Incidental Palmaris Profundus: An Aberrant Structure Encountered During Routine Carpal Tunnel Release

Carpal tunnel release (CTR) for carpal tunnel syndrome (CTS) is one of the most common surgical procedures of the hand. Multiple anatomical anomalies within the wrist and carpal tunnel have been reported in the literature [1]. We describe a case of palmaris profundus (PP) found incidentally during a left open CTR causing compression of the median nerve (MN). A 58-year-old office-worker female presented to the Plastic, Reconstructive and Hand Surgical Clinic with a longstanding history of left-hand paraesthesia, nocturnal discomfort, and reduction in grip strength in the median nerve (MN) distribution of the left-hand. These symptoms interfered with her activities of daily living. With nerve conduction studies confirming median nerve compression at the wrist, the patient as brought forward for a left open CTR. An unusual glistening and white structure was visualised deep to the transverse carpal ligament (TCL) and adherent to the underlying MN. The tendinous structure was within the same fascial sheath as the MN. Distally, it was continuous with the palmar aponeurosis of the hand. This aberrant structure was discrete to the palmaris longus, or other wrist flexors. The origin was not identified as proximal dissection into the wrist and forearm was not indicated. Figure 1A,B. This structure caused visualised compression of the MN and was accordingly meticulously resected. MN was then released. Haemostasis was achieved. The skin was closed directly, and the patient was sent home with a compression-dressing of the left hand on the same day. The patient was reviewed at the 1-week and 6-month follow-up mark. Paraesthesia, pain and functional limitation in the left MN distribution have resolved. She is managing her activities of daily living independently and is pleased with her postoperative progress. Palmaris profundus (PP) is an aberrant structure within the wrist and hand found incidentally during open carpal tunnel release (CTR). This anomaly was first described by Frohse and Frankle in 1908 [2]. Subsequent cadaveric studies indicate an approximate incidence of 0.125% [1]. PP is often encountered within the same fascial sheath as the median nerve (MN), giving rise to the description ‘musculus comitans nervi mediani’ [3]. Given the low incidence of PP, data mainly arises from a series of case reports and anatomical studies. Some authors have classified PP based on a proximal origin. Priola et al. note three distinct variations: Type I: arising from the radius, Type II: from flexor digitorum superficialis fascia and Type III: from the ulnar [4]. Most authors note a volar and radial relationship of PP to MN, particularly at the level of the wrist [4, 5]. Rarely, it may traverse in a radial to ulnar direction [6]. A review of available literature yields 21 case reports noting incidental identification of PP during open CTR. Tracing the origin of PP requires proximal dissection into the forearm. This is often not clinically indicated [4, 6, 7]. Thus data on incidence of proximal insertion is challenging to interpret. In all cases where an aberrant PP was identified, the insertion was noted to be the palmar aponeurosis. We did not identify a proximal origin in our case, as proximal dissection was not required. The PP in our case inserted onto the palmar aponeurosis. CTS is exacerbated by factors which may increase the pressure within the enclosed carpal tunnel. This includes oedema, synovitis or aberrant structures [5]. The presence of an additional muscle, PP, often investing into the same fascial sheath as the MN, is a discrete risk factor in CTS [3, 4, 6]. This is most often diagnosed at the time of surgery [3, 4]. Given a paucity of available literature, there is no clear guideline; management remains contentious. Challenges exist in establishing clear preoperative diagnoses of CTS from aberrant structural compression, including the PP. Preoperative imaging is not routinely undertaken for routine CTR; clinical examination alongside nerve conduction confirmation of MN compression is sufficient to diagnose and manage CTS [6, 7]. Case report data corroborates these inconsistencies. Out of 21 available reports, 18 provided information on whether the PP was directly compressing the MN. 15/18 (83%) cases note visualised compression of the MN. Most authors tend to resect PP upon identification of MN compression. This is often followed by symptom resolution (19/21, 90%). This carries minimal increased risk to the patient. We visualised direct MN compression from the volar MN within the same fascial sheath and resected accordingly. Our case similarly demonstrates post-resection symptom resolution. Only 1/18 (5.6%) cases with visualised MN compression from PP did not undergo resection [8]. Reasons for foregoing resection were not documented. This patient demonstrated recurrent symptoms 1 month later and required repeat open CTR with resection of PP and symptom resolution [8]. Preoperative imaging for establishment of CTS is not routine surgical practice [6, 7]. Specific imaging modalities such as dynamic ultrasound may be indicated in cases of recurrent or variant presentations [7]. Further investigation of the role of targeted imaging is required to establish the importance of preoperative scans in an otherwise routine operation. Our case and review of the literature highlights the importance of appropriate identification of this aberrant structure upon incidental encounter. Excision of the palmaris profundus is recommended to decompress the median nerve. Failure to resect may portend symptom recurrence or requirement for repeat surgery. Further research is required to investigate anatomical subtypes, incidence, and potential causative relationships with CTS. Development of a clear management guideline will be of use to surgeons encountering this variant in an otherwise routine operation. Figure 2 reference is included in the Manuscript V2 as [9]. The authors confirm that the patient has provided their informed and documented consent for the publication of images for the purposes of this manuscript. Data sharing not applicable to this article as no datasets were generated or analysed during the current study.