Ultrasound Guided Posterior Tibial Nerve Block to Facilitate Foreign Body Removal in a School-Aged Child

Highlights

What do We Already Know About This Topic?

Ultrasound is increasingly becoming the standard of care for the detection and localization of radiolucent foreign bodies in the Emergency Department.

How Does Your Research Contribute to the Field?

We describe the technique for using point-of-care ultrasound to detect retained foreign bodies in the sole of the foot, and demonstrate that (posterior tibial) nerve blocks can be successfully performed in relatively young children.

What are Your Research’s Implications Toward Theory, Practice, or Policy?

We believe that many ultrasound applications, including assessment of soft tissue injury, will soon begin to permeate primary pediatrics as well as many other pediatric sub-specialties.

Introduction

Soft tissue foreign bodies are common chief complaints in the emergency department (ED). Ultrasound has been utilized for detecting foreign bodies, both radiopaque and non-radiopaque, and facilitating their removal from soft tissue. ¹ Point-of-care ultrasound (POCUS) offers ED providers a safe and efficient way to determine if a foreign body is present, perform precise 3-dimensional localization, and maintain visualization during removal. ¹ POCUS further enables the practitioner to determine if a foreign body has been removed in its entirety, as wood, thorns, and other objects often fragment with initial puncture and removal. ² Additionally, POCUS can be quickly performed, potentially resulting in a shortened ED length of stay. We report the case of a 9-year-old male who presented to the ED with recurrent foot infections.

Case Report

A 9-year-old-male presented to the emergency department with pain to the sole of his left foot. The patient stated that he was at karate the day prior to presentation when he began to have pain. He had initially injured his foot while playing at his home 3 months prior. The family went to their pediatrician at that time and a small wooden splinter was removed. The patient was discharged with instructions for warm soaks and supportive care. Despite having had the splinter removed, the patient’s pain persisted, and he started to develop erythema and edema to the plantar aspect of his left foot. One week after the initial visit to his pediatrician, he presented to an urgent care where an x-ray of his foot was performed and did not reveal any radiopaque foreign body. The patient was diagnosed with cellulitis and was discharged with a prescription for amoxicillin/clavulanate. The patient’s symptoms gradually improved and resolved by the completion of his antibiotic course.

The patient continued to have intermittent mild foot irritation for approximately 3 months when he developed a recurrence of erythema, edema, and tenderness to the sole of his foot. His mother noted that the area was intermittently draining pus. The family re-presented to the primary care physician who diagnosed the patient clinically with an abscess of his foot. A referral was made to a podiatrist whose next available appointment was over a month away. As a result, the patient’s mother brought him to the ED for further care. At that time, the patient denied fever, chills, nausea, or vomiting. The patient was fully immunized and generally in good health with the exception of mild asthma.

Physical exam revealed a well-nourished, alert school-aged male in no distress. His vital signs were within normal limits. The plantar aspect of the patient’s left foot was significant for a patch of erythema and edema, with 2 punctate lesions within the affected area. The area was indurated, tender to palpation and without purulence. The patient had full range of motion of his left leg, ankle, and foot and his extremity was neurovascularly intact. The remainder of his physical exam was unremarkable.

The treating ED provider performed a POCUS of the plantar aspect of the patient’s left foot using a high frequency linear transducer in a musculoskeletal preset. The POCUS revealed a 3 cm retained soft-tissue foreign body (Image 1A).

Image 1.

Retained foreign body: (A) still frame ultrasound image. Arrows indicate location of foreign body and (B) foreign body status post extraction.

Given the patient’s significant pain and tenderness, anesthesia was provided using an ultrasound guided posterior tibial (PT) nerve block. ³ For the administration of the block, the patient was positioned supine with his left lower extremity externally rotated. An in-plane technique with the same linear transducer in a transverse orientation at the level of distal tibia, just proximal and posterior to the medial malleolus, was used (Image 2A). The medial malleolus and posterior tibial artery were localizing landmarks. Five ml of 0.5% bupivacaine without epinephrine was injected via a 25-gage needle using standard hydro-dissection technique (Image 3). Bupivacaine was used both to facilitate the procedure and aid in post-procedural pain control. After 10 minutes, the posterior tibial nerve distribution had near complete anesthesia.

Image 2.

Posterior tibial nerve: (A) in-plane nerve block technique and (B) sensory nerve distribution.

Image 3.

Posterior tibial nerve block.

The linear probe was then used to guide a horizontal incision over the foreign body followed by the use of forceps to grasp and remove a piece of wood (Image 1B). The technique of using POCUS to identify the longitudinal axis of the foreign body so that traction can be applied in parallel with this axis, thus limiting adjacent soft tissue destruction, has been previously described. ⁴ The foreign body’s length of 3 cm corresponded to our pre-procedure POCUS estimate. POCUS was repeated post-procedure to ensure that the object had been removed in its entirety. The patient’s wound was dressed with bacitracin ointment and he was discharged with a prescription for cephalexin.

Discussion

We describe a case of a retained wooden foreign body in which POCUS was instrumental in all aspects of the patient’s care. Within an hour of being roomed in the ED, our 9-year-old patient who had been intermittently symptomatic over a 3-month period without clear explanation, was diagnosed with a retained foreign body and successfully treated after regional anesthesia. POCUS is increasingly being used for the purposes of foreign body identification and removal as well as to guide nerve blocks. ⁵ POCUS has emerged as a first line imaging modality when attempting to identify and locate radiolucent objects such as wood that cannot be visualized by traditional x-ray. ⁶ In addition, it allows providers to perform precise 3-dimensional localization, and maintain visualization during removal. ¹

Suspected soft tissue foreign body is not only a common concern in emergency medicine, but also in primary care pediatrics. We believe that many POCUS applications, including assessment of soft tissue injury, will soon begin to permeate the primary care setting, and awareness of this emerging technology is important among primary care providers.

Two techniques for performing POCUS on the sole of the foot have been described. ⁷ For the water bath technique, the patient should be positioned prone, with the knee on the affected side elevated. This positioning will allow the patient to comfortably rest their foot in a water basin with their knee in full extension. The probe is placed centimeters from the patient’s skin which allows for the transmission of a precise signal through the water medium. (Image 4A). A second technique, gel submersion, also requires laying the patient prone. The knee on the affected side is bent 90° so that the sole of the effected foot faces the ceiling. A large quantity of ultrasound gel is placed over the affected area. The layer of gel allows for submersion of the ultrasound probe, producing a high-resolution image while not contacting the patient’s tender skin (Image 4B).

Image 4.

POCUS to sole of foot: (A) water bath technique and (B) gel submersion technique.

Although the posterior tibial nerve block has been described using anatomic landmarks, ⁸ an ultrasound-guided approach allows for improved precision in localizing the nerve and intentional avoidance of the posterior tibial vasculature which lies in close proximity. ⁹ The posterior tibial nerve block, when successful, anesthetizes the vast majority of the plantar aspect of the foot (Image 2B). The block is particularly useful for plantar wound management and for calcaneal injuries.^5,9

There are special considerations when performing regional nerve blocks in children. Pediatric patients may benefit from systemic anxiolysis (ie, intranasal versed), or pretreatment of the area to be injected with topical pain control (ie, topical lidocaine cream). Practitioners must be mindful of weight-based dosing recommendations for children as over dosing of anesthetics can lead to significant toxicity. ¹⁰ Additionally, clear discharge instructions should be provided to the patient and caregivers as prolonged lack of sensation due to regional nerve blocks can lead to further injury. Our case demonstrates that (posterior tibial) nerve blocks can be successfully performed in relatively young children. To our knowledge, this technique has not been previously published for this age group.

Conclusion

Retained foreign body in the soft tissues is a common ED complaint. Ultrasound should be the first line imaging modality for the detection and localization of radiolucent foreign bodies. For those foreign bodies identified on the sole of the foot, a posterior tibial nerve block can be utilized to facilitate adequate analgesia and successful foreign body removal.