Sural neuropathy

Sural neuropathy, also known as sural nerve entrapment, can be the result of nerve compression or traction injury of the sural nerve. This nerve is a purely sensory branch, usually formed by a confluence of branches from the tibial nerve and common peroneal nerve that supplies the lateral aspect of the ankle and the foot to the base of the fifth toe ^1,2.

Epidemiology

Associations

Sural nerve entrapment is usually associated with traumatic injuries ^1-3.

Diagnosis

The diagnosis is usually made on clinical grounds and if this is difficult can be confirmed with electrodiagnostic tests like nerve conduction studies and/or small fibre testing. Differential diagnoses, such as common peroneal nerve, tibial nerve entrapment and radiculopathy should be ruled out.  A diagnostic sural nerve block is another option to confirm the diagnosis. Imaging studies, such as MRI, might help to identify the location of the entrapment and provide clues about the aetiology ¹.

Clinical presentation

Presenting symptoms include pain and paraesthesia along the lateral aspect of the foot and ankle including the base of the fifth toe ^1,3 which might be exacerbated by plantar flexion and/or inversion of the foot ². Chronic calf pain exacerbated by physical activity is another symptom ². A positive Tinel sign or point tenderness at the entrapment site might be another clinical clue.

Pathology

Sural nerve neuropathy can be a result of an acute traumatic injury at different sites, chronic compression due to tendon dislocation or entrapment within a scar ⁴ or as a consequence of a traction injury with secondary fibrosis ^2,3.

Aetiology

The cause is usually related to a traumatic injury affecting the sural nerve including ^1-4:

• direct contusion
• distal fibular fracture, talar, calcaneal or cuboid fracture
• fracture of the fifth metatarsal base
• severe lateral ankle sprain
• gastrocnemius injury
• Achilles tendinopathy or peroneus tendon injury
• space-occupying lesions
• iatrogenic injury (surgery)

Location

Sural neuropathy due to a direct traumatic event can happen at multiple different sites along its course ^1-3. A common location is at the level of the fifth metatarsal base where it bifurcates into its medial and lateral terminal branches ^2,3. Another potential site would be the lateral aspect of the calf where the sural nerve leaves the crural fascia ⁵.

Radiographic features

Ultrasound

In the calf, the sural nerve can be visualized on top of the gastrocnemius muscle at a midcalf level and descends with the small saphenous vein ^6,7. Further distally it may be seen between the Achilles tendon and the peroneal muscles and below or posterior to the peroneal tendons and superficial to the peroneal retinacula and the calcaneofibular ligament at the level of the calcaneus ^6,7.

In sural nerve neuropathy, the nerve might be thickened and hypoechoic with partial or complete loss of its normal fascicular pattern ⁷. Local compression with the ultrasound probe might reproduce pain and ultrasound can be utilized for the guidance of a diagnostic nerve block ⁷.

MRI

MRI might visualize displacement or compression of the sural nerve due to traumatic injury or space-occupying lesionlesions ². Since the sural nerve is pure sensory there will not be any denervation changes ².

Radiology report

The radiology report should include a description of the following ⁶:

• abnormal appearance of the sural nerve and location
• neuroma formation
• associated underlying pathology

Treatment and prognosis

Management includes conservative measures including physiotherapy, local anaesthetics, nonsteroidal anti-inflammatory drugs. Surgery might be required for the removal of space-occupying lesions and involves decompression and neurolysis ⁵.

History and etymology

Some of the first reported cases of sural nerve neuropathy were described by R Pringle and colleagues in 1974 ^54,8.

Differential diagnosis

The differential diagnosis of superficial sural nerve entrapment includes:

• common peroneal nerve entrapment
• deep peroneal nerve entrapment
• peripheral nerve sheath tumour
• radiculopathy