High-Stepping Cross-Country Athlete: A Unique Case of Foot Drop and a Novel Treatment Approach

In Brief

Ganglion cyst-induced peroneal neuropathy successfully treated with ultrasound guided cyst aspiration and nerve hydrodissection.

Introduction

Ganglion cysts are benign masses that commonly arise adjacent to joint spaces and can interfere with the function of surrounding nerves and vessels. The most common clinical presentation is the presence of an asymptomatic, palpable mass, but some patients may experience paresthesias, pain, and/or motor weakness due to external compression forces on nearby nerves (^3,11). In rare circumstances, this external compression by the ganglion cyst can induce a peripheral neuropathy and lead to symptoms of nerve palsies (^3,5,6,8,9). The development of foot drop due to ganglion cyst induced common peroneal nerve palsy is relatively uncommon and exceedingly rare in the pediatric population (³).

Magnetic resonance imaging (MRI) is the criterion standard for diagnosis of peroneal nerve palsy and periarticular cystic lesions with excellent soft tissue contrast and visualization of regional anatomy (^3,5,6). Ultrasound further aids in defining these lesions, especially when used as image guidance for corticosteroid injection and/or cyst aspiration. Electromyographical (EMG) and nerve conduction velocity studies (NCVS) assist in further isolating the neuropathy as well as determining severity and chronicity of the lesion.

Standard of care for peroneal nerve palsy due to compression by a ganglion cyst involves surgical decompression and removal of the cyst (^2,5,6,9,11). The rate of cyst recurrence following surgery is as high as 30% and is associated with a substantial risk of permanent nerve injury (^6,11). To decrease the likelihood of cyst recurrence, the articular branch of the peroneal nerve is often ligated. However, evidence for recurrence remains despite this additional measure (^2,3,11).

Ultrasound guidance is used frequently for superficial ganglion cyst aspirations, but to our knowledge, only one group has reported successful management of a common peroneal nerve ganglion cyst with this procedure (^4,6,8,10). Additionally, nerve hydrodissection has proven useful for treating various entrapment neuropathies, but the application of hydrodissection for treating ganglion cyst-induced nerve palsies has not been described (^1,7). We present a case of a pediatric athlete who presented with pain and foot drop due to ganglion cyst-induced common peroneal nerve neuropathy that was managed with ultrasound-guided hydrodissection, cyst aspiration, and corticosteroid injection, allowing the athlete to return to sport.

Case Description

A 15-yr-old white male presented with an 11-month history of left foot drop and ankle pain. The patient first noticed weakness in his left foot after skiing, but both the patient and his mother denied trauma or inciting factors. He initially presented to a neurologist and was diagnosed with left common peroneal neuropathy at the knee via needle EMG and NCVS. The patient was prescribed physical therapy and an ankle-foot orthosis (AFO) for 5 months. Lower-extremity symptoms of weakness and decreased active range of motion improved at the completion of physical therapy, but returned when the patient resumed running cross-country. Subsequently, the patient consulted two orthopedic surgeons and completed two more months of physical therapy without benefit. MRI of the knee was performed revealing a 1.6-cm ganglion cyst anterior and superior to the proximal tibiofibular joint space. During this time, his foot weakness progressed to foot drop, and the patient was referred to our outpatient sports medicine clinic for consultation for conservative management.

Musculoskeletal ultrasound (Sonosite Edge system with a linear HFL 50, 15 MHz probe) in our office demonstrated the ganglion cyst in the left proximal anterior tibiofibular joint space with consequent flattening of the common peroneal nerve near the fibular head (Fig. 1 and Fig. 2). Upon physical examination, no gross deformity was appreciated, the cyst was not palpable, and a positive Tinel sign was found posteriorly over the fibular head. Range of motion testing was consistent with common peroneal nerve neuropathy with the inability to actively dorsiflex his left ankle past 5 degrees of plantarflexion and muscle strength in dorsiflexion rated 3/5. Sensation assessment revealed decreased perception to light touch in the left proximal lower leg posteriolaterally. Before pursuing surgery, the patient opted for a more conservative approach at our clinic with ultrasound-guided ganglion cyst aspiration and hydrodissection of the common peroneal nerve.

Figure 1: Peroneal nerve cyst visualized on short axis view before the procedure. Note the well-circumscribed, anechoic ganglion cyst in the left proximal anterior tibiofibular joint space (yellow arrow).

Figure 2: Compressed common peroneal nerve visualized on short axis view before the procedure (yellow arrow).

Common peroneal nerve hydrodissection was completed first. The patient was placed prone on the table, and the ultrasound probe was held on a short axis view over the posterior proximal fibular head. The nerve was hydrodissected with a mixture of 2 mL of 1% plain lidocaine and 2 mL of sterile saline. Subsequently, a 2-mL solution comprised of 1 mL of 1% plain lidocaine and 1 mL betamethasone sodium phosphate/betamethasone acetate (Celestone) was injected around the peroneal nerve (Fig. 3). The initial injection was used to separate the nerve from the fascia, whereas the injection of corticosteroid was implemented to discourage an inflammatory response.

Figure 3: Decompressed common peroneal nerve visualized on short axis view postprocedure (yellow circle).

Next, the patient was placed supine on the table for cyst aspiration. The ganglion cyst was identified on a short axis view at the proximal tibiofibular joint space. Using ultrasound guidance, aspiration of the cyst was performed, producing 1 mL of thick serosanguinous gelatinous substance. After this procedure, the anechoic area of the cyst appeared decompressed (Fig. 4). The patient was instructed not to participate in physical activity except walking over the next 24h to 48 h.

Figure 4: Decompressed ganglion cyst visualized on short axis view postprocedure (yellow arrow).

Approximately 24 h postprocedure, the patient reported full-resolution of symptoms, including active dorsiflexion past neutral, decreased pain, and the ability to ambulate normally. The patient returned to formal physical therapy 3 d postprocedure. Before the 2-wk follow-up appointment, he resumed running cross-country without return of dysfunction or pain.

Discussion

The unified articular theory attributes the formation of ganglion cysts to synovial fluid extravasation and accumulation from a nearby joint. A capsular defect allows fluid in a synovial joint to track along the articular branch of a nerve and accumulate, coalescing into a ganglion cyst (¹¹). Ganglion cysts formed adjacent to the common peroneal nerve most typically develop due to fluid accumulation from the tibiofibular joint (¹¹). These peroneal nerve cysts are generally asymptomatic, but may present with pain, swelling, and neuropathy, foot drop being a more severe, but rare manifestation (^3,11).

The criterion standard for diagnosis of peroneal neuropathy is MRI, and in our case, MRI without contrast confirmed the presence of a 1.6-cm ganglion cyst in the anterior proximal tibiofibular joint space. In addition to MRI, EMG and NCVS are used to confirm and isolate specific nerve involvement. Ultrasound is further beneficial for both diagnosis and treatment.

Previous case studies have described the use of aspiration for ganglion cysts adjacent to the tarsometatarsal joint, ulnar tunnel, and the tibial nerve (^4,8,10). Only Liang et al. (⁶) has highlighted the utility of this procedure to treat ganglion cyst-induced foot drop. The aforementioned procedure was successful; however, it was performed on a 64-yr-old nonathletic patient, and full recovery took 12 months. While this group revealed the potential for ultrasound-guided cyst aspiration to treat peroneal neuropathies, this approach had not previously been expanded to the pediatric population nor did it use hydrodissection to speed recovery.

Hydrodissection involves injecting a nonirritating solution of saline and lidocaine around the affected nerve to decrease the surrounding fascial constrictions and restore nerve function. In the lower extremity, Clendenen et al. (¹) illustrated the benefits of hydrodissection to treat an infrapatellar saphenous nerve entrapment neuropathy under ultrasound guidance, and Mulvaney (⁷) highlighted its use for meralgia paresthetica. Notably absent in our literature search were specific case studies involving hydrodissection of the common peroneal nerve.

Despite the availability of relatively noninvasive techniques to resolve entrapment neuropathies, surgical decompression remains the standard of care (^2,5,6,9,11). The latest operative technique for common peroneal nerve entrapment entails dissection over the posteriolateral aspect of the leg, removal of the proximal tibiofibular joint synovium (disarticulation), decompression of the affected nerve, and ligation of the articular branch to decrease the likelihood of cyst recurrence (¹¹). Despite efforts to minimize recurrence, rates have been described as high as 30%, especially with failure to ligate the articular branch (^6,11,12). Possible complications of this procedure include permanent nerve damage, injury to the lateral collateral ligament, and anterior tibial vessel damage.

In our case, the peroneal nerve was successfully decompressed, and the cyst aspirated without complication. While we recognize the potential for poor outcomes including infection, recurrence of the cyst, and temporary or permanent neurovascular injury, the decreased relative risk compared to surgery combined with the rapid resolution of symptoms and return to sport highlight the possibilities for using this procedure more regularly. Unfortunately, we were unable to follow up with the patient beyond his initial resolution of symptoms to determine if the cyst recurred.

We acknowledge that current practice standards will not change based on one case report, but the rarity of this pathology complicates clinically controlled trials. We assert that the benefits of this protocol far outweigh the risk of complications and make this treatment choice a viable option for clinicians.

Conclusions

The prompt resolution of symptoms and nerve integrity in this case supports the assertion that ganglion cyst-induced peripheral neuropathies can be managed more conservatively with ultrasound guided hydrodissection, cyst aspiration, and corticosteroid injection. The decreased complication risk, reduced cost, and quicker recovery time afforded by this minimally invasive treatment suggests it to be a viable alternative to surgical intervention, especially in the pediatric and athletic populations. Although additional evidence is required, this novel approach should be considered when dealing with this unique presentation in a young athletic population where a less aggressive procedure and quicker return to play is desired.