Indian Journal of Radiology and Imaging Indian Journal of Radiology and Imaging

: 2002  |  Volume : 12  |  Issue : 2  |  Page : 271--273

Unilocular primary spinal extradural hydatid cyst-MR appearance

S Gupta, V Rathi, S Bhargava 
 Department of Radiology and Imaging, University College of Medical Sciences and GTB Hospital, Shahadara Delhi-110095, India

Correspondence Address:
S Gupta
E-3, GTB Hospital Campus, Dilshad Garden, Delhi 110 095


Primary extradural hydatid disease of the spine is a very rare condition. A 9 year old boy presented to the hospital with paraplegia of sudden onset. MRI study revealed an oval, unilocular, well-defined extradural cyst posterior to the thecal sac opposite to D4-D6 vertebrae in the middle. it was hypointense on T1W and uniformly hyperintense on T2W images. An operation was performed and histopathological findings confirmed the diagnosis of a hydatid cyst. Primary extradural hydatid disease should be considered in the differential diagnosis of a unilocular extradural cyst on MRI.

How to cite this article:
Gupta S, Rathi V, Bhargava S. Unilocular primary spinal extradural hydatid cyst-MR appearance.Indian J Radiol Imaging 2002;12:271-273

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Gupta S, Rathi V, Bhargava S. Unilocular primary spinal extradural hydatid cyst-MR appearance. Indian J Radiol Imaging [serial online] 2002 [cited 2020 Jul 4 ];12:271-273
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Full Text


Hydatid disease occurs either by direct ingestion of parasite eggs from contact with dogs or indirectly from ingestion of contaminated water or food. Hydatid disease of the spine occurs in 1% of all cases with hydatidosis and is most commonly located in the dorsal spine. [1],[2] The disease either occurs by direct extension from a pulmonary infestation [3] or less often begins primarily in the vertebral body. [1].[4] Primary extradural hydatid disease is an exceptional possibility. Previous MR studies reported in literature have revealed primary extradural hydatid cysts manifesting as multiple small extradural cysts or a multiloculated cyst. [3],[5],[6] To the authors knowledge, this is the first time the MR appearances of a unilocular primary extradural hydatid cyst is being reported. The purpose of the present report is that hydatid cyst should be considered in the differential diagnosis if MRI shows a unilocular extradural cyst.

 Case Report

A 9 year old boy presented with paraplegia of sudden onset of 1 day duration. There was a history of low grade fever off and on for a duration of 15 days prior to onset of this illness. Neurological examination revealed the child to be in spinal shock with marked hypotonia and areflexia in both lower limbs. Gradually in the next 2-3 days the muscle tone increased, all the deep tendon reflexes became brisk, ankle clonus was seen and bilateral planters became upgoing. There was loss of sensation below the D6 segment. Haemogram and routine blood chemistry were normal. Plain radiographs of the dorsolumbar spine did not reveal any abnormality and chest radiograph was also normal. MRI of the dorsal spine was performed using SE T1W and SE T2W and TIRM pulse sequences. T1W and T2W images were also acquired after intravenous gadolinium DTPA administration.

MR study revealed an oval, unilocular well-defined extradural cyst posterior to the thecal sac opposite D4-D6 vertebra in the midline. The cyst was hypointense on T1W images. On T2W images it was uniformly hyperintense with a smooth, sharply defined hypointense capsule.

The cystic mass was compressing the underlying spinal cord which showed hyperintensities on T2W images consistent with myelomalacic changes. Post contrast study did not reveal any enhancement of the lesion.

Vertebral bodies, pedicles and posterior elements appeared normal. No pre or paravertebral soft tissue masses were seen. Based on these imaging findings a diagnosis of an extradural arachnoid cyst with evidence of underlying compressive myelomalacia (at D4-6 level) was made.

Patient underwent surgery, Gross pathologic and histologic examination established the diagnosis of hydatid cyst.

Sonography of the abdomen done post operatively revealed no evidence of hydatid disease.


Spinal hydatid lesions are classified into five types7. Type 1 Primary intramedullary cyst Type 2 intradural extra medullary cyst Type 3 extra dural intraspinal cyst Type 4 Hydatid disease of the vertebra Type 5 Paravertebral hydatid disease.

Primary extradural hydatid cysts are usually considered to arise from an undetected bony focus[1]. It is believed that those cases manifesting as a single cyst in the epidural space must be the ones in which bony involvement by multiple cysts cannot be visualized [1]. Rarely the disease begins from the extradural area: the embryo possibly being carried through the portovertebral venous shunts[2].

Extradural compression by hydatid cyst in the spinal canal as a direct spread from pulmonary lesion has also been reported [8].

Though rare primary extradural hydatid cysts have been reported in literature [3],[4],[6],[9].

Previous MR studies [3],[5],[6] of pure extradural hydatid cysts have revealed multiple small extradural cysts or a multiloculated cystic mass in the extradural space.

Ours was a Type 3 extradural intraspinal cyst and was unusual in that the lesion was unilocular and did not involve adjacent vertebrae or paraspinal regions.

The MR features of hydatid disease of spine have been described in literature; however the experience is limited. To date, no pathognomonic feature has been identified. Hydatid cysts usually appear as well circumscribed, cystic lesions with signal intensities similar to cysts on all sequences [6]. The cyst wall is usually thin and regular with no septations.

On T2 weighted imaging the cyst wall or capsule is seen as a low intensity rim which shows mild enhancement following intravenous gadolinium. There was no enhancement of the capsule in our case. The enhancement reflects the vascularity of pericyst [10]. Fahl et al [5] describes that hydatid cysts have two dome shaped ends, no debris in their lumen and look like a flattened sausage as in our case. Occasionally they are spherical. Unlike Fahl et al who believe that extradural hydatid cysts are always multiple and involve bone, our case showed a solitary extradural cyst not involving bone. Multiple cysts (daughter cysts) within a cyst which are of varying signal intensities is the characteristic imaging feature of his disease [10]. On T1 weight images, daughter cysts are more hypointense than the parent cyst and on T2 weighted images the daughter cysts are of slightly higher signal intensity than the parent cyst. Signal intensities may change with coexisting infection, calcification or haemorrhage[7]. Extradural spread of the hydatid cysts through a widened neural foramen into the muscle planes may result in a 'bunch of grapes' appearance [3],[6].

MRI is also useful is determining the viability of hydatid cyst. Jena [3] has pointed out that the intensity differences may account for viability of cyst. On T1 weighted image the viable cyst appeared to contain low intensity fluid with iso to mildly hyperintense wall. The T2 weighted sequence indicated whether a cyst was viable or dying. A decrease in hyper intensity and an increase is hypointensity from collapsed cyst wall were indicated as findings of a succumbed cyst.

Surgery remains the mainstay of treatment of extradural hydatid cyst. Inadvertent injury to the dura during surgical exploration is conducive to intradural spread of disease [5]. So it is highly important to consider the diagnosis of primary extradural hydatid cyst because of danger of dissemination of disease intradurally [5]. CT myelography may fail to demonstrate the extent of pre or paravertebral involvement, so MRI is the modality of choice in evaluating hydatid disease of spine.

The differential diagnosis of an extradural cystic lesion includes an arachnoid cyst, Epidermoid or inflammatory cyst. Though most extradural hydatid cysts are multiple the finding of a unilocular extradural cyst on MRI should also suggest the possibility of hydatid disease, even in the absence of hydatid cysts in the lung or liver and negative skin or serologic tests.


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