An accessory belly of the abductor digiti minimi muscle: a case report and embryologic aspects

Accessory fasciculi of the hypothenar muscles have been involved in vascular and nerve compressions. During a routine dissection an accessory belly of the abductor digiti minimi muscle arising from the tendon of the palmaris longus muscle was found in the lower third of the forearm. The accessory fasciculus ran through Guyon’s canal enclosing the ulnar nerve and vessels. It was attached by means of two tendons where the fibres of the abductor digiti minimi muscle ended in a single pennate form. This anatomic variation was associated with a marked reduction of the caliber of the fourth tendon of the flexor digitorum superficialis muscle and a split of the median nerve. The nerve supply arose from the ulnar nerve. A fibrous band originating from this accessory muscular belly was found covering the median nerve. Based on the development of muscles and fibrous structures within the hand and forearm, as well as on our results, we consider the present anomalies as an unusual persistence of an undifferentiated group of mesenchymal cells. These belong to the superficial muscular anlagen layer of the hand, just between the flexor digitorum superficialis muscle blastema (which has the capacity of migration) and that for the abductor digiti minimi muscle.     

An unusual termination of the ulnar nerve in the palm.

During the dissection of the left forearm and hand of a 57-year old male cadaver fixed in 10% formalin, it has been noted that the medial proper palmar digital nerve to the little finger arose from the dorsal branch of the ulnar nerve, instead of the superficial branch of the same nerve. The dorsal branch, given off by the ulnar nerve in the forearm, coursed distally and dorsally deep to the flexor carpi ulnaris muscle. Some 2 cm proximal to the pisiform, it pierced the deep fascia on the posteromedial side of the muscle to become superficial. At this point, one of the three branches given off travelled distally on the palmar-ulnar side of the hand to the skin of the little finger. There were connections between this branch and the branches of the superficial branch of the ulnar nerve which innervated the skin of the hypothenar eminence. Further, another branch of the superficial branch of the ulnar nerve passed under the fibrous arch of the flexor digiti minimi brevis muscle origin and the opponens digiti minimi muscle to re-unite with its parent nerve.     

Variation in the hypothenar muscles and its impact on ulnar tunnel syndrome

Compression of the ulnar nerve at Guyon’s canal can be caused not only by tumor-like structures, a fibrotic arch, a ganglion, lipoma, aneurysm or thrombosis but also by anomalous hypothenar muscles which are reviewed here. For the search of relevant papers, PubMed and crucial anatomical textbooks were consulted. The abductor digiti minimi is the most variable hypothenar muscle. It can possess one to three muscle bellies. Additional heads can arise from the flexor retinaculum, the palmaris longus tendon, the pronator quadratus tendon or the deep fascia of the palmar side of the forearm. Our own case of an aberrant abductor digiti minimi appearing like connective tissue and originating in the antebrachial fascia is included here. Hematoxylin and eosin staining revealed that macroscopically non-muscle-like tissue contained skeletal muscle tissue. The muscle itself resembled other described cases. In addition, at the flexor digiti minimi accessory heads with origin from the flexor retinaculum, the antebrachial fascia or the long flexor muscles of the forearm can be detected. By contrast, the opponens digiti minimi mostly lacks variations and is sometimes missing. In our opinion, this is due to its hidden location. However, in few cases an additional head can arise from the lower arm aponeurosis. Furthermore, additional (fourth) hypothenar muscles might be expressed. These muscles are characterized by origins in the forearm and insertions on the head of the 5th metacarpal bone or on the 5th proximal phalanx. It must be noted that accessory hypothenar muscles might look like connective tissue at first glance. Often their origin extends to the antebrachial fascia. This can be explained by the phylogenetic fact that all intrinsic muscles of the hand are derived from muscle masses that originated in the forearm. In the opinion of several authors, ulnar nerve compression mostly is evoked by hyper trophied variant hypothenar muscles due to overuse as for example in carpenters. In some rare cases, an aberrant hypothenar muscle can also evoke median nerve compression.     

小鱼际肌肌内神经和肌梭密度分布

目的揭示小鱼际肌肌内神经和肌梭密度的分布规律。方法对成年20具尸体(40例)小鱼际标本用改良Sihler肌内神经染色法和苏木精-伊红(HE)染色法。结果小鱼际肌肌外神经干长0.49～1.64 cm,从肌起端深面入肌。小指展肌内尺侧和桡侧有独立的神经支配,可分为两个神经肌亚部;小指短屈肌的肌内神经干斜行穿越肌实质中央;小指对掌肌内神经吻合形式多样,"U"型吻合更明显。3块肌的桡侧部神经分支分布密集。小指对掌肌肌梭密度最高为(19.33±2.72)个/g;小指短屈肌其次,有(15.79±1.33)个/g;小指展肌最少,为(12.86±1.69)个/g。结论三块肌桡侧部更多地参与精细调节;肌块越小,肌梭密度越高。     

Muscle variations and abnormal branching and course of the ulnar nerve in the forearm and hand

During dissection of the right forearm of a 27-year-old female cadaver, variations in the form and insertion of the palmaris longus muscle were observed. The tendon of the palmaris longus muscle, which demonstrated a centrally placed belly, split into two tendons: one inserted into the palmar aponeurosis and the other into the proximal part of the flexor retinaculum. Additionally, we found an accessory muscle extending between the flexor retinaculum and the tendon of the abductor digiti minimi muscle. This accessory muscle was located deep to the ulnar artery but superficial to the superficial and deep branches of the ulnar nerve at the wrist. Finally, an aberrant branch of the ulnar nerve was identified in the forearm; it traveled distally alongside the ulnar artery and in the palm demonstrated communications with common palmar digital nerves from the ulnar and the median nerves. No variations were observed in the contralateral upper limb.     

Undescribed variant muscle – “Deep abductor-flexor” of the little finger, in relation to ulnar nerve compression at the wrist

During routine anatomical dissection in the hypothenar region of the left hand of a 64-year-old female cadaver, a number of variant structures were observed. The most prominent finding in our case was a supernumerary muscle hitherto unknown in the anatomical literature. This variant muscle had a muscular body formed by the connection of two deeply situated muscular bellies--medial and lateral. The lateral belly originated from the flexor retinaculum, the medial one--from the hamate bone. The common muscular body inserted to the antero-lateral surface of the base of the fifth proximal phalanx. Due to its location and possible function, we named the variant muscle "deep abductor-flexor" of the little finger. The flexor digiti minimi brevis muscle showed two proximal tendons--the medial tendon was attached to the hamulus of the hamate bone while the aberrant lateral tendon originated from the lateral part of the flexor retinaculum. Both, the aberrant lateral tendon of the flexor digiti minimi brevis and the lateral belly of the "deep abductor-flexor", passed over the palmar branch of the ulnar nerve, which define their possible clinical significance in ulnar nerve compression. Therefore, the variations of the hypothenar muscles are reviewed and their relation to the compression of the ulnar nerve is discussed.     

Fiber type composition of monkey forearm muscle

Histochemical staining methods were applied to selected superficial forearm muscles of Macaca mulatta monkeys. The muscles were analyzed with regard to relative percentage distribution of different fiber types. In extensor carpi radialis brevis, extensor carpi radialis longus, and palmaris longus there was an even dispersion of each fiber type from the superficial to the deep part of the muscle. Extensor digiti communis showed a slightly higher percentage of type I fibers and correspondingly lower percentage of type II fibers in its central as compared to its superficial area. Three muscles, bracioradialis, extensor carpi ulnaris, and flexor carpi radialis, displayed marked differences between their superficial and deep areas. All of them contained a higher proportion of type I fibers (and correspondingly lower percentage of type II fibers) in their deep parts than in their superficial areas. Flexor carpi ulnaris (FCU) differed from the other muscles studied in that it showed distinctly different fiber proportions on either side of a central tendon. While the ulnar head of FCU was dominated by type II fibers (71% compared to 27% type I fibers), the humeral head contained a larger proportion of type I fibers (58% vs. 40% type II fibers). This difference in fiber type distribution suggests different functional demands for the two heads of FCU, with the possibility of more sustained activity in the humeral head.     

小指对掌肌腱弓的解剖学研究

目的探讨Guyon管以远尺神经深支卡压征的解剖学基础。方法对20例新鲜成人上肢标本腕部进行显微解剖，肉眼及镜下观测腕部尺神经分支与周围组织的关系。结果尺神经穿出Guyon管后进入小指对掌肌浅、深两头和钩骨钩之间的间隙，该间隙可分为顶、底和桡侧壁，有一个入口和一个出口，暂称其为小指对掌肌间隙。小指对掌肌浅头腱性起点的近侧缘锐利，呈腱弓样，存在率100％，暂称其为小指对掌肌腱弓。结论小指对掌肌腱弓可以卡压尺神经深支，引起除小鱼际肌以外的尺神经支配的所有手内在肌的功能障碍。     

Anatomical variations as potential risk factors for ulnar tunnel syndrome: a cadaveric study

The aim of this study was to assess the anatomical variations, especially the anomalous muscles passing through Guyon's canal and the fibrous arch forming the piso-hamate hiatus, which may play a role in ulnar tunnel syndrome. We have also focused on the relation of these structures with specific concern to the ulnar nerve. Nineteen embalmed cadavers (37 hands and forearms) were dissected. A fibrous arch extending between the pisiform and the hook of the hamate was observed in 21 hands. In majority of the cases flexor digiti minimi muscle was found to originate only from this arch. An anomalous muscle was disclosed in six hands with four of them passing through the piso-hamate hiatus with the deep branch of the ulnar nerve. In two of four cases, the superficial branch of the ulnar nerve was also accompanying the deep branch of the ulnar nerve beneath the anomalous muscle and through the piso-hamate hiatus. Because these anomalous muscles were generally found to course through the piso-hamate hiatus with the branches of the ulnar nerve, we conclude that the distal portion of the Guyon's canal has a relatively higher risk for ulnar nerve entrapment. We believe that surgeons operating on this region should take into account these various anatomic structures.     

Unusual vascularization and nerve supply of the fifth finger.

During a dissection course, an unusual variation in vascularization and nerve supply of the fifth finger of the right hand was found in a 73-year-old human male cadaver. The proper digital palmar artery arose from the superficial branch of the ulnar artery distel to the Guyon's canal. The proper digital palmar artery coursed first palmar to the ulnar nerve then between the abductor minimi and flexor digiti minimi muscles to reach the ulnar side of the fifth finger. Besides this, the ulnar border of the fifth finger was supplied on its palmar side by the dorsal branch of the ulnar nerve, which gave off two tiny twigs for the sensory innervation on the dorsal side of the same finger.     

Accessory muscle in the hypothenar region: a functional approach.

An accessory muscle was found in the hypothenar region on both hands during routine cadaver dissection. This muscle originated from the tendon of the flexor carpi radialis, crossed the palma manus region superficially and inserted together with the abductor digiti minimi muscle into the ulnar aspect of the basis of the fifth proximal phalanx. The muscle was supplied by one branch arising from the main trunk of the ulnar nerve. Abnormalities of the hypothenar muscles have been described by many authors with a focus on their structural aspects, but there is not enough data about the possible functions they could induce. In our study, we try to elucidate the functions of this accessory muscle. We did not name the variant muscle as it has various functions, each similar to that of individual hypothenar muscles.     

Characteristics of the electrophysiological activity of muscles attached to the transverse carpal ligament in carpal tunnel syndrome

The main cause of carpal tunnel syndrome (CTS) remains unknown. Stiffness of the subcutaneous area of the volar aspect of the carpal tunnel is present in many patients and suggests that the stiffness of muscles attached to the transverse carpal ligament is increased. We performed an electrophysiological study to investigate muscle activities and to clarify whether the stiffness of muscles attached to the transverse carpal ligament is involved in the pathogenesis of CTS. The subjects of this study included 16 patients with early CTS showing no motor dysfunction. Both thenar muscles (opponens pollicis, abductor pollicis brevis, and flexor pollicis brevis) and hypothenar muscles (opponens digiti minimi, abductor digiti minimi, flexor digiti minimi brevis) were investigated. Surface electrodes were placed on each muscle, and maximum voluntary contractions with the thumb and little finger in opposition were maintained for 3 seconds in all patients and in 7 control subjects. Electromyographs were subjected to fast Fourier transform analysis, and the root mean square (RMS) and the mean power frequency (MPF) were determined for each muscle. The RMS of the opponens pollicis was significantly less in hands affected by CTS (292.8 μV) than in healthy hands (405.9 μV). The RMS did not differ between affected hands and healthy hands for the other 2 thenar muscles but did differ significantly for the hypothenar muscles. The MPF did not differ between affected hands and healthy hands for any muscle. The results show that electrophysiological differences are present among muscles innervated by the median nerve and that hypothenar muscles originally unrelated to median nerve dysfunction are also affected in early CTS. These results suggest that modulation of muscles attached to the transverse carpal ligament is involved in the pathogenesis of CTS.     

Unusual origin of the flexor digiti minimi brevis muscle

An unusual origin of the flexor digiti minimi brevis muscle of the right hand was discovered during cadaveric dissection. The muscle originated from the anterior aspect of the transverse fibers of the distal antebrachial fascia and inserted onto the base of the fifth proximal phalanx. When traction was applied to the proximal portion of the muscle, flexion was produced at the fifth metacarpophalangeal joint. Other intrinsic muscles of the hand and the muscles of the flexor forearm compartment had normal morphology. Although muscular anomalies in the hypothenar region have been described, the muscular variant described here was distinct in its origin and size from those previously reported. Knowledge of anomalous muscles in the hand has important clinical significance in routine surgery and in determining associated pathology.     

A case of an accessory palmaris longus muscle and a duplicate palmaris longus muscle with special reference to their nerve supply--morphologic significance of a common innervation trunk

During a dissection of the forearm and hand, a duplicate palmaris longus muscle with an accessory palmaris longus muscle was observed on the right side of a 73-year-old Japanese male cadaver. Duplications of the palmaris longus muscle have been reported by many authors. Humphry (1872) suggested the presence of radial, intermediate, and ulnar sectors in the superficial layers of the forearm flexor muscular angulus, based on a comparison of fore- and hind limbs and comparative anatomical theory. The palmaris longus muscle usually differentiates from the intermediate sector but differentiation from the other two sectors may also be possible. Some authors have asserted that a common innervation trunk is critical for determining an ontogenetic relation between the muscles (Fuchino, 1960; Honma, 1980; Yamada, 1986). We examined the nerve supply in addition to scrutinize these anomalous palmaris longus muscles. In our case, the ramification of the innervating nerves was specific. The branches to the second palmaris longus muscle and the flexor carpi radialis arose as a common trunk from the median nerve. The branches to the first palmaris longus muscle and the accessory palmaris longus muscle originated as another common trunk from the median nerve. From these observations, we speculated that the second palmaris longus muscle has differentiated from the flexor carpi radialis, while the accessory palmaris longus muscle has differentiated from the first palmaris longus muscle, based on Humphry's suggestion.     

足底内侧和外侧群肌的肌内神经整体分布模式及临床意义

目的 揭示足底内侧和外侧群肌的肌内神经整体分布模式,探讨其临床意义.方法 24具成年尸体,完整取下足底内侧和外侧群肌,采用改良Sihler染色显示肌内神经分布模式.结果 (足母)收肌的神经支从肌止端的深面入肌,而(足母)展肌、(足母)短屈肌、小趾展肌和小趾短屈肌的神经支常从肌起端的深面入肌.(足母)展肌中有1个半月形和...     

Long Abductor Digiti Minimi Muscle: Variation of the Hypothenar Muscles and Clinical Consequences

Variations in the hypothenar muscles influence the susceptibility of Guyon's canal and surgical procedures in this region. The hypothenar regions of both hands of 38 human donors were dissected and the single muscles identified. A semiquantitative evaluation included 19 donors. The opponens digiti minimi and the abductor digiti minimi were constantly present. The flexor digiti minimi showed various appearances: in 58% it formed one belly, in 21% two bellies, and in 21% it was missing. Rare unilateral supernumerary muscles (2.5% in this study) were associated to the abductor digiti minimi. The variation of the flexor digiti minimi and the possible appearance of additional muscles should be recognized for hypothenar region pathologies. Clin. Anat., 33:643–645, 2020. © 2019 Wiley Periodicals, Inc.     

尺神经深支的应用解剖

目的:为尺神经深支卡压和损伤的临床诊治以及高位尺神经损伤直接移植修复尺神经深支提供解剖学依据.方法:选择30侧自肘以上离断新鲜完整无畸形上肢进行解剖,观察尺神经深支的走行和分支.测量小鱼际各肌支的长度、宽度和厚度;尺神经深支相邻肌支间距离;豌豆骨近缘至深支最末分支间长度;尺神经深支逆向可分离长度和分离后的近端宽度和厚度,骨间前神经旋前方肌支远端宽度和厚度.结果:尺神经深支分支复杂,呈树枝状结构,在其走行过程中经Guyon管、豆钩管、对掌肌管、拇收肌腱弓等,解剖关系复杂.相应骨间肌支和蚓状肌支大部在尺神经深支主干同一平面呈不同角度发出.结论:尺神经深支易卡压部位多,可能是高位尺神经损伤后手内在肌功能不易恢复的原因之一;邻近骨折脱位较易损伤尺神经深支;高位尺神经损伤早期行神经移位直接修复尺神经深支,有利于手功能的恢复.     

A Case of the Bilateral Duplicate Palmaris Longus Muscles Coupled with the Palmaris Profundus Muscle

The palmaris longus muscle is one of the most variable muscles in human anatomy. During a routine anatomical dissection for medical students at Tottori University, we found duplicate palmaris longus muscles in the bilateral forearms together with the palmaris profundus muscle in the right forearm. The bilateral aberrant palmaris longus muscles were observed at the ulnar side of the palmaris longus muscle and their distal tendons were attached to the flexor retinaculum. The palmaris profundus muscle found in the right forearm was located at the radial side of the flexor digitorum superficialis muscle. The proximal tendon was originated from the anterior surface in the middle of the radius, while the distal tendon coursed radial to the median nerve through the carpal tunnel, finally inserting into the distal part of the flexor retinaculum. Both the palmaris longus and aberrant palmaris longus muscles were innervated by the median nerve. The palmaris profundus muscle was presumably supplied by the median nerve.     

肘管综合征的临床和电生理分析

目的：探讨肘管综合征（CUTS）患者的临床和电生理特点,并分析可能的病因。方法：回顾性分析69例CuTS患者（共73侧肢体）的临床和电生理资料。神经传导检测时,采用表面电极刺激和记录。分别于腕部、肘下5cm、肘上5cm刺激尺神经,记录小指展肌复合肌肉动作电位（CMAP）。腕部刺激尺神经,于小指逆向记录感觉神经动作电位（SNAP）。测量不同节段尺神经的运动传导速度（MCV）和感觉传导速度（SCV）。针电极肌电图观察尺神经所支配肌肉静息状态下的异常自发电活动。结果：①前臂尺侧、小指及无名指尺侧半麻木48例（70％）,上肢麻木及力弱19例（28％）,肘部疼痛或活动受限5例（7％）;第一骨间背侧肌萎缩28例（41％）,小鱼际肌萎缩15例（22％）,爪形手6例（9％）;②CuTS原因不明者53例（77％）;长期、频繁操作电脑所致者8例（12％）;③尺神经肘上-肘下节段MCV减慢[（29．91±5．09）m／s],肘下一腕段正常[（50．88±4．63）m／s]。小指展肌CMAP波幅降低,4只上肢未引出CMAP波形;尺神经腕一小指节段SCV正常[（47．43±7．27）m／s],SNAP波幅降低,36只上肢未引出SNAP波形;肌电图显示异常自发活动者,小指展肌和第一骨间背侧肌73肢,尺侧腕屈肌29肢。结论：CuTS临床表现以前臂尺侧、小指及无名指尺侧半麻木为主,伴力弱、疼痛及尺神经支配肌肉萎缩;电生理表现以尺神经肘上、下段MCV减慢为主,伴感觉纤维受损;病因以特发性占多数,长期、频繁操作电脑为另一重要因素;神经电生理检测为诊断CuTS的可靠手段,能明确神经受损受压的部位和程度。