Peripheral Nerve Entrapment
An entrapment neuropathy is defined as a pressure-induced injury to a peripheral nerve in a segment of its course due to anatomic structures or pathologic processes. It occurs at anatomic sites where the nerve changes direction to enter a fibrous or osseofibrous tunnel, or where the nerve passes over a fibrous or muscular band. It occurs in these locations because of an increased risk of mechanical irritation.
Nerve entrapment can occur in tunnels, narrow anatomical spaces, fibrous bands, scar tissue, masses, bony calluses, external compression from casts or braces, oedema, and inflammation.
Prolonged compression causes ischaemia due to compression of vasa nervorum - the blood flow to the nerve itself. There is a mechanical deformation of the myelin sheath, and impairment of axonal transport of nutrients.
The hallmarks are:
- Pain - burning, aching, tingling
- Paraesthesias with compression
- Tinel's sign - represents ectopic excitability
- "Double crush." This is a phenomenon where the clinical effect is greater than the sum of its parts. With the presence of a proximal lesion, the distal nerve is more vulnerable to being compromised. For example a patient with both a C6 lesion and a superficial radial nerve lesion will generally have a worse clinical picture than having either lesion alone.
Pattern recognition is very important. Ask the patient to show you exactly where it hurts, its boundaries, and what direction it goes. This is often more useful than interpreting the patients pain drawing. For example patients will often simply circle a limb on their pain drawings despite careful instructions.
Physical examination allows detecting subtle clues like focal atrophy, or tissue bulging. Sites of entrapment are very tender. Normal nerves are almost insensitive to pain, while entrapped nerves are extremely sensitive.
Under ultrasound nerve entrapment can be visualised by documenting nerve swelling, nerve compression, bone spurs with nerve path distortion, and external compression.
This is best done under ultrasound due to low cost, availability, and good visualisation of pathology. Landmark, fluoroscopic, and CT guided injections are other options. Peripheral nerve stimulators can help to hone in on the nerve.
Once the correct diagnosis has been made, there are various procedural treatment options.
- Therapeutic injections with steroid, 5% dextrose
- Hydrodissection. This is using fluid to dissect apart tissue releasing the entrapment. Fluids find the path of least resistance between tissue planes, mechanically separating fascial layers.
- Surgical release
- Cryoneuroablation. This involves freezing the nerve, leaving the myelin sheath allowing it to grow back along its normal path. A gas flows through the centre of the probe and expands as it cools at a temperature of around -70°.
- Peripheral nerve stimulation. This can be done using ultrasound guidance.
- Dr Andrea Trescot (2020) “The Role of ultrasound in the treatment of Peripheral Nerve Entrapment?,” YouTube. Available at: https://www.youtube.com/watch?v=UTULxC36tjY (Accessed: April 23, 2021).