Case No. :	53
Month :	May
Year :	2003
Contributor :	Dr. Munirpasha Sayed.

Other Cases

A 28-year-old farmer presented with gradually increasing pain on the right side of the back, radiating to the infra-axillary region for a period of six months. There was
history of gradually progressing paraparesis since three months. There was no history of trauma. On examination, a localized bulge was noted over the back on the
right side. This was tender. There was no sensory deficit, but bilateral distal motor weakness was noted. The complete hemogram was normal.

RADIOLOGICAL FINDINGS:

A frontal radiograph of the dorsal spine (Fig 1) showed an expansile lytic lesion on the head and posterior third of the body of the 9th rib on the right side causing destruction of the superior and inferior cortices of the rib.

Fig.1

There was no calcification, sclerosis or any periosteal reaction. Plain and contrast enhanced CT scan of the thorax (Figs, 2,3 ) showed a well-defined, non-enhancing, multiloculated, expansile, lytic soft tissue mass at the same level. This lesion was causing destruction of the transverse process of the D9 vertebra. The soft tissue
mass was seen to extend into the extradural compartment causing compression of the thecal sac. The lung parenchyma and the upper abdominal structures were unremarkable. Ultrasound examination of the abdomen and pelvis was normal. On T1 weighted MRI, there was a well-defined, multiloculated, hypointense lesion of
uniform signal intensity which on T2 weighted images showed a high signal intensity. (Fig 4,5 )

Fig.2	Fig.3
Fig.4	Fig.5

The lesion could be identified to be extending in the extradural space, impinging on the thecal sac. These findings were thought to be classical of hydatid ingfestation
of bone. Fine needle aspiration biopsy of the lesion under CT guidance was performed which confirmed the diagnosis of echinococcosis. The patient underwent complete excision of the cyst and decompression of the cord. The post-operative recovery was uneventful. There was gradual recovery of lower limb weakness and the patient was discharged on the 15th post-operative day on anti-helminthic treatment. Gross pathological examination of the surgical specimen revealed multiple cysts ranging in size from 0.5-1 cm, pearly white in colour containing clear fluid and bone bits within. Histopathology of the specimen showed lamellated cyst wall with inflammatory reaction.

DIFFERENTIAL DIAGNOSIS:

The differential diagnosis of such a radiographic picture includes Giant cell tumour, osteolytic metastases, plasmacytomas, aneurysmal bone cyst and cystic neurofibromas. Biopsy was contraindicated in Echinococcosis due to fear of dissemination of scolices and other potentially fatal complications. However, review of
recent literature showed aspiration cytology is the procedure of choice in suspected cases of skeletal Echinococcosis.

TREATMENT:

The gold standard in the therapy of this disease is the radical resection of the rib(s) involved. It has been proposed that better results are obtained by combining surgery with anti-helminthic drugs like mebendazole or albendazole. Large doses of anti-helminthic drugs for both preoperative treatment and post-operative prophylaxis help in
the reduction of recurrence of this disease.

DISCUSSION:

Human Echinococcosis is a zoonotic infection caused by larval forms (Metacystodes) of Genus Echinococcus inhabiting the small intestine of the carnivores. Echinococcus granulosus is extremely widespread with high rates of infection in eastern and southern Europe, Middle East, North Africa and South America. The
incidence of overall bone involvement in hydatid disease is 1-4% and location in the thoracic cage is less common. The exact incidence of rib Echinococcosis is not known. The natural course of costal echinococcosis starts when the larvae lodge in the rib and buds start vegetating out off the mother cyst to produce a multilocular
cavity with diverticular extensions. This process invades the spongiosa of the bone in all possible directions. The primary rib lesion is multiloculated and osteolytic
which continues to grow slowly. This lesion then may involve adjacent organs such as vertebra, pleura and soft tissues. If this lesion breaks through the cortical portion
of the rib, it produces soft tissue masses or abscesses.

The posterior ends of the ribs are most commonly involved in costal Echinococcosis. Cysts grow along the long axis of the rib causing expansion of the cortex where
it meets more resistance from the solid cortical portion of the rib. Costal Echinococcosis may be classified as an intraosseous form and an extraosseous form. The intraosseous form may be further classified into a solitary costal form and a costovertebral form. The extra-osseous form is secondary which involves the rib by contiguity. The extraosseous form is most commonly seen due to rupture of pulmonary cyst, either spontaneously or during surgery. Plain radiographs of costal Echinococcosis shows an area of multiloculated rib destruction without periosteal reaction or soft tissue swelling. The rib is expanded with preservation of the cortical margins and
absence of sclerosis, but when present, it is suggestive of secondary infection. Pathological fracture of the rib may also occur. The rib lesion may extend further to involve the adjacent vertebra. Plain radiographs may show soft tissue calcification in up to 38% of patients. The appearance of calcification may depend on the duration of the disease and its clinical course. Calcification represents dystrophic changes in dead parasites.

In advanced cases, it is difficult to judge the epicenter of the lesion and most such cases have been reported as primary spinal lesions. A CT scan provides precise anatomical details of the lesion along with bone destruction and depiction of the paraspinal and intraspinal extension of the same. Lesions of Echinococcosis generally
do not enhance following contrast administration. Delineation of the intraspinal extension is also possible with CT myelography although MR scores over CT in this regard. MR is helpful in verifying the extent of the disease, texture of the cyst, degree of medullary involvement and viability of cyst.

On T1 weighted images, there is a mixed morphological appearance. High signal intensity content of the cyst may correlate with high cell or protein content which is suggestive of extensive parasite-host reaction. Daughter cysts are more hypointense than the parent cyst on T1 weighted images. The cyst wall or capsule is seen as
a low intensity rim, which shows mild enhancement following intravenous gadolinium. The enhancement reflects the vascularity of pericyst.

On T2 weighted imaging, the daughter cysts are of slightly higher signal intensity than the parent cyst. Signal intensities may change with coexisting infection,
calcification or haemorrhage. Extradural spread of hydatid cysts through a widened neural foramen into the muscle planes may result in a 'bunch of grapes' appearance. The T2 weighted sequence indicates whether a cyst is viable or not. A decrease in hyperintensity and an increase is hypointensity from the collapsed cyst wall is suggestive of a succumbed cyst. Both CT and MR may show endovesicular daughter cysts, which are frequently observed in hepatic disease but are rare in musculoskeletal manifestations of this disease.

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