottles were numbered, and the key numbers were kept in sealed envelopes to ensure blinding. The nerve specimens were sectioned longitudinally and embedded in paraffin using routine protocols for surgical pathology. Six-micrometer-thick sections were stained with hematoxylin and eosin. For each nerve specimen, at least 3 sections separated by 100 μm were examined for evidence of nerve inflammation by a neuropathologist who was blinded to the nerve location. Nerve inflammation was defined by the presence of perineural or intraneural macrophages, lymphocytes, neutrophils, granulation tissue, or reactive fibroblasts. The presence of each inflammatory marker was graded as “none” (if no inflammatory reaction was seen), “mild” (if the inflammatory infiltrate was seen in 1 high-power microscopic field), or “moderate” (if the inflammatory infiltrate was seen in 2 high-power microscopic field).

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RESULTS

Six posterior tibial nerve specimens were taken from 3 control dogs, and a total of 12 posterior nerve specimens were taken from 6 dogs of the study group. The control nerve specimens (dogs 1–3) and the nerve specimen at the end of 24 hours of gel-nerve contact that belonged to dog number 5 of the study group showed no significant pathology (Fig. 1). However, at the end of 24 hours of gel-nerve contact, the remaining study dogs showed mild focal perineural inflammatory changes with clusters of polymorph leukocytes (Fig. 2). Varying degrees of moderate inflammatory changes were shown in all nerve specimens of the study group (Table 1). At 48 hours of gel-nerve contact, moderate inflammatory changes with clusters of lymphocytes and macrophages around perineural vessels and fat were shown in the nerve specimens of all study group dogs except number 5 (Fig. 3). Long-term neurologic deficit in the form of limping was observed for all dogs.

Figure 1

Figure 1

Figure 2

Figure 2

Figure 3

Figure 3

Table 1

Table 1

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DISCUSSION

This study demonstrated mild inflammatory changes in the first 24 hours of US gel-nerve contact, which became more evident at 48 hours with presentation of perineural clusters of polymorphs and macrophages. It is worth mentioning that all histologic changes happened in the perineural tissue with no evidence of intraneural tissue spread. The use of US gel is a prerequisite for US-guided techniques. The main ingredient of US gel is propylene glycol, which has been reported to be responsible for many cases of contact dermatitis.2,3 During US-guided regional nerve blocks, the US transducer is inserted into a sterile sheath containing US gel. However, many operators prefer to place an additional thin layer of US gel between the draped US transducer and the skin. Therefore, there is a possibility that some US gel may be introduced through the lumen of the needle and along its shaft toward the nerve surroundings or intraneurally.

In conclusion, this is the first animal study to evaluate the pattern of histologic changes secondary to US gel-nerve contact. Histologic features after perineural exposure to US gel are rather nonspecific and likely of no clinical significance. Further studies are needed to determine the effects of US gel injection on intraneural tissues.

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DISCLOSURES

Name: Abdelazeem El-Dawlatly, MD.

Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.

Attestation: Abdelazeem El-Dawlatly has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.

Name: Khalid Kathiry, MBBS.

Contribution: This author helped conduct the study.

Attestation: Khalid Kathiry has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Ammar Al Rikabi, MD, FRCPath.

Contribution: This author helped analyze the data.

Attestation: Ammar Al Rikabi has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Waseem Hajjar, FRCS.

Contribution: This author helped design the study.

Attestation: Wasim Hajjar has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Omar Al Obaid, FRCSC.

Contribution: This author helped conduct the study.

Attestation: Omar Al Obaid has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Tariq Alzahrani, MD.

Contribution: This author helped conduct the study.

Attestation: Tariq Alzahrani has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

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ACKNOWLEDGMENTS

The authors thank Professor Abdulmajeed Fatouh El Mezyen, Centre of Experimental Surgery, College of Medicine, King Saud University, for his kind support.

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REFERENCES

1. Belavy D. Regional anesthesia needles can introduce ultrasound gel into tissues. Anesth Analg 2010;111:811–2
2. Kessler J, Schafhalter-Zoppoth I, Gray AT. Allergic contact dermatitis caused by ultrasonic gel. Reg Anesth Pain Med 2006;31:480–1
3. Khan IJ, Azam NA, Goyal R, Nabi NU. Contact urticaria to ultrasonic gel. Eye (Lond) 2007;21:1016
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