A total of 166 patients with UN mononeuropathy (102 males, mean age = 55 years) were included in the study, and, among them, we identified fifteen (9%; 7 males, mean age = 57 years) cases with SB/DB damage. Seven patients had selective damage of the DB, four patients of the SB, and four patients had a combined neuropathy of the SB and DB. Thirteen (86.6%) patients with SB/DB damage had already been examined with electrodiagnostic tests before US. Seven (46.7%) patients were submitted to MRI to integrate US results. However, in only three (20%) cases, the additional information provided by MRI was judged of some utility in the following work-up of patients. In general, MRI was requested with the following purposes: (i) to characterize solid soft tissue masses; (ii) to identify the origin of ganglia when not possible with US; (iii) to obtain a panoramic view of the regional anatomy and to investigate ancillary findings such as carpal bone fractures, extensive soft tissue derangement, and vessels damage. In all patients, imaging findings significantly impacted therapeutic decisions, particularly guiding the choice between conservative and surgical treatment.
Pathological findingsUS identified seven traumatic nerve injuries, two nerve tumors, and six entrapment neuropathies, including four cases of nerve compression by a ganglion cyst.
DB neuropathies — entrapment syndromesIn six cases, US disclosed an entrapment neuropathy of the DB after its origin from the UN. Fusiform swelling proximal to the site of entrapment and abrupt thinning at the entrapment point represented the hallmarks of US diagnosis. Overall, three levels of potential entrapment of the DB were identified in our series, respectively consisting of (I) proximal to the pisohamate hiatus; (II) from the pisohamate hiatus to the opponens digiti minimi; and (III) from the opponens digiti minimi to the thenar eminence.
In one patient, US disclosed an anomalous course of the DB inside the carpal tunnel with consequent nerve impingement at level one against the flexor retinaculum, the pisohamate ligament, and the hook of the hamate (Fig. 2). MRI was requested after US with the purpose of confirming the anomalous position of the DB inside the carpal tunnel, but the information added by this modality did not impact the following patient management. Three patients had a DB compression at level two by ganglion cysts arising from the pisotriquetral joint. These ganglia are usually a consequence of pisotriquetral joint osteoarthritis or instability and may extend into the pisohamate hiatus where they may impinge against the DB (Fig. 3). One of these patients required further investigation with MRI to better depict the cyst pedicle. In two patients, an entrapment of the DB distal to the opponens digiti minimi (level three) was diagnosed. In the first case, the DB was found entrapped by a fibrous band, and as US was not able to directly demonstrate the fibrous band, MRI was requested in the attempt to directly demonstrate the cause of nerve impingement (Supplemental Fig. 2). However, the revision of images showed that MRI did not add significant information to the one provided by US. The other patient had a mid-palm ganglion arising from the third carpometacarpal joint that squeezed the DB against the flexor tendons. In this case, MRI data were considered useful for surgical planning, as they provided a better depiction of the position of the ganglion and of its pedicle.
Fig. 2Level one entrapment neuropathy in a 29-year-old woman with progressive wasting of the interossei. a, b, c Consecutive short-axis 22–8-MHz US images from proximal to distal demonstrate a hypoechoic and swollen ulnar nerve (black arrow) at the proximal part of the Guyon canal, on the radial side of the pisiform (Pis). Distal to this level, the deep branch (white arrow) appears running in an anomalous position inside the carpal tunnel underneath the flexor retinaculum (thin arrows), on the radial side of a hypertrophied pisohamate ligament (asterisk) and, more distal, of the hamate hook, whereas the superficial branch (outlined arrowhead) keeps on running along the regular path on the side of the ulnar artery (arrowhead). Note the edematous changes and the swollen appearance of the deep branch compared to the regular median nerve (outlined arrow). Sca, scaphoid; FT, flexor tendons; APB, abductor pollicis brevis; PB, palmaris brevis; OP, opponens pollicis; FDM, flexor digiti minimi. d Long axis 22-8MHz US image better demonstrates the thickened pisohamate ligament (arrow). e Transverse turbo Spin Echo T2-weighted MRI scan with fat saturation shows the anomalous position of the deep branch (arrow) inside the carpal tunnel on the radial side of the hamate hook. The superficial branch (arrowhead) is normally positioned over the superficial aspect of the carpal ligament. Outlined arrow, median nerve. f The intraoperative picture confirms the swollen appearance of the deep branch (black arrow) before it engages the carpal tunnel underneath the flexor retinaculum (thin arrow). Arrowhead, superficial branch
Fig. 3Level two extrinsic compression of the deep branch in a 61-year-old man reporting impairment in performing fine movements with the right hand. a, b Consecutive short-axis 18–5-MHz US images and c Transverse turbo Spin Echo T2-weighted MRI scan with fat saturation demonstrate a ganglion cyst (asterisk) that squeezes the deep branch (arrowhead) against the pisohamate ligament (PHL) and the hamate hook (HH). Note mild edematous changes with nerve swelling in b. The superficial branch (outlined arrowhead) is unaffected by the cyst. FT, flexor tendons; thin arrow, flexor retinaculum
DB neuropathies — traumatic injuriesIn one patient with a history of blunt trauma over the ulnar aspect of the palm and progressive motor impairment in the territory of the DB, US diagnosed a hamate hook fracture with DB impingement against the bone fragment (Fig. 4). Following US, MRI was requested with the aim of disclosing eventual associated injuries, but the information added by this modality did not impact the subsequent management.
Fig. 4Deep branch impingement against an unrecognized hamate hook fracture in a 40-year-old male with motor impairment in the territory of the ulnar nerve after a blunt trauma over the palm. a Short-axis 22–8-MHz US image demonstrates severe edematous changes and swelling of the deep branch (arrow) as it runs on the ulnar side of the hamate hook (HH). A small piece of fractured bone (asterisk) is shown avulsed from the tip of the hook. The terminal divisions of the superficial branch (arrowheads) present a normal appearance as they run in between the palmaris brevis (PM) and the abductor digiti minimi (ADM) and flexor digiti minimi (FDM). Outlined arrow, ulnar artery. b Long axis 22-8MHz US image shows a swollen deep branch (arrowheads) presenting a distorted path around the level of the hamate hook. Note the abrupt change in caliper of the nerve at the hiatus (arrow), related to the impingement. c Transverse turbo Spin-Echo T1-weighted and d transverse turbo Spin-Echo fat suppressed T2-weighted MRI scans confirm swelling and edematous changes of the deep branch (outlined arrowhead) and the presence of a fracture of the hamate hook (asterisk). The ulnar artery (arrow) and the superficial branch (arrowhead) have a normal appearance
SB neuropathies — traumatic injuriesHigh-resolution US was able to recognize a selective traumatic injury of the SB in three patients. The first had a complete transection of the SB following surgical release of the flexor retinaculum for carpal tunnel syndrome (Supplemental Fig. 3). In this case, MRI was requested after US, but this modality did not impact the following management. Two patients were diagnosed with overuse compression injury selectively involving the SB. This condition is encountered in patients with chronic trauma over the hypothenar eminence and has been included in the spectrum of the so-called cyclist handlebar palsy, which may involve one or both the terminal divisions of the UN. The detection on US of abnormal nerve enlargement in a patient with a supporting history was considered sufficient for diagnosis.
SB neuropathies — soft tissue tumorsIn one patient, US allowed to demonstrate a millimetric lesion arising from the proper palmar digital nerve for the ulnar side of the little finger, a few millimeters distal to its origin from the SB (Supplemental Fig. 4). US findings allowed to hypothesize a diagnosis of schwannoma, which was confirmed at the histological exams through the identification of high levels of protein S100 expression in the lesion.
Combined neuropathies — traumatic injuriesThree patients were diagnosed with traumatic injuries involving both the SB and the DB. One had a high-energy trauma on the volar aspect of the hand with transection of both the SB and the DB. Two other cases had an overuse injury over the hypothenar eminence with concomitant involvement of both the terminal divisions of the UN (Fig. 5). In these two cases of combined cyclist handlebar palsy, pathological findings affecting the DB were detected at the level of the hamate hook, as a consequence of the close anatomical relationship between the nerve and the underlying bone.
Fig. 5Compression neuropathy of the superficial and deep branches in a 62-year-old amateur cyclist with sensory numbness in the territory of the ulnar nerve. a, b, c, d Consecutive short-axis 18–5-MHz US images show mild edematous changes affecting the deep branch fascicles (arrowhead) at the level of the pisiform (Pis), with a normal appearance of the superficial branch (outlined arrowhead). In this area, the ulnar artery (outlined arrow) appears normal. b Between the pisiform and the hamate, the superficial branch has an anomalous path underneath the abductor digiti minimi (ADM). Note the thickening of the ulnar artery walls as it crosses the edematous subcutaneous tissue of the hypothenar eminence. c At the hamate hook (HH) the superficial branch crosses (dashed arrow) from deep to superficial the proximal part of the flexor digiti minimi (FDM). d At the base of the V metacarpus (V met) the deep branch presents a normal appearance as it runs between the flexor digiti minimi and the opponens digiti minimi (ODM) whereas the superficial branch appears swollen as it crosses the edematous subcutaneous tissues. Note the thrombosed ulnar artery on the radial side of the superficial branch. Arrow, median nerve; thin arrows, flexor retinaculum; APB, abductor pollicis brevis; Ft, flexor tendons; asterisk, pisohamate ligament
Combined neuropathies — soft tissue tumorsIn one patient, US revealed a polylobate mass in continuity with the main trunk of the UN a few centimeters proximal to the Guyon tunnel (Fig. 6). At the distal Guyon, the lesion was demonstrated following the terminal division of the UN, with a deep part of the lesion engaging the pisohamate hiatus and growing around the DB and a superficial part expanding around the path of the SB. In this patient, MRI was requested for further characterizing the mass and to better demonstrate its anatomical location and its relationship with the adjacent structures. The integration of US and MRI data allowed to hypothesize a diagnosis of schwannoma, which was confirmed through histological analysis of the mass.
Fig. 6Ulnar nerve schwannoma involving the superficial and deep branch in a 54-year-old woman with an enlarging soft tissue mass on the ulnar side of the wrist. a The picture shows the appearance of the patient’s wrist at the time of US evaluation. b Short-axis 18-5MHz US image and c transverse turbo Spin-Echo T1-weighted MRI scan demonstrate a bilobate mass located in the soft tissue around the hamate hook (HH). The mass presents a deep and ulnar component (star) engaging the pisohamate hiatus and a superficial and radial component (asterisk) located superficial to the tip of the hook. Note the superficial branch fascicles (black arrowhead) displaced on the radial side of the superficial component of the mass and the deep branch (white arrowhead) compressed between the deep component of the lesion and the hook. d The intraoperative view shows the tumor arising from the main trunk of the ulnar nerve and extending distal with two distinct components into the superficial (black arrowheads) and the deep branches (white arrowhead). Histologic analysis confirmed a schwannoma
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