Mononeuropathies of the distal lower extremity (lower leg, ankle and foot) include the tibial nerve, tibial terminal branches3 i.e. medial and lateral plantar nerves, common fibular (peroneal) nerve with deep and superficial branches, and the sural nerve with contributions from both tibial and fibular nerves. The saphenous nerve branch of the femoral nerve supplies some distal sensory innervation and is addressed under proximal mononeuropathies.
Mononeuropathies occur in many ways including:
- Entrapment: in a fascial layer or a fibrous tunnel
- Internal compression: Bone spurs, bursitis3, nerve tumors,masses such as ganglion cyst1, 2
- External compression/ Biomechanics:2 external factors from braces, compressive clothing or tight foot wear3,5 recurrent ankle sprains or foot/ankle positioning against firm surfaces
- Iatrogenic injury during medical or surgical procedures
- Compartment syndrome from fluid shifts
Epidemiology including risk factors and primary prevention
- Nerve entrapment of the lower leg, ankle, and foot is relatively uncommon.3
- The sural nerve biopsy results in deliberate iatrogenic injury.
- Prevention includes proper positioning of prolonged bed rest patients and avoiding external compression from various sources including medical devices such as braces and casts.
Common fibular (peroneal) nerve:
- Origin: The fibular division of the sciatic nerve arises at about the level of mid-thigh, originates from the L4, L5, S1 and S2 nerve roots.5 In about 80%, the fibular nerve gives off a communicating cutaneous sural branch to join with a branch from the tibial nerve to form the sural sensory nerve.5
- Course: wraps around the fibular head, closely adhered to the periosteum, dividing into the deep and superficial fibular nerves.
- Superficial fibular nerve: motor to muscles of the lateral compartment of the lower leg; sensory to the lower two-thirds of the lateral leg and dorsum of the foot.
- Deep fibular nerve enters the anterior compartment: motor to muscles anterior compartment; sensory to the first web space.
Common fibular nerve injury:
- Direct external compression at the fibular head:1,5 habitual leg crossing, pressure from prolonged bed rest, tall boots 5, casts or braces.
- Stretch injury: with considerable time in the squatting position, known as “strawberry pickers palsy”.
- Frequently injured in exercise involving repetitive inversion or eversion of ankle leading to ankle sprains (in runners) which can injure the fibular nerve by disrupting the vasa vasorum1,2 or following proximal tibiofibular joint dislocation or fracture.1,2
- Deep fibular nerve: anterior compartment syndrome, anterior tarsal tunnel syndrome.1
- Superficial fibular nerve is infrequently injured in isolation, as it pierces the lateral compartment of crural fascia or also seen in chronic exertional compartment (lateral) syndrome.1
- Origin: The tibial division of the sciatic nerve arises at about the level of mid-thigh, originates from the L4, L5, S1 and S2 nerve roots. In the popliteal space it gives of the communicating cutaneous sural branch to join with a branch from the fibular nerve to form the sural sensory nerve.5
- Motor: Superficial and deep posterior compartment of the lower leg
- Motor and sensory to foot: passes posterior to the medial malleolus, giving off the calcaneal branch and then passing through the tarsal tunnel as the medial plantar and lateral plantar nerves.
Tibial nerve injury:
- Rare: relatively well-protected.
- Proximal injury: Popliteal fossa mass, Baker’s cyst, hemorrhage, other space-occupying lesions.
- Distal injury: at the ankle as it passes under the flexor retinaculum, the “tarsal tunnel”, also known as posterior tibial neuralgia 2. In most cases, compression is idiopathic but may be due to ankle trauma, ganglion cysts or lipomas, or due to foot and ankle deformity such as talocalcaneal coalition. This may be the first presentation of a length-dependent neuropathy; so care should be taken to avoid over-diagnosis.
- Origin: purely sensory nerve formed by variable contributions from the tibial and fibular nerves.
- Course: deep to fascia of the posterior compartment in the middle third of the leg, then travels along the lateral border of the Achilles tendon, and then posterior to the lateral malleolus.
- Sensory: postero-lateral aspect of the distal third of the lower leg and lateral aspect of the foot.5
Sural nerve injury:
Although rare, entrapment of sural nerve can occur anywhere in the leg, ankle or foot.3
- Fractures of the calcaneus or fifth metatarsal, history of recurrent ankle sprains, injury of the Achilles tendon, space occupying lesions such as ganglion cysts,3,5 iatrogenic injury during surgical repair of the Achilles, or during arthroscopic surgery of the lateral ankle.
Specific secondary or associated conditions and complications
Ankle contractures from severe foot drop can develop. Complex regional pain syndrome may be a sequela of nerve injury. Falls due to poor foot clearance may occur. Severe sensory loss can predispose to skin breakdown, particularly in patients with preexisting peripheral neuropathy.
2. ESSENTIALS OF ASSESSMENT
Detailed, précised history and physical examination and judicious use of electrodiagnostic and imaging modalities (such as MRI and ultrasonography1,2,5) will help to explain the underlying cause of lower extremity nerve-related symptoms such as chief complains of muscle weakness or numbness (probably more common in fibular neuropathies and tarsal tunnel syndrome), as advance imaging has improved the ability to localized the area of nerve entrapment.3
History for mononeuropathies of the leg may include antecedent injury, trauma, medical condition or surgery. Habitual positions or repetitive motions may be pertinent. Occupational and avocational activities (hobbies, sports such as long distance runner3 etc) and associated shoe wear may be a causative or contributing factor. The patient may present with paresthesias (in distribution of affected nerve), pain (at rest, at night, during or after exertion)1,2 and/or muscle weakness.
- Fibular nerve: a Tinel’s sign over the fibular nerve as it passes around the fibular head if the lesion localizes to this area. Difficult or uncoordinated gait, due to weakness of ankle dorsiflexion (common fibular neuropathy) and steppage gait (increase hip and knee flexion) to compensate for weak or loss of ankle dorsiflexion strength,5 weak great toe extension (deep fibular) and weak ankle eversion (superficial fibular) or both. Foot slap and difficulty heel walking in milder cases where the patient has enough residual strength to clear the foot. Loss of sensation in varying distribution5 in the anterolateral surface of the lower leg and foot. Superficial fibular (including medial dorsal cutaneous and intermediate dorsal cutaneous branches) neuropathy with medial branches affecting sensation to the medial dorsum of the foot, including the great, second, and often the third toe and intermediate superficial fibular branch affecting sensation to lateral third. Fourth, and fifth toes and deep fibular branches with preferential sensory loss in the first web space.5
- Tibial nerve: Sensory changes (dysthesia, tingling, numbness, burning/electrical pain) anywhere along the distribution of tibial nerve or one of its associated major terminal branches (calcaneal, medial and lateral plantar) individually or collectively. Proximal tibial nerve compressed in tarsal tunnel proper ( ie tarsal tunnel syndrome) has clinical finding of localized tenderness to palpation over medial malleoli or abductor hallucis,2,3 Hoffmann-Tinel’s sign may be elicited by tapping or percussing the nerve posterior to the medial malleolus and dorsiflexion eversion test display intensification of symptoms if positive. Compression of medial plantar branch (ie Jogger’s foot) has physical finding of tinel sign at plantar border of the navicular tuberosity and dysthesia along heel, medial arch, and first through third toes. Entrapment of the first branch of the lateral plantar nerve (ie, Baxter neuropathy) has physical finding of tenderness over the nerve deep to abductor hallucis and palpation of this area reproduce symptoms and may cause radiation of the pain proximally or distally 3. Weakness of plantar flexion and inversion of the foot occurs with proximal injuries and toe flexors with mid-leg injuries. Weakness of the gastro-soleus complex may be difficult to appreciate due the power of these muscles; therefore testing unipedal toe raises is critical for subtle lesions. The Achilles reflex may be depressed or absent. Assessing gait pattern, foot deformities (valgus or varus),3 asymmetry of calf bulk and a “sinking” type gait due to poor toe off may be seen. In tarsal tunnel syndrome a localized tenderness to palpation over medial malleoli or abductor hallucis,3,4 Hoffmann-Tinel’s sign may be elicited by tapping or percussing the nerve posterior to the medial malleolus and dorsiflexion eversion test display intensification of symptoms if positive.
- Sural nerve: Sensory abnormalities will be noted in the posterolateral leg and lateral foot, with normal strength and reflexes. Pain to percussion may on rare occasion be seen along the course of the nerve, and stretching the nerve by positioning of the ankle in a dorsiflexed inverted position may provoke symptoms.
- The presence of weakness around the hip or knee would suggest a more proximal lesion such as plexopathy or radiculopathy.
- The presence of lower leg or foot pain and paresthesias, in presence of uncontrolled diabetes mellitus, history of heavy alcohol drinking, cigarette smoking or advanced lumbo-sacral degenerative joint or disc disease, a differential diagnosis of peripheral polyneuropathy, vascular claudication, neurogenic claudication, L5-S1 radiculopathy and plexopathy as well as other quite broad however less likely diagnosis, should be kept in differential diagnosis of lower extremity mononeuropathies 3.
In many cases US and MRI are complementary and selection of either modalities depends on the training and confidence of the imager.7
Supplemental assessment tools
Electrodiagnostic studies identify nerve location, severity, age, prognosis, and should include nerve conduction studies and EMG to rule out proximal injuries such as lumbosacral plexopathies, radiculopathies of the L5 or S1 root levels, or sciatic nerve injuries.6
- Fibular nerve injury: Sural sensory responses will be variably affected and comparison to unaffected side may be helpful. Fibular motor conduction to the extensor digitorum brevis (EDB) should be performed above and below the knee to assess for focal conduction block at the fibular head. If EDB is low or unobtainable, the motor response can be measured at the anterior tibialis. Superficial fibular sensory nerve action potential (SNAP) is also tested (as sensory abnormalities especially loss of amplitudes implies some axonal loss; distal to dorsal root ganglion)5,6 . EMG should sample muscles from both the superficial and deep branches and include the short head of the biceps femoris to differentiate this from a more proximal lesion of the fibular nerve 5.
- Tibial nerve injury: Sural sensory responses will be low in proximal lesions at the popliteal fossa. Side to side comparison should be done, as these responses are affected early in peripheral polyneuropathy. Tibial motor conduction to abductor hallucis (AH) should be performed to asses for prolonged latency, low compound motor amplitude or slow conduction velocity2,6 in suspected tarsal tunnel syndrome, mixed medial and lateral plantar nerve conduction studies, although challenging to obtain in elderly, should be obtained and compared side-to-side.6 EMG should include multiple lower limb muscles to differentiate from radiculopathy, plexopathy, or proximal tibial injury in the thigh, including the posterior thigh, in addition to calf muscles.6
- Sural nerve injury: low sural amplitudes are seen in early peripheral neuropathy. Comparison to the opposite side will reveal asymmetry in unilateral lesions, especially in young people. Use EMG to define this as a pure sensory neuropathy.
- As superficial and deep fibular neuropathy can be associated with chronic exertional compartment syndrome (CECS), compartment pressure needs to be measured if history is suggestive of this diagnosis.1, 4, 5
3. REHABILITATION MANAGEMENT AND TREATMENTS
Available or current treatment guidelines
Treatment is aimed at prevention, managing symptoms and addressing any resulting impairments.
- Prevention/Alleviate compression: Modification of pain-inducing or compression activities3 such as avoidance of squatting and leg crossing, proper positioning of bedridden patients, modification of casts or orthoses, shock absorbing shoe inserts, shoe modifications, running techniques modification1,5
- Orthoses: custom orthoses to support medial and lateral longitudinal arches in tarsal tunnel syndrome2, ankle foot orthoses (AFO) for foot drop in fibular nerve injuries and orthoses for calf weakness in tibial injures may be helpful.
- Physical therapy:2,5 strengthening, range of motion to prevent or address contracture, muscle stretching to help maintain muscle for future reinnervation and effective function, gait retraining, physical modalities, e.g. TENS, soft tissue manipulation and cryotherapy2 for pain management.
- Pharmacology: Non-steroidal anti-inflammatory (NSAID) and neuropathic pain management including topical agents and oral pharmacologic agents, such as GABA analog, tricyclic antidepressant and vitamin B-complex supplements3.
- Ultrasound-guided injection: If conservative treatment fails and an entrapment site is located, an ultrasound guided nerve block can be performed to obtain additional diagnostic information.1
- Surgical intervention: recommended, if failure of conservative methods to provide adequate relief or restoration of function.5 However early surgical intervention is appropriate during urgency and emergency situations such as nerve transection, acute compartment syndrome and space-occupying lesions with progressive neurologic deficit. On other hand in situations such as tarsal tunnel without space-occupying lesion, CECS and chronic nerve injuries which may be amenable to an initial trial of conservative management, before considering surgery.
- resection of mass/tumor
- fasciotomy in the setting of acute compartment syndrome
- immediate nerve repair for transection
- surgical: if conservative measures fail and axonal damage is identified with electrodiagnostic testing neurolysis with or without fascial release (superficial and deep peroneal neuropathy) should be considered.1
- tendon transfers may be considered. One of the indications is, if the foot could not easily be passively dorsiflex to 10 degrees beyond the neutral 1.
Coordination of care
Physical and occupational therapy can address exercise, avocational and occupational needs. The orthotist can evaluate and modify braces or shoe wear. The electromyographer can guide surgical exploration and treatment. Communication with nursing staff and caregivers is important to ensure proper positioning for patients on bed rest.
Patient & family education
Patients and care-givers should be educated on avoidance of exacerbating activities, positioning, daily skin care and precautions for insensate foot
Translation into practice: practice “pearls”/performance improvement in practice (PIPs)/changes in clinical practice behaviors and skills
- With knee and ankle injuries, secondary injury to nerves of the lower extremity can occur, particularly injury to the fibular nerve with fractures of the fibular head.
- EMG is extremely useful in localizing nerve lesion, assessing for signs of reinnervation, determining severity, and prognosis for recovery.
- The fibular nerve is most vulnerable to injury.
4. CUTTING EDGE/EMERGING AND UNIQUE CONCEPTS AND PRACTICE
Tarsal tunnel surgery has a high incidence of postoperative complication. Cryosurgery for tarsal tunnel syndrome is less invasive than conventional surgical techniques, and may be a front line option for treating tarsal tunnel syndrome that has failed conservative therapy. Early published results showed some promising shorter recovery period with cryosurgery for tarsal tunnel; compared to traditional surgical methods, however due to limited numbers of documented cryosurgical trial studies, a broader range of investigational studies are needed in regards to efficacy and appropriate indications for this contentious treatment option.2
Nonsurgical treatment option for chronic exertional compartment syndrome (CECS) involves injection of botulinium toxin A into the muscles of the affected compartments.1
5. GAPS IN THE EVIDENCE-BASED KNOWLEDGE
More future new diagnostic technologies and research studies; hopefully will help with early diagnosis and follow-up on outcome difference between conservative versus surgical treatment outcomes.
- Flanigan RM, DiGiovanni BF. Peripheral nerve entrapments of the lower leg, ankle, and foot. Foot Ankle Clin N Am. 2011;16:255-274.1.
- Rajasekaran S, Finnoff JT. Exertional leg pain, phys Med Rehabil Clin N Am27 (2016) 91-119, also PMR 2012; 4 (12):985-1000
- McSweeney S, Cichero M. Tarsal Tunnel syndrome-A narrative Literature review (2015), http://dx.doi.org/10.1016/j.foot.2015.08.008
- Pomerory G, Wilton J, Anthony S. Entrapment neuropathy about the foot and ankle: An update, Journal of the American Academy of orthopaedic Surgeons. 2015; 23:58-66
- Burrus MT, Werner BC, Starman JS, et al. Chronic leg pain in athletes. Am J Sport Med 2014; 43(6): 1538-47
- Flanigan RM, DiGiovanni BF. Peripheral nerve entrapments of the lower leg, ankle, and foot. Foot Ankle Clin N Am. 2011; 16:255-274.
- Roy P. Electrodiagnostic evaluation of lower extremity neurogenic problems. Foot Ankle Clin N Am. 2011; 16: 225-242.
- Lopez-Ben. Imaging of nerve entrapment in the foot and ankle. Foot Ankle Clin N Am. 2011; 16:213-224.
- Stewart JD. Foot drop: where, why and what to do? Practical neurology. 2008;8: 158-169.
- Yeap JS, Birch R, Singh D. Long-term results of tibialis posterior tendon transfer for drop-foot. Int Orhop. 2001; 25(2); 114-118.
Original Version of the Topic
Anita S.W. Craig, DO. Distal lower extremity mononeuropathies. 08/30/2013.
Poonam Ochani, MD
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