Sciatic neuropathy at the popliteal fossa: clinical, ultrasonographic and computed tomographic diagnosis

Transcription

1 Clinical rheumatology, 1986, 5, N~ Case Report Sciatic neuropathy at the popliteal fossa: clinical, ultrasonographic and computed tomographic diagnosis W. ESSELINCKX 1, J.P. TRIGAUX 2, B. MALDAGUE 3, J. MALGHEM 3, P. KNOOPS 4, C. THAUVOY 5 Sections de Rhumatologie 1, et de Radiologie 2, Cliniques UCL de Mont-Godinne, Yvoir ; Services de Radiologie 3, M6decine Physique 4 et Neurochirurgie 5, Cliniques UCL Saint-Luc, Bruxelles; Unive~sit6 Catholique de Louvain, Belgique. SUMMARY A 22-year-ol~l female had been suffering from sciatica-like pain in the left leg for four years. Clinical findings strongly directed further investigations to the popliteal fossa. Ultrasonography located a hypoechogenic mass in the upper lateral popliteal space. Guided by these data, computerized tomography (CT) with vertical reconstructions made the tentative diagnosis of a common peroneal nerve tumor, which was confirmed at operation. Microscopic examination showed a neurinoma of the mixed neurilemomaneurofibroma type. In the presence of atypical features of sciatica, a high index of suspicion seems advisable. Emphasis is laid on the complementary contribution of ultrasonography and CT in this type of ill-defined lower limb pain. Key words: Sciatic Nerve Tumor, Popliteal Ultrasonography, Reconstruction Computed Tomography, Neurinoma. INTRODUCTION Lumbosacral radiculopathy is the most common cause of "sciatica". We report here on a case of lower limb pain originating at the popliteal fossa. The diagnosis was initiated on clinical suspicions, and subsequently aided by the ultrasonographic findings, which made possible a precise computed tomographic study. The complementary roles of these techniques are demonstrated. CASE REPORT For the past 4 years, a 22-year-old female had been suffering from pain in the left Received 21 June 1985 Revision-accepted 3 July 1985 Correspondence to: DR. W. ESSELINCKX, Cliniques UCL de Mont-Godinne, B-5180 Yvoir, Belgium. lower limb, predominantly located in the calf, radiating to the toes, without paresthaesias. There was a past history of occasional low back pain. On examination, left straight leg raising produced discomfort in the left lower limb. Exquisite tenderness was found in the upper lateral area of the left popliteal fossa, without any palpable swelling ; the knee and other joints were normal. Plain film radiography of the lumbar spine and pelvis were normal. Nerve conduction and electromyographic studies showed no abnormality. Ultrasonography revealed a hypoechogenic lesion of spherical shape as shown by the combined longitudinal and transverse sections, measuring 2.5 cm in diameter, situated in the upper lateral area of the left popliteal fossa, about 1 cm below the skin

2 108 W. Esselinckx, J.P. Trigaux, B. Maldague, et al. surface (Fig. 1). There was no back wall enhancement and a few internal echoes were clearly visible, Computerized tomography (CT) was then selectively performed at the level of this lesion. Noncontrast CT through both upper Fig. I : Transverse ultrasonographic section through left upper popliteal space shows hypoechogenic mass, without back wall enhancement, containing internal echoes (open arrow). Distance from + to + : 2.5 cm ; L = lateral ; M = medial; F = femur. Fig. 2: Precomrast CT at level of both upper popliteal spaces shows well-defined mass (A) with lower attenuation (35 H.U.) thin semimembranosus muscle (B :59 H.U.), lodged between biceps femoris muscle (C) and vascular bundle (V). By comparison with the fight side, the mass seems to be located on the course of the common peroneal nerve (short arrow), lateral to the tibial nerve (long arrow).

3 Sciatic nerve tumor: ultrasonography and computerized tomography 109 popliteal spaces showed a well-defined mass interposed between the biceps muscle laterally and the vascular bundle medially (Fig. 2). This mass had an attenuation value of 35 H.U., which was slightly less than the surrounding muscles (59 H.U.), and by com- parison with the unaffected right side, seemed to occupy the distribution of the common peroneal nerve. Postcontrast CT showed inhomogeneous enhancement (Fig. 3). Oblique vertical reconstruction was performed along the plane shown in Figure 4a. This made the 3 x mass appear as an enlargment of the presumed common popliteal nerve, with which it would be connected at both its proximal and distal poles. These findings were entirely confirmed at surgery. The tumor was completely excised and microscopic examination showed a grade I mixed neurilemoma-neurofibroma. After operation, pain in the leg almost fully receded. There was a slight transient weakness of the peroneus muscles, with mild electromyographic changes. DISCUSSION Fig. 3: Postcontrast CT through left upper popliteal space demonstrates inhomogeneous enhancement of mass. Initially, this patient's lower limb pain was viewed in relation with her past history of low back pain, and diagnosed as radicular sciatica. Later on, owing to the exquisite derness at the posterior aspect of the knee, the diagnosis of a synovial popliteal cyst, possibly causing nerve entrapment, could Fig. 4: a) Detail of left postcontrast CT. Dotted line indicates plane of oblique vertical reconstruction shown in b). Fig. 4: b) Oblique vertical reconstruction shows mass occupying space between vascular bundle (V) and biceps femoris muscle (C), and connected at both upper and lower poles with presumed course of common peroneal nerve (arrows).

4 110 W. Esselinckx, J.P. Trigaux, B. Maldague, et al. have been entertained. However, several facts were not accounted for by this hypothesis. The vast majority of popliteal synovial cysts arise from the gastrocnemio-semimembranosus bursa, and are therefore medially located; in the rare event of a lateral popliteus bursa cyst (1), the lesion is to be found in the lower part of the popliteal area, whereas in our patient i0alpation pointed to a distinctly more proximal lesion. In addition, the intense pain elicited on palpation stood out in contrast to the absence of posterior swelling or other abnormality at the knee joint proper. Once aroused on clinical grounds, these suspicions were confirmed by the ultrasonographic examination, which definitely located the lesion in the lateral uppermost part of the popliteal fossa. Furthermore, the sonographic findings were not entirely in keeping with the usual features of cystic lesions at the popliteal fossa (2). The lesion was spherical ; there were several internal echoes, and similar to a reported case of plexiform neurofibroma of the popliteal fossa (3), there was no back wall enhancement. However, classical cystic features - echopenia, sharply defined walls, acoustic enhancement - have been noted in a neurinoma of the thigh (4). Precisely directed by the ultrasonographic findings, CT could readily be conducted through the optimal levels. By comparison with the unaffected side, horizontal sections showed a mass situated on the presumptive course of the common peroneal nerve. This appearance was similar to the transverse CT scans reported in one case of tibial nerve schwannoma of the popliteal fossa (5), and in six cases of sciatic nerve tumors at various levels of the thigh (6). We were able to proceed further into the CT study by performing vertical reconstructions (sagittal, frontal and oblique), which provided additional suggestive features of the correct diagnosis. Thomas et al. (6) described 35 cases of sciatic nerve tumors, which they had traced from their pathology files over the preceding 60 years. There were 21 fibrosarcomas, 7 neurofibromas and 7 neurilemomas (schwannomas). Absence of.r~6cturnal exacerbation of pain (19 cases) artd positive straight leg raising test (10 cas~) were noteworthy misleading features. A palpable tumor and neurologic deficits were more often noted in the neurofibrosarcoma group. Indeed, these signs eventually developed in an initially deceptive case of neurofibrosarcoma occurring at the popliteal fossa in a patient suffering from rheumatoid knee joint arthritis (7). When no palpable tumor can be found, as in 13 out of the 35 patients reviewed by Thomas et ai. (6), other means of locating the lesion must be resorted to. These authors (6) did not find electrophysiologic techniques very satisfactory for that purpose. Ultrasonography, which was not available in their records, proved extremely helpful in our case, and can be relied upon for searching the whole distribution of the sciatic nerve, including the pelvis, as demonstrated by the imaging of a neurofibroma of the pelvic floor (8). With such valuable preliminary information on their location, CT is the procedure of choice for diagnosing sciatic'nerve tumors. CT assesses the content of the mass, and displays its extent and relations to surrounding structures. Additionally, as in our ease, vertical reconstruction may succeed in suggesting even more precisely the exact nature of the lesion. Thus combined, ultrasonography and CT greatly help the aware clinician in making the difficult diagnosis of sciatic nerve tumor, sparing the patient with unresolved lower limb pain many unnecessary and often invasive (myelography, arthrography) procedures. Acknowledgement: We wish to thank Professor J.M. Brucher (Service d'anatomie Pathologique, Cliniques UCL St-Luc) for the microscopic examination of the surgical specimen.

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