Continuing Education Activity

Tarsal tunnel syndrome is a relatively rare mononeuropathy of the lower extremities that can be difficult to diagnose. Clinical evaluation, electroneurography (ENG), and electromyography (EMG) are used to diagnose this posterior tibial nerve neuropathy, allowing for more accurate treatment and management plans to help alleviate the patient’s symptoms. This activity will briefly discuss the etiology and pathophysiology of tarsal tunnel syndrome and describe how ENG and EMG can be used to identify this condition.

Objectives:

• Assess the diagnostic findings of tarsal tunnel syndrome found on ENG and EMG.

• Identify common pitfalls when conducting ENG of the tibial nerve and its branches, and EMG of the intrinsic foot muscles.

• Determine the appropriate recording and stimulation sites of the medial and lateral plantar nerves.

• Communicate the importance of communicating the findings of the electrophysiological study with the interprofessional team to better enhance patient outcomes.

Introduction

Often, it is difficult to fully elucidate a neuromuscular diagnosis with history and physical examination alone. Furthermore, many neuromuscular diagnoses cannot be identified on an MRI or CT scan and thus require an alternate modality to identify—this is the role of the electrodiagnosis techniques: Electroneurography (ENG) and electromyography (EMG). Specifically, ENG and EMG are useful in identifying peripheral neuropathies caused by tibial nerve compression. Neuropathy of the posterior tibial nerve is a relatively rare condition that causes foot and ankle pain, paresthesia, and even weakness of the intrinsic foot muscles.

When this neuropathy occurs in the tarsal tunnel, it is known as tarsal tunnel syndrome. External compression by ill-fitting footwear or tight plaster casts most frequently causes this condition. Other causes are posttraumatic fibrosis and acute trauma (such as those from sprains, strains, and fractures). Still, they can also result from lipomas, cysts, tumors, soft tissue infections, and inflammatory arthropathies. A true entrapment from a thickened flexor retinaculum is rare.[1][2][3] Electromyography can identify the specific muscles affected by a nerve lesion. If a nerve lesion localizes to the tarsal tunnel, the practitioner must determine the cause of the compression.[4]

Anatomy and Physiology

The tibial nerve is the continuation of the medial trunk of the sciatic nerve. At the ankle, it runs posterior and inferior to the medial malleolus, passing through a tunnel created by the flexor retinaculum. Distal to the tunnel, the tibial nerve divides into the calcaneal sensory branches and the 2 plantar nerves, which are mixed nerves (containing both motor and sensory fibers). The calcaneal nerves are 2 sensory branches (medial and lateral) that provide sensation to the sole's heel. The medial and lateral plantar nerves are the 2 mixed motor and sensory branches.

These nerves provide sensory innervation to the two-thirds of the sole and motor innervation to the intrinsic foot muscles: the lateral plantar nerve supplies motor innervation to the abductor digiti quinti pedis and sensation to the lateral plantar aspect of the foot, including the lateral 1.5 toes. The medial plantar nerve supplies motor function to the abductor halluces brevis, flexor halluces brevis, and flexor digitorum brevis. It provides sensation to the medial plantar aspect of the foot sole and the first 3 toes. Tibial nerve compression at the level of the tarsal tunnel leads to sensory and motor disturbances according to its branches' distribution.[5][6]

Indications

The patient complains of unilateral pain in the foot and ankle and numbness in the sole. Besides, calcaneal sensory brunches are affected in lesions within or proximal to the tarsal tunnel, presenting sensory loss in the heel. A physical examination may reveal a positive Tinel’s sign over the tibial nerve at the medial malleolus and sensory changes along the plantar surface of the foot. Gastrocnemius normal strength and normal ankle reflex with weakness of the intrinsic foot muscles help localize the lesion.[7]

Contraindications

EMG-related complications include skin or soft tissue infections at the anatomic study site, cardiac pacemakers, coagulopathies, lymphedema, and low patient tolerance.

Equipment

Equipment used in the electrodiagnostic evaluation of tarsal tunnel neuropathy includes:

• ENG/EMG machine
• Surface electrodes for ENG
• Disposable concentric EMG needle.
• Needle cable
• Ground electrode

Personnel

Certified and specialized clinical neurophysiologist physicians must perform electrophysiological studies. 

Preparation

Before scheduling the electrophysiological test, the procedure, including anticipated sensations and duration, should be discussed. The patient should also be informed of the rare risk of infection and bleeding, as a needle breaks the skin, and the need not to use skin cream before the test.

Further preparation includes the location of anatomic landmarks and reviewing with the patient how to contract the target muscles. It is equally crucial to ensure the ambient room temperature of the examination room. Just before beginning the exam, it is essential to clean the skin with an alcohol swab with time to allow for air drying.[8]

Technique or Treatment

The patient should be supine, with the legs resting comfortably on the examination table. The EMG study of the intrinsic foot muscles completes the test. Nerve conductions show the reduced amplitude of the tibial nerve's compound muscle action potential (CMAP) stimulated at the ankle and recorded from the abductor hallucis and abductor digiti minimi. Sensory and mixed conduction studies are performed, usually with averaging techniques. Due to the wide range of normal values, comparing the results with the other foot (only if unaffected) is useful.

Following needle EMG analysis of muscles innervated by medial and lateral plantar nerves must be performed. These intrinsic foot muscles include abductor digiti minimi, abductor hallucis brevis, flexor hallucis brevis, and flexor digitorum brevis. Some neurogenic findings may appear in these muscles in normal subjects, so comparing the findings with the unaffected foot is best. Additionally, to complete the study, EMG examination of proximal muscles in the calf innervated by the tibial nerve (proximally to the tarsal tunnel) and the peroneal nerve, such as the gastrocnemius and the tibialis anterior muscles, respectively, must be normal.[9][10]

Complications

Risks associated with EMG include bleeding, infection, and nerve injury. Technical complications can include differentiating between normal and abnormal intrinsic foot muscle activity. 

Clinical Significance

If tarsal tunnel syndrome is identified early in the disease course, the patient can benefit from a conservative treatment plan, depending on the cause, if found. If the syndrome is due to tight footwear, the simple fact of changing it may be sufficient. Surgical exploration of the nerves in the tarsal tunnel and distally may be necessary for progressive tarsal tunnel syndrome. When done efficiently and effectively, ENG and EMG can lead to the early diagnosis of tarsal tunnel syndrome.[11]

Enhancing Healthcare Team Outcomes

In many instances, the disease progression of this mononeuropathy is reversible if addressed early on. Efficient interdisciplinary and interprofessional communication is crucial to treating patients with neuropathy. Appropriate referral by the first medical professional encountering a patient’s complaint can be the difference between an acute disease process and a chronic complication.

By identifying the location of the nerve lesion to the tibial nerve at the level of the medial malleolus, the EMG practitioner can address the causative agent. Initially, conservative measures require clinical attention, including footwear changes, physical therapy, and anti-inflammatory medications. Through effective interdisciplinary and interprofessional communication, the patient’s medical providers can monitor the patient’s progress.

If the patient does not demonstrate adequate progress, surgical intervention is needed. Collaboration and shared decision-making ultimately allow the patient to have the best possible outcome. The earlier the signs and symptoms of neuropathy are identified and the electrodiagnosis test performed, the better the prognosis and outcome.[12][4]

Nursing, Allied Health, and Interprofessional Team Interventions

Depending on the concrete circumstances, a collaboration between the clinicians to hand in technical material, ensure the correct humidity of electrodes and alcohol swabs, and prepare the patient is of great help.