Malignant nerve sheath peripheral tumours (MNSPTs), a type of soft tissue sarcoma, arise from Schwann cells of peripheral neural tissue or within existing neurofibromas (in patients with existing neurofibromatosis). A malignant triton tumour (MTT) is a histological subdivision of these tumours, with diagnosis based on the finding of rhabdomyosarcomatous differentiation alongside malignant Schwannoma on biopsy. In 50–69% of patients, MTT is diagnosed in the presence pre-existing neurofibromatosis type 1 (NF-1 or Von Recklinghausen’s disease).1,2
The annual incidence of soft tissue sarcomas in the USA—from data of the 2006 report from Surveillance Epidemiology and End Results branch of National Cancer Institute—is estimated as 12.8 per million per year in the 25–29-year-old age group for the year 2000 (11.3 per million in period 1975–2000).3
One estimate is that malignant peripheral nerve sheath tumours constitute 5–10% of tumours within this classification group,4
or 0.0001% in the general population,5
and that ‘on occasion’ they can be shown to be a MTT.6
Beyond this, the precise incidence of MTT remains unknown. It is considered to be relatively rare and as a result has been described predominantly in single case reports and small case series.7
Classification criteria for MTT were proposed by Woodruff et al
in 1973 following early case reports of MTT. To determine whether or not a tumour is truly a MTT, three criteria have been proposed: (1) the tumour arises along a peripheral nerve, in a ganglioneuroma, or in a patient with NF-1 or represents a metastasis from such a tumour; (2) the tumour has the growth characteristics of a Schwann cell tumour; and (3) rhabdomyoblasts can be demonstrated arising within the body of the tumour.8
On diagnosis, the grade of tumour can be further divided as described in .
Table 1 American Joint Committee on Cancer Staging System for Soft Tissue Sarcomas (6th edition)9
The treatment of MTT is primarily surgical, although in certain cases (dependent on the above grading system) adjuvant chemotherapy or radiotherapy may be appropriate to prevent recurrence or propagation in situations of incomplete resection. Adjuvant chemotherapy has been described as ‘ineffective’,10
although one case was reported in which recurrent MTT was found to express retinoic acid receptors α and γ and the patient treated successfully (to 3 years with no evidence of recurrence) with targeted isotretinoin and interferon-α.11
Radiotherapy is also considered to be less useful for retroperitoneal soft tissue sarcomas compared to limb extremity cases and local recurrence rates for retroperitoneal tumours range from 40–80% overall. Local recurrence in the absence of metastasis can be and is often the primary cause of mortality in these patients. The prognosis for MPNSTs (as a disease group) is poor, with death occurring in 63% within 2 years of diagnosis.12
MTTs behave more aggressively than classic MPNSTs, with crude 2-year and 5-year survival rates of 15% and 11%, respectively.13
Psoas sign (psoas stretch test)
Eliciting the psoas sign exploits the irritation of psoas in its course through the retroperitoneal space.
Psoas is a retroperitoneal muscle that originates from the lateral borders of the twelfth thoracic to fifth lumbar vertebrae. Psoas has two fibrous and one fascial attachment on the spine. Fascicles attach posteriorly to the L1–L5 transverse processes and anteriorly from the T12–L1 disc to the L4–L5 disc. In 70% of people, it is a single structure (psoas major), but 30% also have a smaller psoas minor muscle, which lies anterior to the psoas major along the same course. It is innervated by ventral rami of the lumbar spinal nerves T12–L4.14
Psoas has attachments to the diaphragm and pelvic floor. At the inferior aspect of the muscle (within the pelvis) it combines with the iliacus muscle (arising from the anterior and medial surface of the ilium) to form the iliopsoas. The iliopsoas then traverses the pelvis, exits anteriorly over the superior pubic ramus and inserts into the lesser trochanter of the femur.15
The primary function of psoas is as a hip flexor—that is, when the body is fixed bringing the upper leg towards the body or when the leg is fixed bringing the body towards the leg (sitting from a supine position). Psoas is also selectively involved in sitting with a straight back and in contralateral loading situations requiring stabilisation of the spine in the frontal plane—that is, as a muscle maintaining truncal posture.16
Psoas also performs minimal external rotation and adduction when the femur is moving from an abducted position at the hip and minimal adduction when the hip is fully flexed.17
Eliciting psoas sign
In order to elicit the psoas sign, it is necessary to both resist activity of psoas and induce stretch by passively extending the muscle. To test active movement have the patient lying supine and, with the pelvis remaining in neutral alignment and the contralateral leg extended on the bed, ask the patient to bend the knee towards the chest, flexing at the hip. Once the lower leg is parallel to the bed the examiner places one hand on the patient’s distal thigh (resisting hip flexion), while stabilising the lower leg with the other hand (). If psoas irritation is present, the additional load from resisting flexion will induce or reproduce pain. To complete examination of psoas ask the patient to roll over and lie in the lateral decubitus position the legs extended (the side being examined should be uppermost; thus, to examine psoas on the right the patient should be in the left lateral decubitus position and vice versa. Next the examiner, keeping the pelvis stabilised with one hand, extends the leg at the hip (while maintaining extension at the knee; ). Again, passive extension stretches psoas and its overlying fascium (anterior thoracolumbar fascium) causing irritation and pain.
Figure 2 Eliciting the psoas sign. (a) With the patient supine resist active hip flexion. (b) In the lateral decubitus position passively extend outstretched leg at the hip. In both instances the presence of or increase in pain on these manoeuvres represents a (more ...)
Significance of a positive psoas sign
In the assessment of the patient in this report the identification of psoas irritation prompted the appropriate use of CT as the initial imaging modality to investigate retroperitoneal pathology,18
and ultimately led to the diagnosis and definitive treatment of malignant neoplasm over 1 year after the onset of symptoms, after assessment by a number of medical professionals. To this end, it was an invaluable part of the physical examination. A review of the literature reveals a well described phenomenon, ‘malignant psoas syndrome’, first described by Stevens et al
in which psoas irritation can represent the initial manifestation of a primary malignant process within psoas or as a result of metastatic spread to psoas due to a range of primary malignancies. It can be characterised by proximal lumbosacral plexopathy, painful fixed flexion of the ipsilateral hip and radiological or pathological evidence of ipsilateral psoas muscle malignant involvement.20
How useful is the psoas stretch test in other clinical contexts? Use of the psoas sign is only commonly suggested as an addition to abdominal examination in two clinical situations: suspected psoas abscess and acute appendicitis.21
In the context of appendicitis, pain results because the psoas borders the peritoneal cavity posteriorly so stretching (by hyperextension at the hip) or contraction (by flexion of the hip) of the muscles causes friction against inflamed peritoneum. The right iliopsoas muscle lies under the appendix when the patient is supine; thus, a positive psoas sign on the right may suggest acute appendicitis. Wagner et al
(1996) completed a comprehensive systematic review in which various physical symptoms and signs were assessed and then correlated to diagnostic specificity and sensitivity after post-operative confirmation of appendicitis. The data from three separate trials (assessing a number of pre-operative diagnostic examination techniques to aid identification of appendicitis) were analysed retrospectively. In the context of the diagnostic effectiveness of the psoas sign in evaluating acute appendicitis the authors identified 450 cases in which psoas sign had been recorded prior to surgery and made an assessment of its suitability as a useful clinical sign. In this combined series, the sensitivity is quoted as 16% and specificity as 95%. This in combination with calculated positive and negative likelihood ratios led the authors to conclude that psoas sign is helpful if present in the context of acute appendicitis (but not helpful if absent).22
No such comprehensive data exist regarding the use of psoas sign in suspected psoas abscess, although case reports would suggest that psoas sign is only positive in up to 66% of cases.23
Thus, absence of psoas sign is not discriminatory in the assessment of psoas abscess. Despite this, Taiwo (2001) argues that the psoas test should form an integral part of the examination, particularly in cases where symptoms could be attributed to other more common causes of abdominal, pelvic or hip pain.24
- While ‘common things are common’, rare conditions can frequently present to the emergency department.
- The addition of the psoas stretch test can add additional information to the physical examination allowing a degree of assessment of the retroperitoneal space.
- Classically psoas stretch test is of use in the assessment acute appendicitis and psoas abscess, although the sensitivity in these conditions is low.
- CT is the appropriate initial choice of imaging modality to visualise the retroperitoneum.