Uncommon Anatomical Causes of Ulnar Compression

Objectives Some patients develop ulnar nerve compression due to rare anatomical variations or malformations. The aims of this review are to provide a comprehensive overview of anatomical structures and variations that can cause ulnar nerve compression and to evaluate treatment options. Methods Case reports and case series about rare cases of unusual ulnar nerve compression published from January 2000 until April 2022 were obtained from databases Embase, MEDLINE, and Web of Science. A total of 48 studies describing 64 patients were included in our study. Results The following structures have proven to cause ulnar nerve compression: anconeus epitrochlearis, accessory abductor digiti minimi, vascular anomalies, palmaris longus, fibrous bands, and flexor carpi ulnaris. All cases except one have had a surgical release of the ulnar nerve resulting in diminished symptoms or complete recovery at follow-up. Conclusions In addition to considering common compression points, it is important to be aware that proximal compression symptoms, such as pain and a positive Tinel sign at the medial elbow, may be attributed to a hypertrophic AE or vascular anomaly. Distal compression symptoms encompass swelling, along with pain and a positive Tinel sign at the distal forearm. Various structures contributing to distal compression include an accessory abductor digiti minimi muscle, an accessory or anomalous palmaris longus muscle, or an accessory or hypertrophic flexor carpi ulnaris muscle. The occurrence of fibrous bands exhibits variability, manifesting in diverse locations across the arm. Level of Evidence: IV


METHODS
Three authors conducted a systematic search in Medline, EMBASE, and Web of Science.The search was last updated on May 2022.Guidelines provided by the Preferred Reporting Items for Systematic Reviews and Meta-analyses statement 7 were followed (Fig. 1).Furthermore, this study is in compliance with the ethical standards in the 1964 Declaration of Helsinki.

Eligibility Criteria
Studies were excluded if they concerned compression of the ulnar nerve at the following sites: the arcade of Struthers, exit of the ulnar nerve out of the FCU muscle, proximal of the medial epicondyle, and the sulcus of the ulnar nerve, under the aponeurosis that connects the 2 FCU heads (Osborne ligament) and Guyon canal.Furthermore, studies describing compression as a result of anomalous ulnar nerve predisposition and UNE caused by metabolic and endocrine diseases, neoplasms, ganglion cysts, traumatic causes, connective tissue, and infectious diseases were also excluded, as well as anatomical cadaver studies.The search was limited to articles in the English language.
Case series and case reports conducted between January 2000 and May 2022 were included.Checklists for case reports and case series formulated by Moola et al 8 were used to perform the critical appraisal.
The following study outcomes and measures were extracted: age, dominant hand, unilateral or bilateral, cause of compression, motor symptoms, sensory symptoms, presence of Tinel and Froment provocative tests, diagnostics performed and follow-up, signs, and symptoms during the follow-up.subsequent full-text articles were reviewed for eligibility, independently by 2 authors.The process of article selection is illustrated in (Fig. 1).Included studies showed a mean score of 75.1% on critical appraisal evaluation (scores on reporting of complications not included).Characteristics of the 44 included case reports and four case series are shown respectively in (Tables 1, 2).Fifty-six percent of the patients were male, and 44% were female.Patients had a mean age of 38.0 years.The following subgroups of compressors were identified: accessory abductor digiti minimi (AADM), anconeus epitrochlearis (AE), fibrous bands, palmaris longus (PL), vascular anomalies, FCU, and singular cases other than mentioned subgroups (Table 3).

Anconeus Epitrochlearis
The AE is a muscular structure located between the olecranon and the medial epicondyle.We identified 9 case reports [15][16][17][18][19][20][21][22][23] and 3 case series [54][55][56] describing a total of 23 patients (11 female, 12 male) with a mean age of 36 years reporting UNE caused by the AE muscle.In 22% of the patients, the AE was present bilaterally.All patients complained of sensory disturbances, such as numbness and tingling of the little finger and ring finger, and 52% of all patients (n = 12) also showed signs of motor impairment.Diagnostics were reported in 20 patients.In 15 patients, NCS or electromyography was performed, which showed abnormalities in nerve conduction in 93% of cases.Imaging studies were performed in 74% of the cases (MRI n = 5, radiographs n = 4, US n = 8) with results of MRI being inconclusive in one patient.The AE was excised in 87% (n = 20) of patients.In 2 patients (9%), subcutaneous anterior transposition of the ulnar nerve was performed 19,20 while the remaining 1 patient (4%) received hydrodissection. 23Overall, 52% of patients (n = 12) underwent neurolysis of the ulnar nerve.Mean recovery time was 11.3 months.At follow-up, 87% of all patients (n = 20) had fully recovered from their symptoms.The remaining patients, all of whom underwent excision of the AE, experienced improved but persistent numbness or weakness.(Table 4) 19,54,56 Accessory Abductor Digiti Minimi In 6 case reports [9][10][11][12][13][14] and 1 case series 53 describing 10 patients in total (7 male, 3 female, mean age, 32.7 years old), an accessory abductor digiti minimi was reported as a cause of ulnar nerve compression.In all patients, symptoms were unilateral and consisted of sensory complaints as, for example, numbness and pain.Clinical examination confirmed sensory alterations (paraesthesia, abnormal 2-point discrimination) in all patients.Nerve conduction studies showed abnormalities in 50% of the cases in which it was performed.Ultrasound and MRI were performed in 70% of patients and had a sensitivity of 29%.Upon surgical exploration, the AADM was originating from the antebrachial fascia (75%) or the PL tendon (25%) and inserted into the hypothenar musculature.In all cases, a (partial) excision of the anomalous muscle was performed resulting in complete recovery from their symptoms in all patients.
Electrophysiological studies were performed in 90% of all patients and were positive for sensory and motor disturbances in all but 1 patient.Excision of vascular malformation was performed in all 5 cases.In 1 patient, pain and paraesthesia had improved at 9-month follow-up, but motor recovery was not observed.In all the other patients, symptoms disappeared completely in a period of 1 to 14 months after treatment.
Four patients had arterial anomalies, which all presented different because of different compression points in the hand/forearm.

Palmaris Longus
A total of 6 reports 14,[32][33][34][35][36] (5 male patients, 1 female patient, mean age 36.2 years) have had UNE by an anomalous or accessory PL muscle presenting as either a 3 headed or reversed PL.In 83% of cases, symptoms were aggravated by some form of exertion.On physical examination, a mass was seen on the ulnar side of the forearm during wrist flexion in 50% of patients.Paraesthesia and hypesthesia in the distribution of the ulnar nerve were found in all cases.Electrodiagnostic studies were performed in 50% of patients, with EMG showing slowing motor and sensory signals at the wrist in 2 patients and no abnormalities in 1 case.All patients underwent surgical excision or transection of the aberrant PL muscle and recovered completely in an average of 7.6 weeks.

Fibrous Bands
Four case reports [24][25][26][27] described patients (1 male, 3 females, mean age 37.3 years) having fibrous bands upon surgical exploration.All patients experienced weakness, clumsiness, or difficulty in performing daily life activities.In all cases, electrodiagnostic studies were performed.Decreased nerve conduction velocity is mentioned in 3 cases.In 2 cases, different thick fibrous bands other than Osborn ligament were found compressing the ulnar nerve at the level of the FCU muscle heads.The third case showed a constricting band extending from the pisiform bone to the hook of the hamate bone, compressing the deep

Singular cases
Continued next page motor palmar branch of the ulnar nerve.In the fourth patient, an abnormal scar tissue functioning as a constricting fibrous band was originating from the transverse carpal ligament and extended to the palmar carpal ligament, causing subtotal obstruction of the ulnar artery.The dilated ulnar artery was impinging on the ulnar nerve proximal and distal to the site of compression.Symptoms disappeared in all patients and strongly diminished in 1 patient after excision of the fibrous bands.
No recurrence of symptoms was described.

Flexor Carpi Ulnaris
Four studies [28][29][30][31] described 5 patients (3 females, 2 males, mean age 32.8 years) experiencing UNE by an anomalous or hypertrophic FCU.All patients described weakness and numbness, while 2 cases showed a mass on the distal forearm and 3 cases had a positive Tinel sign.In 4 cases, an accessory or hypertrophic FCU was found by means of US and/or MRI and in 1 patient a hypertrophic FCU was ascertained  Ulnar Nerve Compression during surgical exploration.In all cases, surgical exploration and subsequent (partial) dissection of the FCU tendon were performed.Complete recovery was described in 3 patients and improvement of symptoms was described in one case.The remaining case report did not provide follow-up information due to relocation.

Conservative Treatment
Twenty of all 64 patients (31.3%) initially received conservative treatment, consisting of immobilization, analgesia or rest or a combination of these.No patients are described to have beneficiary effects of this treatment.

DISCUSSION
When seeking typical compression points, it is essential to remain vigilant for unusual cases.
According to our results, the nature of symptoms may be an indicator to a certain cause.Intermittent or postactivity complaints point to the presence of an AADM or PL.At last, clinical onset of symptoms at elder age may be indicative of a vascular anomaly causing UNE.Compression as a result of fibrous bands produces symptoms that are too variable to be clinically distinctive seeing as these bands can arise anywhere.This applies to the singular cases group as well.
Furthermore, during the examination of proximal and distal compression points, the following associated symptoms may be observed.Symptoms indicative of proximal compression, such as pain and a positive Tinel sign at the medial elbow, primarily suggest a hypertrophic AE or a vascular anomaly, particularly when ruling out most common compression points.
On the other hand, distal compression symptoms encompass swelling, along with pain and a positive Tinel sign at the distal forearm.Various structures contributing to distal compression include an AADM muscle, an accessory or anomalous PL muscle, or an accessory or hypertrophic FCU muscle.The manifestation of fibrous bands is exceptionally variable, and these bands can manifest anywhere in the arm.
Despite the various possible locations for compression, it is important to conduct a comprehensive evaluation of the entrapment site during the physical examination.The assessment of the compression site during the physical examination is crucial to focus subsequent medical imaging (such as ultrasound or MRI) or surgical exploration effectively.This not only ensures a more precise diagnosis but also facilitates targeted interventions, leading to improved patient outcomes and, for instance, preventing unnecessary surgeries that may not provide relief.

CONCLUSIONS
Recently published uncommon sites/and or structures causing the ulnar nerve compression include the following: accessory abductor digiti minimi, AE, fibrous bands, PL, vascular anomalies, flexor carpi ulnaris, and singular cases other than mentioned subgroups.To prevent delay in recovery a thorough physical examination of the complete upper extremity must be performed and documented.
When expanding list for the differential diagnosis beyond the common compression points, consider the following: proximal compression symptoms, such as pain and a positive Tinel sign at the medial elbow, can be attributed to a hypertrophic AE or vascular anomaly.Distal compression symptoms involve swelling, along with pain and a positive Tinel sign at the distal forearm.Various structures contributing to distal compression include an AADM muscle, an accessory or anomalous PL muscle, or an accessory or hypertrophic FCU muscle.The occurrence of fibrous bands has variability, manifesting in diverse locations across the arm.

FIGURE 1 .
FIGURE 1. Study inclusion process: during the identification phase, 2087 records were found through Embase, Web of Science, and MEDLINE.No other sources were used.Throughout the screening phase, the remaining 1612 studies were superficially screened and excluded if necessary based on the title and abstract.During the eligibility phase, the remaining records were thoroughly analyzed and excluded if they met the exclusion criteria we maintained.In the final phase, the remaining articles were included.

TABLE 1 .
Characteristics of Included Case Reports

TABLE 3 .
Summary of the Different Compressors and Numbers of Cases Found

TABLE 4 .
Schematic Overview of Rare Causes of UNE and Their Clinical Characteristics