Pressor neuropathies of the upper limb: Modern diagnosis and treatment

What are pressure neuropathies?

Pressor neuropathies or peripheral nerve entrapment syndromes are a large class of conditions that belong to the peripheral neuropathies. The most common compressive neuropathies seen in the upper extremity are carpal tunnel syndrome and ulnar neuritis, while rarer conditions are radial canal syndromes, Guyon's ulnar canal syndrome, sternal pronus, suprapatellar nerve entrapment, thoracic outlet syndrome, etc.

What are they due to?

Trapping neuropathies are caused by pressure exerted on the peripheral nerves by various anatomical structures adjacent to them, which are overgrown or swollen due to overgrowth, or by pathological processes such as ganglia, lipomas and various other tumours, or by rare anatomical variations such as accessory muscles, vascular strains, etc. The external pressure exerted on the nerves reduces the blood flow in the vessels supplying them. This causes local ischemia that disrupts the function of the nerve. Over time, if the condition is not treated, demyelination occurs, permanent damage to the neuraxons and eventually the nerve scarring occurs.

How do they manifest themselves?

The most common symptoms of compressive neuropathies are pain, numbness (tingling), tingling and muscle weakness. The symptoms occur in the limb and more specifically in the area innervated by the nerve being pressed. 

How is the diagnosis made?

The diagnosis is primarily clinical, i.e. based on taking a good history of symptoms, but also on a detailed clinical examination. The hitherto established method of diagnosing a trapping neuropathy has been neurophysiological testing and electromyography (gold standard). 

However, neurophysiological testing/electromyography has two major disadvantages 1) by definition it can only highlight trapping lesions in which the nerve is functionally affected (destructive type of lesion) and not irritant type of lesions, in which the functionality of the nerve has not yet been affected; 2) they are invasive, painful methods (since needles and electrodes are used), which are technically difficult to repeat at regular intervals. 

Thus, for several years, more rare pathological entities involving nerves located in deep anatomical locations (anterior median nerve branch or radial nerve) or nerves pressed on specific limb locations (ulnar nerve in the homonymous groove of the elbow) could not be diagnosed and treated appropriately. Neurohypersonography has provided a solution to this problem in recent years.

What is neurohypersonography?

Neurohypersonography is a new, specialized, non-invasive, non-invasive ultrasound method in the diagnostic approach of patients with diseases of the peripheral nervous system (PNS), which is performed using a special transducer head (frequency > 12 Mhz) and enables detailed imaging of the structure of peripheral nerves and muscles.

In 1988, Fornage and Rifkin1 described for the first time using ultrasound the pathological lesions of trap neuropathy of the median nerve in the carpal tunnel, essentially paving the way for a pioneering method in the diagnostic elucidation of trap neuropathies. Over the years and with the help of technological advances in ultrasound equipment, numerous ultrasound studies of trapping neuropathies have been published, increasing scientific interest in this method.

How does neurohypersonography help us diagnostically?

Neurohypersonography offers us useful information around:

• the morphology of the peripheral nerves, revealing acute or chronic pathological lesions (e.g. stenoses, oedemas or hypertrophies)
• the exact cause of a nerve entrapment (e.g. from a haematoma, ganglion, pathological thickening of ligaments)
• revealing to the nearest millimetre a typical (e.g. within the carpal tunnel or ulnar groove) or atypical anatomical nerve entrapment site (e.g. forearm or around the head of the fibula)
• the detection of significant anatomical variations (e.g. persistent median artery, spina bifida, presence of accessory muscle within the ulnar groove) of interest to the treating surgeon prior to decompression
• the detection of autoimmune lesions of nerves (e.g. chronic inflammatory demyelinating polyneuropathies of the CIDP type) or muscles (myositis, myopathies)

What preparation is needed for the test?

For neurohypersonography no preparation whatsoever is needed. The examination is performed in a supine or sitting position, at complete rest and is completely painless.

When do I need to have a neurohypersonography?

The most important indications for neurohypersonography are:

• symptoms of pressure or injury to a nerve (entrapment neuropathy) to reveal the severity, cause and exact location (e.g. finding the cause of carpal tunnel syndrome)
• in symptoms of muscle disease, to reveal inflammatory or atrophic lesions
• symptoms suggestive of possible autoimmune polyneuropathy to reveal the distribution and severity of structural lesions of the nervous and muscular system

What are the advantages of neuroimaging over conventional neurophysiological testing?

Among the most important advantages of neurohypersonography, the following have been documented in the literature:

• earlier detection of pathological lesions, particularly in irritant-type trapping neuropathies, compared to neurophysiological testing which can only detect functionally significant destructive lesions
• the documentation of both the severity and the exact cause of a peripheral nerve entrapment lesion
• the differential diagnosis of axotomy from complete neurosection in traumatic lesions, information that has a direct therapeutic impact on the surgical or conservative treatment of nerve attacks
• the detection of important anatomical variations for the attending surgeon that will affect the decompression technique
• the absolutely painless and easily repeatable nature of the test

What are the most common findings in trapping neuropathies?

The main pathological lesions observed ultrasonographically in trapping neuropathies are focal thickening of a nerve and disturbance of its echogenicity. 

Newest data in the diagnosis and treatment of trapping neuropathies 

Carpal tunnel syndrome

Carpal tunnel syndrome is the most well-studied ultrasonographically studied trapping neuropathy. Literature review studies have found that increased nerve cross sectional area at the entrance of the carpal tunnel is the earliest and most common finding of this neuropathy, with sensitivity ranging from 70 to 88% and specificity from 57 to 97% among studies2. 

In studies by Kele and Reimers, a diagnostic algorithm for this syndrome was proposed by ultrasound assessment of the median nerve at both the carpal tunnel and forearm level. This algorithm increased the sensitivity of the method to 89.1% and its specificity to 98%3. This method was compared in literature with neurophysiological testing in terms of sensitivity and specificity showing similar results4.

In 16% of patients with carpal tunnel syndrome, the persistent median artery, which is usually located lateral to the median nerve, can be identified as an anatomical vascular variant by ultrasound. The significance of this finding for the syndrome has not been investigated in the literature, but case reports of thrombosis of this artery as a cause of carpal tunnel syndrome have been described in the literature5.

From a therapeutic point of view, the information provided by neurohypersonography of the median nerve is valuable for the treating surgeon, especially in case of endoscopic carpal tunnel opening. The endoscopic technique according to the latest meta-analysis of 28 studies provides faster and better recovery of daily activities, greater patient satisfaction, shorter return to work time and fewer complications from the surgical trauma6. Our personal experience supports that when endoscopic opening is combined with preoperative assessment of the syndrome with median nerve neurohypersonography, the risk of complications is eliminated and efficacy is assured7,8.

Of great importance is the help of neurohypersonography of the median nerve and in cases of relapse of the syndrome after previous surgical opening. In this case, neurohypersonography can give us an answer as to whether the opening that was made is incomplete, whether scar tissue has formed around the nerve or whether an injury to the nerve has occurred that has led to the formation of a painful neuroma. This allows the surgeon to do the appropriate preoperative planning, inform the patient of the treatment to be applied and determine the prognosis9. 

Ulnar nerve entrapment neuropathy in the elbow

Entrapment of the ulnar nerve in the homonymous groove of the elbow is the second most common neuropathy of the upper limb after carpal tunnel syndrome. The use of neurophysiological testing is of major importance for the localisation and severity of the lesion, but its sensitivity in this neuropathy is lower than in carpal tunnel syndrome10.

The ability of neurohypersonography to visualise the precise anatomy of this area is perhaps the main benefit, particularly for the surgeon performing the decompression.

Beekman et al. showed that the addition of ultrasound to neurophysiological testing increases the sensitivity of syndrome diagnosis from 78% to 98%11. Abnormal nerve enlargement on ultrasound shows a statistical correlation with the findings of neurophysiological testing, and the site of this enlargement may precisely match the site of conduction block on neurophysiological testing12.

From a surgical point of view, neurohypersonography of the ulnar nerve provides valuable information to the treating surgeon, such as the presence of accessory muscle, the existence of a hypertrophied medial head of the triceps, the documentation of whether the nerve is under pressure within the groove, centrally or peripherally and whether it is unstable. With this preoperative information, the surgeon can perform targeted decompression with a much smaller incision, which was not possible in the past and the nerve had to be decompressed over its entire extent. This greatly increased the recovery period and complications. Finally, the number of cases of anterior nerve transposition, which is a non-anatomical solution and is accompanied by a significant degree of recurrence due to nerve decompression and subsequent scarring, is greatly reduced.

Radial nerve neuropathies

Unlike the median and ulnar nerves, detailed ultrasound imaging of the radial nerve is a technical challenge of neurohypersonography. 

The most common cause of neuropathy of this nerve in the upper limb is a fracture of the humerus, a condition that has been extensively studied ultrasonographically. Bodner et al. specifically examined the radial nerve in patients with neuropathy from a closed fracture in the middle third of the humerus describing pathological findings such as swelling, torsion or even the development of post-traumatic neuroma13. Cases of radial nerve palsy due to pressure on the spiral groove have also been reported in the literature14 .

The study of the motor branch of the radial nerve in the forearm presents particular difficulties due to its small diameter and the adjacent muscle structures that show similar echogenicity. However, irritant neuropathy in the hypogastric muscle canal is a common ultrasound finding in patients with localized pain in the upper forearm.

Neurohypersonography of the radial and posterior median nerve can help to differentiate between persistent external epicondylitis and radial tunnel syndrome, confirming in these cases the clinical suspicion of nerve compression and giving the surgeon the right to perform decompression.

Rarer upper limb neuropathies

With neurohypersonography of the upper limb it is now possible to confirm other rarer trapping neuropathies. In recent years, therefore, cases of median nerve pressure between the heads of the proximal pons (pronator syndrome) or from the denervation of the scythe (lacertus syndrome), ulnar nerve pressure in Guyon's canal, etc. have been diagnosed and treated in our clinic.

Bibliography

1) Fornage BD, Rifkin MD. ultrasound examination of the hand and foot. radiol Clin North Am. 1988 Jan;26(1):109-29

2) Beekman R, Visser LH. Sonography in the diagnosis of carpal tunnel syndrome: a critical review of the literature. Muscle Nerve. 2003 Jan;27(1):26-33

3) Kele H, Verheggen R, Bittermann HJ, Reimers CD. The potential value of ultrasonography in the evaluation of carpal tunnel syndrome. 2003 Aug 12;61(3):389-91.

4) Padua L, Pazzaglia C, Caliandro P, Granata G, Foschini M, Briani C, Martinoli C. Carpal tunnel syndrome: ultrasound, neurophysiology, clinical and patient-oriented assessment. Clin Neurophysiol. 2008 Sep;119(9):2064-9

5) Kele H., Verheggen R., Reimers C.D.: Carpal tunnel syndrome caused by thrombosis of the median artery: the importance of high-resolution ultrasonography for diagnosis, case report. 97:471-473

6) Yueying Li, Wenqi Luo, Guangzhi Wu et al: Open versus endoscopic carpal tunnel

release: a systematic review and meta-analysis of randomized controlled trials.BMC Musculoskeletal Disorders, 2020; 21:272.

7) Kerasnoudis A., Venouziou A. et al. "Ultrasound Anatomy of the Median and Ulnar Nerves." Compressive Neuropathies of the Upper Extremity, 2020, pp. 11-26.

8) Venouziou, A. and Kerasnoudis A.: "Endoscopic Carpal Tunnel Release." Compressive Neuropathies of the Upper Extremity, 2020, pp. 53-60.

9) Kerasnoudis A., Venouziou A., et al. "Ultrasound, Clinical, and Electrophysiological Findings in Persistent Carpal Tunnel Syndrome." Journal of Neuroimaging, vol. 29, no. 2, 2018, pp. 218-222.

10) Kim B.J., Koh S.B., Park K.W. et al: Pearls and oysters: false positives in short-segment nerve conduction studies due to ulnar nerve dislocation Neurology 2008; 70:e9-e13.

11) Beekman R, Visser LH, Verhagen WI. ultrasonography in ulnar neuropathy at the elbow: a critical review.Muscle Nerve. 2011May;43(5):627-35.

12) Bayrak A.O., Bayrak I.K., Turker H., et al: Ultrasonography in patients with ulnar neuropathy at the elbow: comparison of cross-sectional area and swelling ratio with electrophysiological severity. Muscle Nerve 2009; 41:661-666.

13) Bodner G., Buchberger W., Schocke M. et al: Radial nerve palsy associated with humeral shaft fracture: evaluation with US-initial experience.Radiology 2001;. 219:811-816.

14) Lo Y.L., Fook-Chong S., Leoh T.H. et al: Rapid ultrasonographic diagnosis of radial entrapment neuropathy at the spiral groove J Neurol Sci 2008;. 271:75-79.