Introduction
The common peroneal nerve (CPN) is the most commonly damaged nerve in the lower extremity.
1 The nerve damage mostly occurs around the fibular head level, where the nerve is the most vulnerable as it passes through fibular tunnel and due to its superficial location. Common peroneal neuropathy at the fibular head can result from various causes such as trauma (e.g. fracture, forcible stretch injury), prolonged compression (e.g. casting, surgical procedure under heavy sedation) and structural mass lesions, either intrinsic or extrinsic.
1 Although it is a rare entity, ganglion cyst can act as a mass lesion that can lead to peripheral neuropathies. Hereby, we report two cases of common peroneal neuropathy caused by intraneural ganglion cyst (IG) and their detailed neurophysiological findings.
Discussion
Ganglion cysts are cystic lesions that originate from tendon sheath or joint capsule. IGs are fluid-filled formations within the epineural sheath of peripheral nerves and can cause nerve compression as in our cases.
3,4 Although the mechanism is not fully understood, the most accepted hypothesis on formation of the peroneal IG is the ‘articular theory’ by Spinner et al.
2 CPN has been reported to branch out at least three articular branches during its course and one of which provides sensory information from the proximal tibiofibular joint.
5 According to the ‘articular theory’, the IG is formed by one-way communication between the proximal tibiofibular joint and the articular branch of CPN. Due to predisposing conditions such as a traumatic event or abnormal joint pathology, the capsular defect at the proximal tibiofibular joint might have been formed and the articular branch of CPN becomes a conduit. Consequently, the cystic fluid enters the epineurium of the articular branch of CPN and extends towards the less resistant CPN and its branches as pressure is applied. And it has been reported that IG can even extend proximally to the sciatic nerve.
2 In both of our cases, the articular branch of CPN was observed in the radiographic imaging studies and confirmed intraoperatively. Resection of the articular branch stalk to the proximal tibiofibular joint was performed and no sign of recurrence was observed during the follow up period.
In both cases, patients experienced pain in the upper lateral calf region before the foot drop symptom started. Young et al. reported that the patients with common peroneal neuropathy due to the IG were significantly associated with pain at the lateral knee in comparison with the patients with no evidence of IG on MRI.
6 Spinner et al. described that this poorly localized lateral knee pain can be caused by pathologic conditions of the proximal tibiofibular joint.
2 Although it is a rare disease entity, the history of upper lateral calf pain may be the clinical feature that requires further evaluation such as sonography or MRI, if there is no clear proximate cause for common peroneal neuropathy.
The CPN derives from the dorsal branches of L4, L5, S1 and S2 and it descends as a main division of the sciatic nerve. It courses obliquely along the lateral side of the popliteal fossa and winds around the fibular head where it is vulnerable.
1 When exiting the fibular tunnel, the CPN is generally known to bifurcate into the DPN and SPN. However, some studies report that the CPN bifurcation be trifurcation, by including the articular branch. The idea of trifurcation is based on the cadaveric study on branching patterns of CPN. In the study, the articular branch branched out from DPN just within 3mm distal to the bifurcation point in all the limbs examined.
2
At the fibular head level, the intraneural topography of CPN is that the fascicles destined for DPN has tendency to lie more medial, whereas the fascicles destined for the SPN are more lateral. And the fascicles to the articular branch is situated medially, close to the DPN and it sometimes merges with the fascicles to the tibialis anterior and even innervates the tibialis anterior alone in some cases .
2,7 Due to this proximity, predominant involvement of DPN innervated muscles such as tibialis anterior muscle has been reported on previous cases of peroneal IG.
3,8 And this also applies to our cases and supports the previous observations that fascicles of the CPN comprising the DPN may be more vulnerable than those of the SPN. In case 1, IG extended proximally up to 5 centimeters of the CPN portion (Stage III), but only the DPN innervated muscles showed denervation potentials in needle EMG and the mixed and superficial peroneal sensory responses were within the normal range in NCS. Predominant involvement of the fascicles to DPN is also shown in the case 2. However, the ankle eversion was weak and involvement of the SPN was also observed in the NCS and needle EMG. The surgical findings with diffuse adhesion of the CPN and SPN due to the ruptured IG and the mixed common peroneal SNAP abnormality which indicate the lesion extending proximally to the fibular head might explain the involvement of the laterally located fascicles of SPN in the CPN.
As mentioned ahead, CPN originates from the sciatic nerve with the root values of L4, L5, S1 and S2.
1 As a result, foot drop may result from the proximal lesions such as sciatic neuropathy, lumbosacral plexopathy and radiculopathy. In a retrospective series of 217 patients presented with foot drop, common peroneal nerve lesions (30.6%), L5-radiculopathies (19.7%) were the two most common subgroups among the peripheral neurogenic origin group.
9 Therefore, to accurately distinguish the causes of foot drop is often difficult. Both of our patients had no history of low back pain nor the radiating pain and SLR was negative in the initial physical examination. NCS of the tibial motor and sural sensory responses were within the normal range and the peroneal division of the sciatic nerve innervated muscle (short head of biceps femoris muscle) and non-peroneal L5-innervated muscles (tensor fascia lata and flexor digitorum longus) showed normal EMG potentials.
A very few cases of CPN due to IG have been reported with detailed neurophysiology. This case report emphasizes correct diagnosis based on electrophysiologic, radiographic and operative findings with the follow up studies. Although the radiographic and surgical findings were compatible with the lesion extending into the CPN, the precise analysis of the electrophysiologic findings could differentiate the fascicular involvement within the CPN in two cases, deep peroneal portions only versus predominant involvement of DPN of CPN.
Intraneural ganglion cyst (IG) is not an entity that we encounter often. However, an early diagnosis of the IG and to rule out the other general causes of common peroneal neuropathy is important, because early surgical treatment of the IG is crucial to the prevention of the neurological deterioration with improved outcomes. In our cases, no recurrence was observed and improvement in the clinical symptom and the electrophysiologic findings were obtained in the follow-up studies after the surgery.