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 compression of the tibial nerve. 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. This condition is most frequently caused by external compression by ill-fitting footwear or tight plaster casts. Other causes are posttraumatic fibrosis and acute trauma (such as those from sprains, strains, fractures) but can also result from lipomas, cysts, tumors, soft tissue infection, and inflammatory arthropathies. A true entrapment from a thickened flexor retinaculum is rare.
Electromyography can be used to 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.
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 two plantar nerves, which are mixed nerves (contain both motor and sensory fibers). The calcaneal nerves are two sensory branches (medial and lateral) that provide sensation to the heel of the sole. The two mixed motor and sensory branches are the medial and lateral plantar nerves.
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 three toes. Tibial nerve compression at the level of the tarsal tunnel will lead to sensory and motor disturbances according to its branches' distribution.
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.
Contraindications for EMG include skin or soft tissue infections at the anatomic study site, cardiac pacemaker, coagulopathies, lymphedema, and low patient tolerance.
Electrophysiological studies must be performed by certified and specialized in clinical neurophysiologist physicians.
Discussion of the procedure, including anticipated sensations and duration, should be discussed before scheduling the electrophysiological test. It is also essential to discuss with the patient the rare risk of infection and bleeding as a needle will break the skin and the need for not using skin cream previous to 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.
The patient should be in a supine position with the legs resting comfortably on the examination table. Following, the EMG study of the intrinsic foot muscles will complete the test.
Nerve conductions show the reduced amplitude of the compound muscle action potential (CMAP) of the tibial nerve stimulated at the ankle and recorded from the abductor hallucis and abductor digiti minimi. Sensory conduction studies and mixed conduction techniques are performed, usually with averaging techniques. Due to the wide range of normal values, comparing the results with the other foot (only if it is unaffected) is useful.
Following needle EMG analysis of muscles innervated by medial and lateral plantar nerves must be performed. These muscles are the intrinsic foot muscles and 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 it is best to compare the findings with the unaffected foot. 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.
Risks associated with EMG include bleeding, infection, and nerve injury. Technical complications can include differentiating between normal and abnormal intrinsic foot muscle activity.
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 foot-wear, the simple fact of changing it may be sufficient. For progressive tarsal tunnel syndrome, surgical exploration of the nerves in the tarsal tunnel and distally may be necessary. ENG and EMG, when done efficiently and effectively, can lead to the early diagnosis of tarsal tunnel syndrome.
In many instances, the disease progression of this mononeuropathy is reversible if addressed early on. Efficient interdisciplinary and interprofessional communication is a crucial component of treating patients with neuropathy. Appropriate referral by the first medical professional who encounters 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—these include changes in footwear, 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. Ultimately, it is through collaboration and shared decision making that allows the patient to have the best possible outcome. The earlier the signs and symptoms of neuropathy identified, and the electrodiagnosis test performed, the better the prognosis and outcome. [Level 4]
Nursing or health auxiliary collaboration, depending on the concrete circumstances, with the clinician, to hand in technical material, assure the correct humidity of electrodes and alcohol swab, prepare the patient, etc. is of great help.
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