Ultrasound in Peripheral Nerve Surgery

How to Start (Basic Principles)

One of the basic principles of US is the fact that high frequency (17–15 MHz; ▶ Fig. 8.1) leads to high resolution but low tissue penetration. Therefore, the visualization of superficial peripheral nerves is decently feasible. However, for the presentation of deeper lying nerves (for instance, sciatic nerve), the application of lower frequency transducers (10–12 MHz) becomes necessary.

Fig. 8.1 The 15-MHz transducer, called ice hockey stick (left-hand side); the 17 MHz transducer (right-hand side).

To start examination, the authors would recommend obtaining transverse US images. At the beginning, it is difficult to distinguish nerves from tendons (i.e., median nerve at the wrist) ( ▶ Fig. 8.2). However, when the US probe is slightly tilted, nerves will not change their shape, while tendons will become either hyperechoic or hypoechoic. The typical transverse picture of a healthy peripheral nerve is similar to a honeycomb, meaning that single fascicles are hypoechoic, those are surrounded by hyperechoic membranes of the perineural sheath. The whole nerve is coated by epineural tissue, which is also hyperechoic. Note that the real anatomical existing number of fascicles is usually much higher. It does not correspond to the number of fascicles depicted by HRU.

Fig. 8.2 Transversal plane of the median nerve. RCFT, flexor tendon of the M. carpi radialis muscle; RF, retinaculum flexorum.

The investigator should start US at an anatomical relevant area, where the nerve is lying superficially or nearby a typical bone structure, for example:

Median nerve: carpal tunnel/wrist.
Ulnar nerve: cubital tunnel/elbow.
Radial nerve: humerus/middle upper arm.
Peroneal nerve: fibula/knee joint.
Tibial nerve: tarsal tunnel/medial malleolus.

HRU should be a dynamic examination. It is important to examine the whole or at least a long trail of the nerve, sometimes even to compare the healthy side of the body with the lesioned one. By slightly increasing the pressure of the transducer, the investigator can even provoke a Tinel sign; or by asking the patient to flex and stretch the elbow, a subluxation or complete luxation of the ulnar nerve can be seen by HRU. Last but not least, one should also evaluate the structure of the different surrounding tissues, i.e., muscles, bones, and tendons, especially in trauma.

Note:

High frequency means less tissue penetration.

Start examination in transverse plane on an anatomical relevant area.

HRU is a dynamic examination, keep moving.

8.3 Compression Neuropathies

8.3.1 Compression Neuropathies of the Upper Limb

Bearing in mind that the carpal and the cubital tunnel syndrome (CTS and CUTS) are the most frequent entrapment syndromes, clinical examination and electrophysiology are indispensable and usually sufficient. However, if the visualization of the affected nerve becomes necessary, for instance, in cases of suspected recurrence, HRU is the suitable diagnostic tool to provide additional morphological information.

Historically, the first HRU studies were dealing with CTS and CUTS. Already in 1991, only 3 years after Fornage first described a neural structure in US, Buchberger and his colleagues ⁴ published their findings in patients with CTS. The study group pointed out that the median nerve appeared enlarged before entering the entrapment zone. One possible pathological explanation for the development of this swelling is postulated to be caused by compression of the vasa nervorum. As a result, an ischemia and venous congestion may occur, which in turn leads to neural edema. ⁵ This suspicious surface area has to be compared to the more proximal or distal parts of the nerve. If the ratio is 2:1, one can speak of a pseudoneuroma ( ▶ Fig. 8.3a–c). Kele et al ⁶ examined 110 median nerves of patients suffering from CTS via US. A cross-sectional area (CSA) of > 0.11 cm² was considered to be highly predictive for the diagnosis of CTS (sensitivity 89.1% and specificity 98%). Therefore, diagnostic accuracy of HRU is comparable to electrophysiological studies in CTS. ⁷ Another group reported that by combining HRU with nerve conduction studies, sensitivity and specificity increased to 98 and 91%, respectively, in patients with CUTS. ⁸

Fig. 8.3 Transversal plane of the ulnar nerve of a patient suffering of CUTS. (a) At the level of the epicondylus medialis, showing a hypoechoic enlargement of 0.146 cm². (b) At the middle of the upper arm with a CSA of 0.068 cm². (c) At the middle of the forearm next to the ulnar artery with a CSA of 0.067 cm².

For all other rare entrapment syndromes of the upper limb, such as, supinator syndrome, thoracic outlet syndrome, Guyon’s syndrome, etc., HRU is even more essential, and it provides helpful additional information (existence of cysts, lipoma, cervical ribs) since electrophysiology examination is often challenging in those cases.

8.3.2 Compression Neuropathies of the Lower Limb

Entrapment syndromes of the lower limb are quite rare, but peroneal and tibial nerves in particular can be compressed by extraneural or intraneural cysts. Therefore, for surgical planning, HRU is extremely helpful ( ▶ Fig. 8.4). ⁹ Even Morton’s neuroma of the interdigital plantar nerve can be visualized as a spherical swelling. Also, a hypoechoic enlargement of the lateral femoral cutaneous nerve (LFCN) is a secure sign of entrapment at the anterior superior iliac spine in patients with meralgia paresthetica, but since those patients are often obese, an examination via HRU is in general difficult. ¹⁰ Infiltrations of the LFCN as a nonsurgical treatment option are firmly performed by US guidance. ¹¹

Fig. 8.4 Tibial nerve next to an extraneural cyst at the malleolus medialis.

8.3.3 Recurrent Compression Neuropathies

HRU is of great importance and indispensable in cases of recurrent entrapment syndromes. Before a surgical redo surgery is decided, the visualization of new postoperative morphological changes, such as, scars, epineural fibrosis ( ▶ Fig. 8.5), partial neuromas, cysts, nerve kinking after transposition, etc. have to be depicted and evaluated. Also the CSA of the pseudoneuroma before and after surgery can be compared. Tas et al ¹² described a decrease in CSA after CTS surgery in asymptomatic patients, postulating that after decompression the swelling of the affected median nerve is reversible.

Fig. 8.5 Median nerve of a symptomatic patient, who was previously operated for CTS, showing epineural fibrosis.

Note:

Pseudoneuroma means CSA 2:1.

HRU is above all useful for “failed surgery” and rare compression neuropathies of the upper or lower limb (e.g., detection of cysts).

8.4 Trauma

8.4.1 Preoperative HRU

HRU in the recent past had a significant impact on peripheral nerve surgery, especially in peripheral nerve trauma.

Besides medical history, clinical examination, and electrophysiological studies, HRU provides morphological information of the lesioned nerve segment and its surrounding tissues (bone, tendons, muscles). Before, clinical and electrodiagnostic examination were the only tools to evaluate patients. In some instances, MRN was performed, but in the majority of cases, due to the presence of osteosynthetic material, nerve structures could hardly be recognized.

Now, via HRU, the injured area can be examined as a whole. Tendons, muscles, bones, osteosynthetic material, hematoma, and nerves become visible. ¹³

Since morphological nerve damage is visualized and registered by HRU, this diagnostic tool has obtained an outstanding role especially in regard to iatrogenic nerve lesions ¹⁴ ( ▶ Fig. 8.6a,b).

Fig. 8.6 (a) Preoperative ultrasound revealing the exact location where the radial nerve slides under the screw. (b) Intraoperative picture of osteosynthesis after a fracture of the left humerus; radial nerve is lying under a screw.

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