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Diagnosis and Management of Hydatid Cyst of the Central Nervous System: Part 2: Hydatid Cysts of the Skull, Orbit, and Spine

Abbassioun, Kazem*; Amirjamshidi, Abbass

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Hydatidosis may affect any place in the central nervous system (CNS) with or without brain involvement, such as the skull, orbit, vertebral column, and spinal cord. The authors review briefly the characteristics and management of such cases of hydatidosis.

*Sina Hospital and †Arad General Hospital, Tehran University of Medical Sciences, Tehran, Iran

Address correspondence and reprint requests to A. Amirjamshidi, MD, Associate Professor of Neurosurgery, Sina Hospital, Tehran University of Medical Sciences, P.O. Box 19585–678, Tehran, Iran. E-mail: Glavijeam@Yahoo.com

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HYDATID DISEASE OF THE SKULL

Pathogenesis

The changes visible in skull bones are usually secondary to the affliction of the brain, and cranial bones are rarely involved primarily by hydatid disease. They become involved by hydatid lesions when the larvae of Echinococcus granulosus are carried by the general circulation and lodged in the diploe of the one of the bones. This has been demonstrated experimentally 1,2 by injecting scolices of Echinococcus into the carotid artery of rabbits, leading to the development of hydatid cysts in the spongiosa of the cranial bones, as it may grow in the same manner in other bones. Unilocular cysts are rare, and the cysts are nearly always multilocular without the presence of a mother cyst. Because the hard walls of the bone are strong barriers to the expansion of the cyst, this can cause rupture of the mother cyst, leading to the development of several microcysts in the bone, partially separated by trabeculae. Usually a single bone of the skull is affected and the condition does not spread to the adjoining bones. Trauma to the skull can cause fracture of the inner table and perforation of the dura. In this way, the cysts may extend extradurally or into the subdural space. The extradural cysts produce certain reactions in the host tissues, and an adventitial membrane is formed covering the dura. Development of this membrane can be another obstacle to the spread of the disease into the cerebral tissues. 3

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Clinical Presentation and Diagnosis

In our series, we encountered 7 cases of cranial hydatid cysts among more than 130 cases of craniocerebral hydatidosis. Reviewing all similar reports, we concluded that cranial hydatidosis accounts for no more than 5% of all craniocerebral cases, which account for 3% to 4% of bony hydatids. 2,3 The condition is seen in older patients than the cerebral hydatids, the average age being 18 years. The mass lesion can be extracranial or intracranial. The length of history varies from 2 to 4 years. It is covered by normal scalp with no change in skin vascularity. Occasionally the patient reports a headache, when the inward bulge becomes sufficiently large. Intracranial masses seen in hydatid disease of the basal bones produce signs and symptoms according to their size and location. Occasionally, hydatid cysts erode the inner table early in their development and extradural cysts appear with only minor pathologic changes in the bone. Primary extradural cysts are rare: only four cases have been reported in the literature. 2,3 All four appeared to be primary extradural cysts, but erosions and adhesions in the adjacent bone and multiplicity of the cysts were all indications pointing to an osseous nature.

Plain radiography of the skull is a valuable investigation, showing a neatly demarcated cavity in the diploe in smaller cases. As this cavity enlarges, the outer and inner tables become thinner and bulge outward or inward (Fig. 1). In the fully developed case, there is a second curved line representing the calcified adventitial membrane in the outer and inner surface of the extruded cysts. These two curvilinear calcified lines in the radiographs are pathognomonic of hydatid disease of the skull. Fully calcified masses represent calcified extruded dead cysts. The same phenomenon may appear in cases of monostotic fibrous dysplasia of the skull. 4 In such cases, computed tomography (CT) scanning is helpful because it demonstrates a destructive lesion of the bone with expansion and areas of low density. Other conditions to be differentiated from hydatid cyst of the skull are dermoid cysts, cavernous hemangiomas, and aneurysmal bone cysts. These last two are easily differentiated by CT because their contents have the density of blood, which is much higher than the hydatid cyst.

FIG. 1.

FIG. 1.

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Treatment

Total extirpation of the lesion without rupturing any of the cysts is the treatment of choice and can be easily achieved if both tables are intact. A large enough scalp flap should be raised to allow some healthy bone to be included in the excised part. The exposed bone should be carefully examined and the bur holes should be placed in such a way that at least 1 cm of healthy bone is included, to obviate an accidental rupture of a peripheral cyst. If the outer table is eroded and some cysts have appeared under the scalp, care should be taken that they are not ruptured. The area should be covered by gauze soaked in 2% formalin solution. If the inner table is eroded and some of the cysts have gained access to the extradural space, it is then practically impossible to remove the bone flap without rupturing one or more of the cysts. After removing the affected bone, the area should be irrigated with 2% formalin solution. No attempt should be made to remove the adherent adventitial membrane, and the dura should be left intact. It is not wise to anchor the dura to the surrounding bone because some fertile cystic fluid may reach the subarachnoid space. The bone defect can be repaired in the same session with a suitable material.

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Prognosis

With total extirpation of the lesion, the prognosis is very good. There has been no report of recurrence in such cases, but with perforation of the dura, the prognosis is poor.

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HYDATID DISEASE OF THE ORBIT

Pathology

Orbital cysts are usually primary, and it is rare to have one occur secondary to hydatidosis of the orbit bones. They are usually solitary; in only one case did we encounter multiple orbital hydatid cysts. They grow within the retroglobal soft tissue, and intraocular cysts are rare. 5–8 Unlike brain cysts, an adventitial fibrous membrane surrounds the orbital cysts in at least 50% of cases. They are usually ovoid and vary in size according to their age. It has been said that they do not recur because they are acephalocystic, but they recur occasionally and at least some of them contain scolices and are fertile. 7,9 Suppuration and death can occur in the orbital hydatid cysts, as elsewhere in the body. To the best of our knowledge, no case of bilateral orbital hydatid cyst has been reported.

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Clinical Presentation and Diagnosis

Unilateral exophthalmos and visual impairment are the characteristic signs of orbital hydatidosis, as in most other retroglobal space-occupying lesions. Proptosis is slowly progressive, painless, and irreducible. Movements of the globe are often limited in patients with exuberant proptosis (Fig. 2). In a few patients, especially after trauma, it may progress rapidly and become painful, which would need urgent treatment. Vision is affected as a result of compression of the optic nerve or interference with its blood supply. Frank papilledema is not a common finding, but venous engorgement plus differing degrees of optic atrophy may be detected on fundoscopy.

FIG. 2.

FIG. 2.

Biologic tests are unreliable for the diagnosis of orbital hydatid cysts, as in cases of brain hydatidosis. Destruction of the orbital wall, seen both on radiographs and CT scans, and a homogenous retrobulbar mass with regular contour and fine rim enhancement after contrast injection on the CT scan 5,6,9–13 are characteristic features of orbital hydatidosis (Fig. 3). A young patient from an endemic area with a painless and slowly growing proptosis is highly suspicious of harboring orbital hydatidosis. The conditions to be eliminated in the differential diagnosis are mucocele, epidermoid cysts, aneurysmal bone cyst, and fibrous dysplasia of the surrounding structures. This can be done using a good-resolution CT scan with bone window images and contrast injection. Aspiration of the cyst is a method for differential diagnosis, but it is not recommended because it may cause an anaphylactic reaction as well as possible contamination of orbital tissues.

FIG. 3.

FIG. 3.

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Treatment

Hydatid cysts of the orbit represent 0.5% to 0.8% of all the hydatid lesions of the body and account for 2% to 3% of all cases of unilateral exophthalmos. Our experience is based on 28 cases encountered during the past 23 years, one of the largest series reported in the literature. In this series, there was a slight preponderance of males, and the age range varied from 5 to 54 years, with a mean of 15. Most of the patients lived in the sheep-raising regions of the country.

At present, the only effective treatment is surgical removal of the cyst, performed by orbitotomy or modified low basal craniotomy techniques. It is difficult or impossible to deliver an orbital cyst without rupturing it, especially if there is an adventitial membrane adherent to important tissues. In these circumstances, the adventitial membrane is incised and the cyst wall is removed after aspiration of its contents. After removing the cyst, the area should be irrigated with a weak solution of formalin or silver nitrate. Craniotomy is advised only when the cyst is situated in the posterior part of the orbit; otherwise, considering the chance of intracranial spread, orbitotomy is the preferred surgical approach.

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Prognosis

Exophthalmos disappears within a few days after surgery, but the two eyes do not become symmetrical; in many patients the affected orbit remains larger than the normal side. Vision, if not severely affected, improves after surgery, but no change occurs in those with poor vision. A certain amount of ptosis may occur after removal of a large cyst, but it recovers in 2 to 3 weeks. Recurrence of the lesion is uncommon, partly as a result of the sterility of the orbital cysts.

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HYDATID DISEASE AFFECTING THE VETEBRAL COLUMN, SPINAL CORD, AND ITS ROOTS

Pathology

The symptoms of spinal cord compression in hydatid disease are due to local extension of the vertebral lesion into the spinal canal in more than 90% of cases. They may be due to the affliction of the head of the rib or paravertebral soft tissues. Primary extradural hydatid cyst is rare, primary intramedullary lesion extremely rare. 14 The vertebral column is the site of primary hydatid lesions in 0.5% to 1% of all echinococcal lesions of the body, but it accounts for 50% of cases of hydatid disease of the bone. 15–22 The ratio of cases of hydatid disease of the spine to those of the brain is 1:2. In all the series reported, there is a slight preponderance of male patients to female patients. The age distribution in all the series show that the patients become symptomatic between 20 and 50 years of age; at this time, other organs are involved in about 25% of the patients. The most common site of involvement is the lower dorsal and lumbar regions (65%). The primary site of involvement in the vertebra is the more vascular part (i.e., the vertebral body); otherwise, the hydatid process in the vertebra is no different from hydatid lesion in other bones. This process leads to a microvesicular polycystic vertebra as a result of the growth of several daughter cysts. Whenever the life of the hydatid organism is threatened, many small daughter cysts are developed inside the mother cyst; this is an attempt at survival by the parasite. The confined space of the bone and its trabeculae is such a threat.

In intraosseous collections, most of the cysts are spherical and only a few millimeters in diameter, with transparent walls. Others are disklike with opaque walls, and some are even biconcave. There is always colorless fluid between the cysts, which is usually clear.

The most interesting phenomenon in osseous hydatidosis is the complete absence of any reaction by the host tissues to this alien organism. This phenomenon explains the absence of any pain or discomfort until the bony cortex is eroded and the cysts enter the spinal canal. The collapse of the affected vertebra is more often like a concertina, because the whole body of the vertebra is destroyed. This is unlike the vertebral collapse in tuberculosis of the spine, in which a wedge-shaped vertebra results, with the appearance of a gibbus at the back. In hydatid disease of the spine, gibbus is unusual, and severe kyphosis is seen in less than 10% of patients. Extension of the lesion to the adjacent vertebra is not uncommon, but involvement of more than two vertebrae is rare. The intervertebral disk is a strong barrier to the extension of the hydatid lesion, and it actually remains intact for many years. When the bony barrier and its periosteum are penetrated and the cysts escape into the canal and come into contact with the dura, a foreign body reaction sets in and a thick adventitial membrane is formed. In the spinal canal, the cysts are usually found posterior or posterolateral to the cord and are of various sizes, ranging from pinpoint to 1cm in diameter; they are bathed in a clear or turbid fluid. Cysts in the spinal canal may occupy the length of up to five vertebrae, an important point to remember when planning surgical treatment. 16,18,23–27

Superimposed blood-borne infection of hydatidosis of the spine is rare, but postoperative bacterial infection is not uncommon. This can kill the hydatid lesions, which can cure the disease if such an infection is controlled. In practice, despite antibiotic therapy, a fistula persists that is extremely difficult to eradicate. As in brain hydatids, calcification occurs in old cysts; it always starts in the adventitial membrane of extraosseous collections, and ultimately the whole membrane and some of its contents become calcified. 26,28,29

The cysts can be extradural, subdural, or subarachnoid. A single extradural cyst in the absence of an adventitial membrane should be considered definitely a primary cyst. When an adventitial membrane is present, and especially if this membrane is adherent to the bone, it is difficult to exclude a bony origin. 17,22,25

Of the few cases of subdural cysts reported, they were always multiple and of different sizes. On the outer side, the dura is thick, whereas the inner surface is covered by an adventitial membrane. The cysts protrude into the subarachnoid space, producing some degree of arachnoiditis. The cyst is known as a primary subarachnoid cyst when the dura is not involved and there is no lesion in the posterior fossa. 14,19,27

Intramedullary cysts are extremely rare. It is likely that the vascular pattern of the spinal cord does not allow small parasitic emboli to reach it, and for the same reason, hydatid cysts appear more often in the conus medullaris than other parts of the cord. 14,27

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Clinical Presentation and Diagnosis

In vertebral hydatidosis and during the intraosseous phase, no symptoms are produced. Thereafter, the initial symptoms are either radicular pain or muscle weakness. In vertebral echinococcosis, three types of syndromes are described: pseudotumoral, pseudopottique, and pseudodiskal. In practice, neurosurgeons encounter mostly pseudotumoral cases and orthopedic surgeons see mostly pseudopottique cases. Neurologic signs appear when the cysts are extruded into the canal or compress the roots at their exit and depend on the level of the affected vertebrae as well as the stage of the disease. Paravertebral swelling should always be sought, because the cysts may be too small to be detected. Trauma may play a part in precipitating the invasion of the canal and the production of acute neurologic signs. 18,20,21,23–25

Negative results on serologic tests have no significance in spinal hydatidosis, but a positive reaction with marked eosinophilia adds a few points to the diagnosis of hydatidosis. In plain radiographs, changes depend on the stage of the lesion. The vertebra retains its shape in the early microvesicular stages, but as the cysts gradually expand, lytic changes appear and resemble a bunch of grapes. In the advanced stage, honeycomb lesions extend into the pedicle, the head of the rib, and even the adjacent vertebra. Changes in the disk space and adjacent vertebra appear in 5% of cases and occur under the ligaments. This can produce a paraspinal shadow presenting as an ossifluent abscess containing patchy calcification of the adventitial membrane. In the final stage, partial or complete collapse of the affected vertebra is detectable.

CT scanning seems to be the most reliable diagnostic method in spinal hydatidosis. The cysts are seen as hypodense lesions, with the adventitial membrane shown as a dense calcified rim. Any erosion and destruction of the bone can be seen, and the extent of the lesion is clearly demonstrated. Hydatid lesions of the adjacent viscera can also be discovered in appropriate cuts (Fig. 4). CT scanning after intrathecal injection of metrizamide can clearly demonstrate small amounts of spinal cord compression. CT scanning is also useful in recurrent cases, in which previous laminectomy permits the recurrent cysts to attain a larger size before becoming symptomatic. 19,20,26,27,29

FIG. 4.

FIG. 4.

Magnetic resonance imaging can demonstrate any cord compression throughout the length of the spinal cord, but bone involvement is not identified as clearly as on CT scans. 27–29 Magnetic resonance imaging has replaced myelography in nearly all the similar cases with both extradural or intradural manifestations (Fig. 5).

FIG. 5.

FIG. 5.

Aspiration of a painless subcutaneous fluctuating mass as a diagnostic procedure may lead to complications, such as fistula formation and subsequent bacterial infection, if it turns out to be a hydatid cyst. The following precautions are suggested: meticulous aseptic technique, a small incision of the skin 2 cm away from the dome, oblique insertion of the needle so that as it is removed, a valvelike action is produced, and closure of the skin incision with one or two stitches.

Hydatid cyst of the spine should be differentiated from tuberculosis of the spine, hemangioma of the vertebra, malignant lesions, aneurysmal bone cyst, fibrous dysplasia, osteoblastoma, plasmacytoma, herniated intervertebral disk, and cysticercosis. Each of these pathologies has specific features that are useful for differential diagnosis before any surgical intervention. 19,20,26,29

We found that hydatid disease was responsible for 2% of all cases of spinal cord compression. There were 36 cases of spinal hydatidosis, with a preference for male patients (20/16), the same as reported by other authors. 18,19,23 The parts of the vertebral column were not uniformly involved; the pattern of distribution was 5% in the cervical region, 60% in the thoracic region, and 35% in the lumbosacral region. The higher incidence in the thoracolumbar region (T7–L3), accounting for about 65% of cases, may be due to the higher vascularity of these vertebrae. This series included one subarachnoid cyst, one subdural cyst, and one intramedullary hydatid cyst. Radicular pain was the initial symptom in 75% and weakness of the extremities was present in 20% of patients. A paravertebral swelling attracting the attention of friends was present in some cases. Eight percent of the patients in this series underwent surgery months to 2 years before being referred to our center. The best imaging study in our hands has been CT scanning before the use of magnetic resonance imaging, which has been the diagnostic method of choice for preoperative evaluation of similar cases in recent years.

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Treatment

Using laminectomy in nearly all these patients, we have been able to evacuate the offending cysts as much as possible, even in recurrent cases. Chemotherapy was used for at least 1 year in all the patients. Using chemotherapy in treatment of spinal hydatidosis has been disappointing. This is partly due to the unpredictability of spinal hydatidosis, in which the chance of remission is extremely variable. Different approaches have been used to eradicate the lesions as completely as possible using laminectomy, costotransversectomy, or vertebrectomy with or without fusions. At the completion of surgery for extradural lesions, the area should be thoroughly irrigated with a parasiticidal solution (2% formalin, 0.5% silver nitrate, or 10% hypertonic saline).

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Prognosis

Immediate postoperative results depend on the degree of preoperative neurologic deficit. No improvement can be expected in patients with complete paraplegia, unless it is of acute onset and the patient undergoes surgery within a few hours. In other patients, the short-term outcome is gratifying. Because spinal hydatidosis is a locally malignant disease (le cancer blanc), recurrence of symptoms and signs can be expected in most patient within 4 years. 17,19,21,25 With the aid of CT scanning and magnetic resonance imaging, recurrence can be diagnosed before clinical signs appear, so it is tempting to perform “preventive surgery.” However, clinical symptoms may not appear for a long time, even years after the CT changes confirming the cyst recurrences. It is therefore more prudent to await the appearance of early symptoms before undertaking a second operation. In spinal hydatidosis, a complete cure is possible in only a few instances—that is, in patients with a single cyst that has been delivered intact. The same is also true in intradural and intramedullary cysts. The subdural cysts are usually multiple and extend over several levels. A long laminectomy is usually necessary until normal dura is visible at both extremities. The prognosis in multiple cysts is always poor. 20,24,26,30–32

Medical treatment is another milestone in the management of spinal hydatidosis. We prescribe the antihelmintic drugs in the group of benzimidazole compounds such as mebendazole and albendazole for at least 3 months, in the hope that they may prevent or delay recurrence in a few patients. 31–36

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CONCLUSION

Spinal hydatidosis is a locally malignant disease that is nearly impossible to eradicate even after repeated surgical interventions and chemotherapy. Every case of spine deformity with or without neurologic deficits encountered in endemic areas should be considered hydatid disease until proved otherwise.

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Keywords:

Hydatid cyst; Orbit; Skull; Spine; Spinal cord

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