Ultrasound case report of a palmar lipoma causing carpal tunnel syndrome

Introduction

Lipomata are benign mesenchymal tumours that can develop in virtually any organ within the body. Soft tissue lipomata account for 50% of all soft tissue tumours, with deep-seated lipomata of the hands, feet, chest wall and retroperitoneum accounting for approximately 1% of lipomata. ¹ Of all benign tumours of the hand, lipomata are relatively rare, representing less than 5%. ² The exact aetiology of lipomata remains uncertain; however, there have been some suggestions of a link between trauma and subsequent lipoma formation, although this has not been proven. ¹ It is unclear whether a soft tissue lipoma represents a benign neoplasm, a local hyperplasia of fat cells or a combination of both processes.

The ultrasound appearance of a lipoma is typically that of a hyperechoic mass, which is thought to be as a result of the fat content. However, lipomata can have variable appearances and can be hypoechoic or isoechoic; a significant number can be ill-defined. Although colour Doppler can help exclude liposarcoma, it is difficult to to be certain with ultrasound, as lipomata can often demonstrate some internal vascularity. Magnetic resonance imaging (MRI) is superior for a confident identification of adipose tissue within these lesions. ¹

Carpal tunnel syndrome is common and results from compression of the median nerve at the level of the wrist. It is usually idiopathic; ² any condition that leads to an increase in the pressure within the carpal tunnel can compress the median nerve, resulting in neuropathic changes. Secondary compressive causes are rare. ³ Predisposing factors may include anatomical variations, susceptibility of the nerve to pressure or space-occupying lesions. ⁴ The incidence of lipomata related to the flexor tendon and causing carpal tunnel syndrome is uncertain but is considered rare.^5,6

Advances and refinement of ultrasound transducer technology have enhanced the ability to detect subtle abnormalities in carpal tunnel syndrome, such as changes to the median nerve. The advantages of assessing palpable lumps with diagnostic ultrasound are now well documented. The use of high-frequency transducers allows excellent resolution in the near field, making it the preferred initial imaging technique for superficial soft tissue structures, such as those in the hand and wrist. It can be used to assess mass size, appearance and location in relation to adjacent structures. Internal vascularity can also be assessed using colour or power Doppler imaging, the presence of which can be suggestive of a neoplastic mass.

However, many cases reported within the literature still advocate the use of MRI to provide clinical information. A recent case report of a giant lipoma of the hand concluded that MRI was an essential part in preoperative planning to ensure a successful outcome. ³ This case report highlights that ultrasound can provide key diagnostic information without the need for further imaging.

Case report

The patient presented originally with a palpable lump in the volar aspect of the right hand, which had been present for 6 months. There was some pain around the wrist, which would radiate up towards the shoulder. The pain was not associated with any movement or action but occurred unpredictably and sometimes awoke the patient at night and could present when just sitting.

The patient was referred to the ultrasound department for assessment of a likely ganglion associated with the volar aspect of the right wrist. The ultrasound was performed using a Philips IU22 system (Philips, Eindhoven, Amsterdam) with a 17-5 MHz linear transducer. The scan revealed a well-defined, homogenous, echogenic structure measuring 9 × 19 × 8 mm (Figure 1). No vascularity was demonstrated within the lesion using power Doppler. The structure was seen to move freely from the palm into the carpal tunnel on flexing of the wrist. With dynamic scanning and by independently flexing the distal and middle phalanges, it was possible to identify that the structure appeared intimately involved with the flexor digitorum superficialis tendon, rather than the flexor digitorum profundus tendon of the middle finger. Ultrasound appearances were suggestive of a synovial sheath lipoma. The flexor tendons appeared otherwise normal. An incidental finding of a bifid median nerve was identified (Figure 2). The nerve appeared otherwise normal. OPEN IN VIEWER

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Figure 2

Bifid median nerve indicated by the blue arrows

On presenting back to the doctor, approximately one month later, there had been some progression of symptoms with paraesthesia radiating into the median nerve distribution. The patient was referred for surgery for removal of the probable lipoma.

At pre-assessment for surgery, two months later, the symptoms had progressed further with a clunk at the front of the wrist with flexing of the middle finger. There was also some associated soft tissue swelling. The symptoms of carpal tunnel syndrome, in addition to symptoms from the mass, were established.

The patient underwent surgery. Under local anaesthetic, the transverse carpal ligament and distal palmar fascia were released longitudinally, revealing marked tenosynovitis of the flexor tendons and a well circumscribed lump within the flexor digitorum superficialis tendon sheath (Figure 3). This was excised and submitted for histology and was found to be a simple lipoma. This patient made a full recovery, with no further carpal tunnel symptoms. OPEN IN VIEWER

Discussion

Ultrasound can be used as a first-line imaging technique for the assessment of wrist and hand masses. ⁷ Previous case reports of palmar lipomata causing carpal tunnel syndrome or trigger wrist, however, have only described the use of MRI for imaging investigations.

Lipomata of the joint or tendon sheath are rare. The ultrasound appearances of the lipoma in this patient are typical of a benign lipoma. It was solitary, well-defined, echogenic, ovoid in shape, compressible with gentle transducer pressure and demonstrated no vascularity within with power Doppler. Lipomata can, however, have a variety of appearances. Hand lipomata have been described as presenting in two ways. ¹ First, in its rarest form, it may present as a discrete solid fatty mass in the affected joint or tendon sheath, as in this particular case. Second, and more commonly, it may present as a lesion composed of hypertrophic synovial villi, distended with fat, called lipoma arborescens. This type of lipoma extends in an elongated manner among muscles and flexor tendons, with ill-defined margins, which can make differentiation with ultrasound difficult, especially in the absence of a palpable mass. In addition, the echogenicity of the lipoma can vary depending on the amount of cellularity, fat and water within the mass, again making it difficult to differentiate from surrounding tissues.

Lipomata can be characterised as superficial or deep. The exact definition of deep-seated lipoma is uncertain. However, it has previously been defined as any lesion below the superficial fascia. ¹ Superficial lipomata, such as those commonly found within the subcutaneous tissues, are usually straightforward to assess with ultrasound. Deep-seated lipomata, however, may be more difficult to differentiate. MRI is useful in cases of suspected occult lipomata. However, in this particular case, the palpable mass, although deep seated, was easily identifiable on ultrasound.

Arguments for the use of MRI for deep-seated structures have usually been around the usefulness to assess masses in relation to adjacent structures, which is an important factor for surgical resection. Ultrasound in this case was able to accurately assess with dynamic scanning and flexion of the fingers that the lipoma was intimately involved with the flexor digitorum superficialis tendon of the middle finger. It is also important to note that an anomalous bifid median nerve was also identified with ultrasound, which was confirmed during subsequent surgical management.

The cause of this patient’s neurological symptoms, from carpal tunnel syndrome, were also established with ultrasound. Carpal tunnel syndrome is usually spontaneous and about a half of cases are unilateral. A space-occupying lesion should therefore be considered in patients with unilateral symptoms. ² Diagnosis of carpal tunnel syndrome is generally based on clinical assessment and occasionally neurophysiology tests; it usually does not require imaging. The advances in image resolution of ultrasound have led to the ability to detect fine changes in the median nerve as a result of carpal tunnel syndrome.

However, although subtle changes of the median nerve can be identified with ultrasound in carpal tunnel syndrome, very often the nerve will appear normal. ⁴ No obvious changes to the median nerve were identified on ultrasound in this patient, although the cause of neurological symptoms was clearly established. As with dynamic scanning on wrist flexion, the lipoma could be seen to enter the carpal tunnel, causing momentary stenosis of the tunnel, with compression of the adjacent tendons, the median nerve and bulging of the flexor retinaculum (Figure 4). OPEN IN VIEWER

Trigger wrist is a relatively rare phenomenon and the pathological entities to which the term trigger wrist is applied are not well defined in the literature. ⁸ The term trigger wrist has been applied in this case, with clicking heard on flexing of the finger, although it has been suggested that trigger wrist should only be used when triggering occurs specifically on wrist movement. ⁸ The causes of trigger wrist have been classified into three categories: tumour originating from the flexor tendon; an anomalous muscle and a combination of both. ⁹ Triggering of the flexor tendon at the wrist is rare. ⁶ There have only been a few reported cases within the literature of a lipoma causing trigger wrist. As far as we could establish, there are only two previous reported cases of a synovial sheath lipoma causing trigger wrist and carpal tunnel syndrome simultaneously.^6,10

It was interesting that at the time of the scan, there was no evidence of flexor tenosynovitis, although at surgery this was found and seen to be quite significant. The surgery was however approximately five months after the ultrasound examination. A similar finding was reported by Imai et al. ⁶ There was no evidence of triggering of the wrist either at the time of the scan. Imai et al. ⁶ suggested that the intrasynovial lipoma may have contributed to the development of adhesions. This present case would concur as there were no signs of inflammation of the tendons at the time of the scan, yet the ongoing lipoma was the eventual likely cause of the tenosynovitis.

Conclusion

This case report highlights all the advantages of using ultrasound, especially the benefits of dynamic scanning, in the assessment of common as well as the more rare soft tissue masses. It can be used as a first-line imaging investigation and in many cases other imaging techniques may not be required. MRI can be reserved for cases where the diagnosis is uncertain, where accurate definition of local anatomy is required or if malignancy is suspected.