Spontaneous radial nerve palsy with hourglass-like constriction

Abstract

Hourglass-like constriction (HLC) is an uncommon spontaneous mononeuropathy that is typically characterised by a sudden onset of pain followed by palsy, affecting branches of the radial (posterior interosseous nerve) and median nerves (anterior interosseous nerve). HLC of the radial nerve (RN) is rare, with only a few reported cases. Here, we report a case of a man who presented with acute wrist and finger drop due to the HLC of the RN. Surgery was recommended 5 months after clinical observation, when the lesion was resected and primarily repaired, resulting in satisfactory recovery. There is still much that remains unknown about HLC, especially for RN. The current understanding points out an inflammatory disease that should be treated conservatively for 3–7 months. The surgical technique depends mostly on the severity and extent of constriction; however, considering only RN constrictions, primary repair by neurorrhaphy or nerve grafts resulted in better functional outcomes.

Background

In a patient facing spontaneous non-traumatic mononeuropathy, several pathophysiological mechanisms have been proposed, including compression neuropathy by normal and aberrant anatomy,^{1 2} mass lesions,² inflammatory neuralgic amyotrophy (NA)^{3 4} and hourglass-like constrictions (HLC).^5–19 According to McGraw,¹² HLC was first described in English literature in 1976 by Englert. HLC is an uncommon spontaneous mononeuropathy that is typically characterised by a sudden onset of pain, followed by palsy in the territory of a specific peripheral nerve, generally around the joints of the upper limb.^5–19

There is still much that remains unknown about HLC, including its pathophysiology, diagnostic criteria and appropriate management. In addition, the special predilections in affecting branches of the radial (posterior interosseous nerve (PIN)) and median nerves (anterior interosseous nerve (AIN)) are yet to be elucidated.^{15 16} In this context, radial nerve (RN) involvement is rare. To the best of our knowledge, only 34 cases have been reported.3 5–8 16 19 Here, we report a patient with spontaneous RN palsy due to HLC (non-traumatic neuroma). In addition, we conducted a literature review, emphasising all aspects, from the clinical presentation to the management of such unusual lesions of the RN.

Case presentation

A man in his mid-60s presented with a 3-month history of right RN palsy. Wrist and finger drop developed acutely along with sensory dysfunction of the superficial RN territory without any precipitating events. The patient sought medical attention and was prescribed complex B vitamins (B₁, B₆ and B₁₂). Symptoms progressively improved over 3 weeks when the patient experienced acute aching pain at the anterolateral aspect of the right arm, followed by wrist and finger drop. The second event was associated with a strenuous effort using a sledgehammer.

The muscle strengths on the Medical Research Council Scale were 5/5 in the triceps, 3/5 in the brachioradialis (BR), 2/5 in the extensor carpi radialis longus (ECRL), extensor carpi radialis brevis (ECRB), and extensor carpi ulnaris (ECU), and 2/5 in the remaining finger extensor muscles, including extensor digitorum communis (EDC), extensor digiti minimi, extensor pollicis longus and extensor pollicis brevis. In addition, we found a strong Tinel sign approximately 8 cm above the elbow and a positive rule of nine for RN injuries. Sensory dysfunction was observed in the superficial RN, whereas median and ulnar nerve functions were not affected. The initial electrophysiological study performed 3 months after symptom onset revealed an amplitude reduction of compound muscle action potentials and motor conduction block at the level of the humeral spiral groove without changes in the sensory nerve action potential. EMG examination revealed denervation of the affected muscles with scarce neurogenic motor unit potentials in BR and EDC. The patient was administered oral steroid therapy for 4 weeks without success. In addition, worsening of motor function was observed in the wrist (ECRL/ECRB/ECU grade 0/5) and finger extension (EDC 1/5) during follow-up.

MR neurography (MRN) demonstrated oedema of the extensor compartment compatible with denervation changes and a diffusively thickened contrast-enhanced RN, except for a small nodular non-enhancing focal area approximately 8 cm above the elbow (figure 1).

Figure 1.

Sequential proton density-weighted contrast-enhanced images with fat suppression at the distal upper arm showing nodular hypointensity of the radial nerve (red circle) and marked signal elevation distally (yellow circle).

Differential diagnosis

The differentials for diagnosis of HLC are broad and include RN entrapment after muscular overexertion,¹ unusual tumours² and inflammatory NA.^{3 4} The acuteness of symptoms development, as well as progression and fluctuation, indicated that the neurological injury was more compatible with a focal inflammatory lesion of the RN.

Treatment

Therefore, RN exploration 5 months after symptom onset was recommended because of clinical and electrophysiological deterioration during conservative treatment. During surgery, the firm HLC was found to be intensely adherent to the BR muscle and closely related to the radial collateral artery. Internal neurolysis was unsuccessful, and resection of the neuroma (approximately 1 cm) was performed. The RN was primarily repaired without tension.

Outcome and follow-up

Eighteen months postoperatively, the patient experienced complete motor recovery for wrist extension, and incomplete, but functional recovery of small extensors and sensory function (figure 2). Histological sections demonstrated a complete loss of the architectural pattern, showing clusters of nerve fibres distributed in different directions, with fibrous tissue and dense scarring around the remaining fascicles. No signs of vessel occlusion or inflammation were observed. Therefore, a diagnosis of a neuroma was made (figure 2).

Figure 2.

Intraoperative photographs (upper row) demonstrating an HLC of the radial nerve at the distal upper arm. A severe constriction was revealed right after focal enlargement (white arrow). A 1 cm long specimen was resected and the nerve was primarily repaired. The patient experienced satisfactory RN motor recovery on the right side after 18 months (lower left). Histological sections obtained at the level of HLC confirmed complete loss of nerve fibres and intense fibrosis, leading to the diagnosis of a neuroma (lower right). HLC, hourglass-like constriction; PABCN, posterior antebrachial cutaneous nerve; RN, radial nerve.

Discussion

HLC of the RN is a rare disease, and as mentioned above, much remains unknown regarding its pathophysiology, diagnostic criteria and appropriate management. The association with a torsional deformity during intraoperative inspection has led some authors to credit the development of HLC to mechanical neuropathy caused by repetitive arm movements, such as pronation and supination, with anchoring at the take-off of branches from the major nerves.^{1 19}

Previous reports and recent literature indicate that the primary mechanism of HLC is potentially inflammatory, since nerve dysfunction occurs after a consistent progression of symptoms, typically, but not always starting with arm pain, followed by motor weakness and atrophy with limited or no sensory involvement.3 5 13 16 18 19 Such a clinical development resembles NA, also known as Parsonage-Turner syndrome, as it is hypothesised to be part of the NA spectrum.^{3 5 16} Approximately one-third of patients affected by NA experience atypical phenotypes involving major peripheral nerves, alone or in combination, that do not fit the typical clinical presentation of muscle weakness, pain or sensory dysfunction in diverse nerve territories.⁴ This association can be more problematic because patients with isolated spontaneous PIN palsy might have clinical and electrophysiological disturbances beyond PIN function in 36% of cases,²⁰ while patients who fulfil the diagnosis of typical NA might develop HLC on the peripheral major nerves outside the brachial plexus.³

Pan et al observed inflammatory cell infiltration (CD8-positive T-lymphocytes) in the nerve fibres and fascicles of HLC in histological specimens.¹⁶ In this context, nerve inflammation has been considered a triggering event, leading to intrafascicular oedema, local adhesion, scarring, fibrosis and endoneural ischaemia.2 6 13 17 20 21 Classic hourglass-like lesions are observed in cases of prolonged ischaemia,² with the peak in the inflammatory response at 4 months.¹⁶ In our patient, the typical progression of symptoms pointed out to an inflammatory pathogenesis that was secondarily exacerbated by mechanical RN anchoring due to local adhesion and scarring. Perhaps, the co-occurrence of inflammatory and mechanical pathogenesis might explain the steroid resistant progression of neuropathy.

From a clinical standpoint, pain is a sine qua non condition of the HLC involving the RN. In addition to muscle weakness, sensory dysfunction was documented in 5 out of 15 (33.3%) patients,3 6 7 19 which corresponds to a slightly higher prevalence when considering the whole group of upper-limb HLC (approximately 20% of the patients).¹⁶ Bilateral involvement and multiple constrictions were also reported.^{5 19} Five precipitating events have been were described: such as upper-arm trauma,⁵ spider bite,⁵ skiing,⁸ strenuous work³ and influenza-like syndrome.³ Table 1 provides a detailed description of previous studies on the HLC of the RN.

Table 1.

Summary of hourglass-like constriction of the radial nerve in English Literature

Author, year	No of Patients	Symptoms	Time to surgery	Surgical technique	Results
Belsole et al, 19785	1	Pain, muscle weakness	30 days	External neurolysis	Incomplete recovery (tendon transfer)
Burns et al, 19846	3	Pain, muscle weakness, Sensory dysfunction (1 patient) Precipitating event (2 patients)	3 weeks to 3 months	Internal neurolysis	Incomplete recovery (tendon transfer)
Fernandez et al, 20018	1	Pain, wrist and finger drop Precipitating event	40 days	Internal neurolysis	Complete recovery
Yongwei et al, 200319	5	Pain, muscle weakness, Sensory dysfunction (1 patient)	Within 3 months	Neurolysis in 3 Neurorrhaphy in 2 Nerve graft in 3	Complete recovery in 7 Incomplete recovery in 1 (neurolysis)
Pan et al, 20113	4	Pain, muscle weakness, Sensory dysfunction (2 patients) Precipitating event (2 patients)	2–10 months	Neurolysis in 1 Neurorrhaphy in 1 Nerve graft in 2	Complete recovery in 2 Incomplete recovery in 2 (neurorraphy and graft, 1 each)
Pan et al, 201416	19	Pain, muscle weakness, sensory dysfunction (21.4%)*	1–15 months	Neurolysis in 6 Neurorrhaphy in 8 Nerve graft in 5	Complete recovery (neurolysis 4, neurorraphy 3, graft 2) Incomplete recovery (neurolysis 2, neurorraphy 4, graft 2)
Ebrahimpour et al, 20137	1	Pain, motor weakness, sensory dysfunction	N/A	Neurorrhaphy	Complete recovery
Bretas et al, 2023	1	Pain, motor weakness, sensory dysfunction	5 months	Neurorrhaphy	Complete motor recovery for wrist extension, and incomplete, but functional recovery of small extensors and sensory function

*Both studies reported radial nerve constrictions indistinct from other upper limb lesions.

Imaging is generally not routine for investigating HLC since most of the previous reports performed surgical treatment relying on clinical and electrophysiological data.6 7 16 19 The use of high-resolution ultrasound (HRUS) and MRI offers an effective means to confirm electrophysiological findings and rule out structural lesions compressing the RN.^{20 22 23} HRUS has become a first-line modality for studying peripheral nerve disease because of several benefits over MRI, such as greater soft-tissue resolution, dynamic evaluation and the ability to scan the entire affected and healthy extremities,²² especially in experienced hands. Recently, MRN has had a progressively increasing diagnostic and therapeutic role for neuromuscular diseases, given that it provides a direct assessment of intraneural architecture and integrity.²³ Our imaging findings confirm the usefulness of MRN considering that different previous reports demonstrated normal RN appearance or distal diffuse nerve enlargement with indirect signs of denervation.1 2 8 20

The management of HLC is still controversial, but it generally involves a period of clinical observation, followed by surgery in patients who do not demonstrate signs of recovery during follow-up. The optimal timing for observation is yet to be defined; however, considering that natural recovery can be expected in half of the patients affected by spontaneous AIN and PIN palsy within 3 months and that a period greater than 7 months is associated with worse outcomes in PIN palsy,^{14 17} it would be reasonable to wait for spontaneous recovery between 3 and 7 months before proceeding with surgery.

Another controversial issue is the surgical technique. The choice of surgical technique should consider the severity and extent of nerve constriction so that mild to moderate constrictions might be treated by neurolysis, while severe constrictions are best managed by neurorrhaphy and/or nerve grafts.¹⁷ Regarding RN constrictions, the rates of complete recovery were 50%–70% (table 1), which is considerably lower than the surgical results obtained for PIN and AIN constrictions reaching 80%–100% complete recovery regardless of the technique.^{16 17} Even though, the reasons for these poor outcomes remain unclear, it could be likely related to the close proximity to the target muscles.

Conclusions

There is still much that remains unknown about HCL, especially for RN, a rare disease presented mostly in case reports. The current understanding points out that an inflammatory disease should be treated conservatively for 3–7 months before proceeding with surgery for patients who demonstrate no signs of recovery. The surgical technique depends mostly on the severity and extent of constriction. However, considering only RN constrictions, primary repair by neurorrhaphy or nerve grafts results in better functional outcomes.

Learning points.

• Hourglass-like constriction (HLC) is a rare spontaneous mononeuropathy characterised by sudden onset of pain, followed by palsy, with a special predilection affecting the posterior and anterior interosseous nerves.

• The current understanding suggests that an inflammatory disease should be treated conservatively for 3–7 months before proceeding with surgery.

• The surgical technique depends mostly on the severity and extent of constriction, but generally includes neurolysis, neurorrhaphy or nerve grafting.

Ethics statements

Patient consent for publication

Consent obtained directly from patient(s).