Posterior Fossa Infections

Vijaysaradhi Mudumba, Challa Sundaram, and Prasad Vannemreddy

Infections in the posterior fossa are ominous not only due to its small dimensions, but also due to the close proximity of the brainstem with the resultant mass effect, occlusion of the cerebrospinal fluid (CSF) pathways, and hydrocephalus. Because of the adjacent middle ear and mastoid air cells, any contiguous infections can easily spread to the posterior fossa. Infections can also spread hematogenously from a distant source. Rarely, penetrating trauma to the posterior fossa, such as depressed fractures and gunshot wounds, may cause infections by contiguous means. The risk of development of brain abscess after penetrating craniocerebral trauma is 3 to 4.2%.1 Bacterial brain abscess are rare in the United States and they occur approximately in 1500 to 2500 infections each year.2 Improvements in the management of ear, sinus, and orofacial infections were responsible for the reduction of intracranial infections.3

♦ Classification of Posterior Fossa Infections

1. Bacterial infections

    A. Pyogenic infections

    B. Mycobacterial infections

2. Fungal infections

3. Parasitic infections

♦ Bacterial Infections

Pyogenic Infections of Posterior Fossa

Cerebellar Abscess

Of all the sites in the posterior fossa, the cerebellum is the most common site for a brain abscess. Cerebellar abscesses comprise 6 to 35% of all brain abscesses. They are often ominously silent and carry a significant mortality.4 Middle ear infections are the commonest source of cerebellar abscess. The majority of otogenic brain abscesses are located in the temporal lobe followed by the cerebellum.1 The infection spreads to the posterior fossa in contiguity, leading to either abscess formation or subdural empyema. The cerebellar abscess usually has a connection to the petrous bone via a fibrous track consisting of adhesions between the arachnoid and the dura overlying the abscess. In 6% of otogenic abscesses, normal cerebellar tissue lies between the abscess and the petrous bone.4

♦ Causative Organisms of Otogenic Cerebellar Abscess

Understanding the pathogenesis of brain abscesses is important in determining the most likely causative microorganisms and subsequent treatment.3 The causative organisms of bacterial brain abscess depend on geographic distribution, patient age, underlying medical or surgical conditions, and the mode of infection.5 Aerobic or microaerophilic streptococci account for approximately 50% of the aerobic isolates and have been present in approximately 70% of all brain abscess patients. Anaerobic bacteria (e.g., Bacteroides spp., Prevotella spp., Peptostreptococcus, Fusobacterium spp., and Actinomyces spp.) are another major cause of brain abscesses.3 Staphylococcus aureus is the most common cause of traumatic and postoperative brain abscess.1 In 30 to 60% of cases pyogenic brain abscesses are due to mixed infections. Viridans streptococci and Klebsiella pneumoniae are associated with hematogenous spread. K. pneumoniae is a common cause of brain abscess in patients with diabetes mellitus.6 V. streptococci is the most prevalent pathogen in infection of the paranasal sinuses. Head trauma and postneurosurgical states have become important predisposing factors and nosocomial infections also played an important role.4 A majority of cerebellar abscesses are associated with ear or mastoid infections and are usually solitary.5

Fig. 8.1 Normal cerebellar cortex with abscess. (Hematoxylin and eosin [H&E] ×100.)

Fig. 8.2 Normal cerebellar cortex with abscess. (H&E ×400.)

♦ The Stages of Abscess Formation(Figs. 8.1, 8.2, 8.3, and 8.4)

Based on histology criteria, development and progression of the brains abscess are divided into four stages7–9: early cerebritis, late cerebritis, early capsule formation, and late capsule formation. Each stage can be identified on radiologic imaging.

Early Cerebritis (1–3 Days)

This stage is characterized by perivascular polymorphonuclear leukocyte infiltration with swollen endothelium, which results in focal inflammation and edema.

Late Cerebritis (4–9 Days)

A necrotic center develops during this stage. Necrotic debris is converted to pus by enzymatic digestion.

Early Capsule Formation (10–14 Days)

This stage is characterized by the appearance of reticulin fibers, which are converted into collagenous capsule. A vascularized capsule enhances with contrast, resulting in a ring-enhancing lesion.

Fig. 8.3 Stage III abscess showing early capsule formation. (H&E ×400.)

Late Capsule Formation (Beyond Day 14)

At this stage the abscess develops a mature collagenous wall surrounded by a mild degree of cerebritis.

♦ Clinical Features

Clinically the posterior fossa abscess presents with the classic triad of increased intracranial pressure, focal neurologic deficits, and fever. The history is of short duration. Presenting features include the syndrome of the cerebellopontine (CP) angle, cerebellar compression, and cranial nerve deficits including the Gradenigo syndrome, papilledema, and features of hydrocephalus. The most common symptom is headache, and the most common abnormality on physical examination is a decrease in the level of consciousness.10

♦ Laboratory Investigations

Blood cultures should be obtained in all patients, especially for the uncommon sources as in infective endocarditis. Cerebrospinal fluid (CSF) analysis is rarely diagnostic, and lumbar puncture is not indicated, especially in the setting of posterior fossa mass, which might result in herniation. Pus should be obtained during surgery or by stereotactic aspiration and it should be cultured. Other laboratory studies, such as the leukocyte count, erythrocyte sedimentation, and C-reactive protein rate may be normal and not helpful in clinical management.

Fig. 8.4 Mature collagenous wall in stage IV abscess. (Masson Trichrome ×400.)

♦ Radiology

Radiologic findings vary with the stage of abscess. Repeat imaging at regular intervals is recommended to follow the response to therapy. In the cerebritis stage, a computed tomography (CT) scan shows an irregular area of low density. In the early cerebritis stage, there may or may not be contrast enhancement.9Contrast enhancement starts to appear in the late cerebritis stage.9 Once the capsule is formed, it is well appreciated as a thick ring surrounding a central hypodense or hypointense core. Usually the capsule is thinner toward the ventricular surface. The earliest stages of cerebritis usually show some amount of vasogenic edema. Ependymal enhancement suggests ventriculitis. The differential diagnosis of ring-enhancing lesion by CT includes tumor, necrotizing encephalitis, infarction, granuloma, as well as abscess due to toxoplasmosis and fungi (Fig. 8.5). On radiologic studies, multiple ring lesions favor fungal or tubercular lesions, metastatic deposits, and toxoplasmosis. Tuberculomas have an isodense center in contrast to an abscess, which has a hypodense core on CT scans. A pyogenic abscess is usually single but can have lobules and coalescing cavities. Abscesses secondary to infective endocarditis in congenital cyanotic heart disease do not form a thick capsule because of the anaerobic nature of the bacteria and the ischemic origin of the cavity of the abscess. Diffusion-weighted (DW) magnetic resonance imaging (MRI) is helpful in diagnosing a brain abscess and differentiating it from a neoplasm.1

♦ Treatment

A posterior fossa abscess with clinical features of compression is a surgical emergency, and unless treatment is initiated promptly, the neurologic condition may deteriorate. Radiologic findings are helpful in making the decision about the extent of the surgical procedure, which varies from a bur hole to a standard suboccipital craniotomy. Compared with primary aspiration, primary excision is the preferred method for treating a cerebellar abscess.11 Excision may not be possible in all cases. When the capsule is ill-formed, stereotactic or image-guided aspiration yields a specimen for diagnosis and achieves the decompression of the posterior fossa contents. Accompanying hydrocephalus may require external ventricular drainage. In cases of congenital cyanotic heart disease, a corticotomy must always be done under direct vision, to avoid dilated surface veins. There are reports where the endoscope was used in aspirating an intracranial abscess.12,13

In a chronic abscess or in cases of recurrent collections of pus requiring repeated aspiration, excision of the abscess is necessary. Excision of the abscess is also recommended in cases of abscess due to penetrating trauma. There is a well-defined plane of cleavage between the capsule and the adjacent brain parenchyma. Image guidance greatly helps in cases of abscesses that are deeply located. Care must be taken when excising the abscess close to the ventricular system so as not to rupture into the ventricular system. Small fragments of capsule adherent to the surrounding structures such as the neurovascular bundle in the CP angle or the fourth ventricle can be left behind. Persistent hydrocephalus is treated with a shunting procedure, once the infection is controlled.14

Fig. 8.5 Magnetic resonance imaging (MRI) contrast scan showing well-loculated abscess with peripheral ring enhancement.

Treatment does not stop with the brain abscess. The underlying condition responsible for the abscess needs to be dealt with as well, otherwise the infection might recur again.

Subdural Empyema

Subdural empyema is defined as a purulent infection of the space between the dura and the arachnoid membrane.3 Subdural empyema accounts for approximately 13 to 23% of all intracranial infections.15 With the wide availability of computer-based imaging and good medical facilities, and with the possibility of prompt surgical intervention, the mortality rate has dropped from as high as 40% down to 12.2%.16 With prompt diagnosis and treatment, the mortality should be less than 10%.17,18 Macroscopically one can find thickened arachnoid and thrombosed meningeal veins. Microscopically, subdural empyema is characterized by various degrees of organized exudates, with infiltration of the arachnoid with mature polymorphonuclear leukocytes. Superficial layers of the cerebellum may show ischemic necrosis. The main causes of subdural empyema are chronic paranasal sinusitis, otitis media, and mastoiditis. Other causes include complications of cranial surgery and compound depressed fracture. Subdural empyema is a surgical emergency; while awaiting operative intervention, antibiotics should be chosen based on the suspected source of infection. Treatment consists of a suboccipital craniotomy and drainage of pus, and institution of appropriate antibiotics. Prognostic factors include the preoperative level of consciousness, the timing, the aggressiveness of treatment, and the rapidity of progression of the disease.17 Poor prognostic factors are grade III and grade IV coma at admission and age younger than 2 years.19

Brainstem Abscess

Solitary brainstem abscess accounts for less than 4% of all posterior cranial fossa abscesses.20 The pons is the most common location followed by the midbrain and medulla.21 Most infections reach the brainstem through hematogenous spread or from the middle ear by contiguous spread. In a third of cases, the origin of infection is uncertain, and without treatment the outlook is poor. The treatment of choice is surgery for aspiration of the abscess to confirm the diagnosis and to obtain pus for culture and sensitivity. Usually 6 to 8 weeks of parenteral administration of sensitive antibiotics is required.22

Tubercular Infections

Central nervous system (CNS) tuberculosis occurs in approximately 1% of all patients with active tuberculosis.23 Tuberculous infections of the posterior fossa usually manifest as meningitis or as a mass lesion (tuberculoma and tuberculous abscess formation). Tuberculosis of the CNS is most commonly seen in developing countries. In the developed world, infection with human immunodeficiency virus (HIV) has caused an increase in mycobacterium infection, especially of the atypical strains. Intracranial tuberculosis takes several forms:

1. Tuberculous meningitis

2. Tuberculoma

3. Tubercular abscess

4. Vasculitis

Tuberculous meningitis (TBM) is the most serious clinical form of extrapulmonary tuberculosis and is a medical emergency.23 In TBM, proliferative inflammatory arachnoiditis is most marked in the basal and perichiasmatic locations. In chronic forms, it resembles a fibrous mass engulfing the cranial nerves. Vasculitis of the posterior circulation is rare. Deranged dynamics of CSF results in communicating hydrocephalus due to inflammatory exudates blocking CSF absorption sites. In the majority of patients, CSF analysis shows pleocytosis, low sugar levels, and elevated proteins. The yield of Mycobacterium tuberculosison Lowenstein-Jensen medium from CSF is around 50%.24 The diagnostic yield of CSF microscopy and culture for M. tuberculosis increases with the volume of CSF submitted, and the British Infection Society recommends repeated lumbar puncture if the diagnosis remains uncertain23 because the diagnosis of TBM is made with lumbar puncture and examination of the CSF.23 Less often, hydrocephalus may result from blockade of the aqueduct or the fourth ventricular outlet by inflammatory exudates. Treatment consists of the CSF diversion procedure to relieve raised intracranial pressure (ICP). Contrast CT may demonstrate basal exudates and other coexistent lesions such as tuberculomas. The role of neuroendoscopy in patients with TBM with hydrocephalus is not yet established.25,26The raised ICP in TBM could be due to hydrocephalus, focal brain lesions, or cerebral edema. Mannitol is currently the most commonly used agent to treat the cerebral edema.27 The Cochrane Review28 recommends the use of corticosteroids in HIV-negative patients with TBM to reduce death and disability.

Tuberculomas

Tuberculomas are a conglomeration of caseous foci that develop from multiple tubercles seeding through hematogenous spread, with a centrally located active focus and fibrous capsule formation. When the host resistance is poor, this process may result in a focal area of cerebritis and tuberculous abscess formation. Tuberculomas mimic tumors both clinically and radiologically. The characteristic CT finding is a nodular enhancing lesion with central hypodensity and peripheral contrast ring enhancement. In later stages, well-encapsulated tuberculomas appear iso-to hyperdense on plain scans. The incidence of tuberculomas is high among children.29,30 In contrast to adults, posterior fossa30 tuberculomas are common in children.

Conservative management with antitubercular therapy (ATT) is indicated for intracranial tuberculomas because they respond very well. The indications for surgery are raised ICP and CT evidence of a mass effect. Lesions showing a poor response to ATT should be subjected to biopsy/surgery and histopathologic confirmation.31

Posterior fossa tuberculomas most often require surgical intervention unless they are very small and peripherally located. A standard suboccipital craniotomy and excision of the lesion is performed for both diagnosis as well as treatment. As the lesions are avascular and well encapsulated, the plane of dissection is easy, and the lesions are totally excisable, although excision is not mandatory. The aim of surgery is to release the obstruction to the CSF pathways. Typically, the granulomas are characterized by the presence of epithelioid cells and Langhans' cells with inflammatory exudates (Figs. 8.6 and 8.7). The caseous necrosis shows multiple granulomas with these cellular contents (Fig. 8.8).

Fig. 8.6 Caseous necrosis surrounded by epithelioid histiocytes and lymphocytes. (H&E ×100.)

Fig. 8.7 Granuloma comprised of Langhans' giant cells, epithelioid cells, and lymphocytes. (H&E ×400.)

Tuberculous Abscess

Dandy32 in 1932 noted the presence of pus in tuberculomas. Instead of forming caseating granuloma, the brain parenchyma liquefies and becomes an encapsulated sterile pocket of pus. The wall of the tuberculous abscess does not contain the epithelioid granulomas, but is made up of fibrous tissue, as in a pyogenic abscess. The pus of the tuberculous abscess shows the presence of numerous tubercle bacilli, as has been reported by various authors.31,33–35Tuberculous brain abscesses commonly occur in patients with abnormal cell-mediated immunity and are mostly focal. The criteria for diagnosis of a tuberculous abscess are the presence of pus, isolation of acid-fast bacilli from pus, and histologic confirmation of the abscess.

♦ Bacteriology

The majority of intracranial tuberculomas are caused by the human strain of tubercle bacilli.

♦ Radiologic Findings

Computed Tomography Scan Findings36

1. Isodense lesion or slightly hyperdense lesion with strong contrast enhancement

2. Disk-like shape

3. A nodular mass with irregular margins

4. Combination of disks and ring

5. Nonenhancing lesion

6. Target sign with central calcification

7. Approximately 50 to 60% of tuberculomas are multiple.

Magnetic Resonance Imaging

On T1-weighted MRI, tuberculomas show a hyperintense rim surrounded by complete or incomplete hyperintensity, and the central part may be variable in signal intensity.

Fig. 8.8 Caseous necrosis with multiple well-formed granuloma. (H&E ×100.)

On T2-weighted MRI, the granulomas are usually hyperintense and the variable intensity on MRI represents layers of collagen fibers and inflammatory cellular infiltration surrounding the active caseous focus. MR spectroscopy demonstrates a characteristic lipid peak in the central liquefaction area.

Medical Treatment

The first-line antitubercular drugs that are most commonly used are isoniazid, rifampicin, and pyrazinamide; all are bactericidal. Ethambutol, a bacteriostatic drug, is included as a complement in some cases. In view of the lack of wide data from well-controlled trials, presently four drugs are administered for the initial 3 to 4 months and two drugs for another 14 to 16 months. Most intracranial tuberculomas start to regress after 4 to 6 weeks of antitubercular therapy, and most resolve within 12 to 14 months. In one study, more than two thirds of patients with partially excised or biopsied tuberculomas exhibited persistent lesions on CT scans36 even after 18 months of ATT. Some tuberculomas show an initial increase in size after starting ATT and then regress after the treatment continues for a time, before the onset of any neurologic deficits or altered sensorium; the outcome is excellent. An unusual phenomenon, paradoxical progression of lesions, has been observed in certain cases,37–39 but this does not represent treatment failure.

Surgery is indicated if the tuberculoma is obstructing CSF pathways in the posterior fossa, leading to hydrocephalus, or when the diagnosis is in doubt and in certain drug-resistant cases. Total excision of a tuberculoma may be advocated whenever feasible. As reported by Poonnoose et al,36 only 18.2% of patients demonstrated complete resolution of their tuberculomas after 9 months of ATT. Even after 18 months of ATT, approximately 70% of patients had residual lesions on imaging. By 24 months, only 54% of patients demonstrated complete resolution of their tuberculomas. Some authors feel that the duration of therapy should be based on the radiologic response of the lesion.36

♦ Posterior Fossa Fungal Infections

The causative agents include coccidioides, histoplasma, paracoccidioides, and blastomyces. Opportunistic fungi include aspergillus, candida, cryptococcus, and mucormycosis. In HIV-infected individuals, the incidence of cryptococcal infection in the era of highly active antiretroviral therapy (HAART) is between 0.1% and 1%.40 Central nervous system fungal infections are secondary to hematogenous dissemination from a focus elsewhere in the body. CNS fungal infections are increasing due to the use of immunosuppressive drugs in oncology, the high prevalence of HIV, poor control of diabetes mellitus, malignancy, and following organ transplantation.41 Fungal infections of the CNS may cause leptomeningitis, cerebral abscess, granuloma, and obstruction of vasculature by hyphae causing infarcts.

These fungi gain entry into body through the respiratory system, paranasal sinuses, the middle ear, and the skin. Invasion of the CNS may lead to granuloma formation or meningoencephalitis.

Fungal granulomas are most commonly due to aspergillus42 (Figs. 8.9 and 8.10). Aspergillus is highly angiotropic. It can also lead to a mycotic aneurysm. Aspergillus spp. have an angioinvasive nature due to their ability to produce the enzyme elastase. The elastin present in arterial walls and anatomic barriers offers resistance to bacterial invasion but cannot protect against fungi that produce elastase.43 The pseudomycetes are all capable of producing granulomas except for Cryptococcus. The exact incidence of isolated posterior fossa fungal abscesses or granulomas is not known. However, a high index of suspicion and the use of imaging can help in diagnosing them preoperatively.44 In the study by Gasparetto et al,45 the incidence of CNS paracoccidiomycosis affecting the cerebellar hemisphere was 35%

Surgical Treatment

There are three commonly performed procedures: (1) surgical excision to remove or reduce the mass effect, (2) ventricular shunt placement to treat hydrocephalus, and 3) stereotactic biopsy as a means for establishing the diagnosis and for identifying the organism.

Fig. 8.9 Multiple hyphal forms of Aspergillus. (SM ×100.)

Hydrocephalus may result from basal arachnoiditis or intraventricular granulomas. The CSF diversion procedure is warranted if the raised intracranial pressure is symptomatic. The incidence of shunt obstruction is quite high following placement of a shunt. Occasionally, cryptococcal meningitis mimics pseudotumor cerebri, which necessitates a bilateral subtemporal decompression procedure.46

Fungal abscesses are encountered with aspergillosis, cladosporiosis, mucormycosis, candidiasis, and nocardiasis. Usually abscesses are multiple, preceded by vasculitis and hemorrhagic infarction. A well-formed abscess requires open or stereotactic drainage, with institution of antifungal therapy. When accessible, a fungal abscess should be excised totally.

Fungal granulomas are produced by aspergillosis, histoplasmosis, blastomycosis, paracoccidiomycosis, and cryptococcosis. They mimic tuberculomas but are firmer in consistency, and they often necessitate the use of a knife to cut open the lesion and excise it. The plane of cleavage is difficult, but, in contrast to tuberculomas, fungal granulomas require total excision whenever possible.

The treatment of choice in fungal granulomas or abscesses is antifungal therapy after surgical excision of the lesion, whenever feasible and safe for the patient. The most frequently used agents in this clinical setting are amphotericin B and flucytosine. The usual duration of therapy is 8 to 10 weeks. The recommended dosage of amphotericin B is 0.5 to 0.7 mg/kg. As with its use for other conditions, amphotericin B treatment requires monitoring of renal function and good hydration. The recent addition, liposomal amphotericin, is showing promising results.

Flucytosine is a synthetic oral drug useful in cryptococcus, candidiasis, and chromoblastomycosis. Within the fungal cell, flucytosine is converted to the antimetabolite 5-fluorouracil. Drug resistance appears rather rapidly when flucytosine is used alone. For this reason the drug is generally used in combination with amphotericin B. The usual dose of flucytosine is 25 to 37.5 mg/kg every 6 hours. It is well absorbed from the gastrointestinal tract and has good penetration into the CSF. Normal renal functions are mandatory before this drug is administered.

Fig. 8.10 Slender septate, acute-angle branching hyphal forms of Aspergillus. (SM ×400.)

♦ Parasitic Infestation of Posterior Fossa

Parasitic infections involving the posterior fossa include cysticercosis, hydatidosis, schistosomiasis, paragonimiasis, trichinosis, filariasis, angiostrongyliasis, and gnathostomiasis. Most of these infections are not surgically relevant, but cysticercosis is the most prevalent.

Neurocysticercosis

Human cysticercosis results when a person serves as the intermediate host of Taenia solium, a tapeworm whose larvae develop in various body tissues including the brain. Neurocysticercosis (NCC) is a growing public health problem in the United States.47,48 It has four forms morphologically: racemose, cystic-intracerebral, intraventricular, and spinal. The racemose form of cysticercal cysts occupies the basal cisterns. It produces chronic meningitis, a process leading to communicating hydrocephalus. Intraventricular cysticercal cysts may block the outlet of the fourth ventricle, causing obstructive hydrocephalus, which may require surgical diversion procedure. They also induce aqueduct stenosis of an obstructive and inflammatory nature. Intraparenchymal cysticercosis of the cerebellum and brainstem occurs as part of a generalized disease spectrum (Fig. 8.11). Dead cysts undergo calcification.

Diagnosis

The diagnosis of NCC is based mainly on imaging studies and serologic tests. CT scans show features of various stages of the disease. A hyperdense cystic lesion, a hyperdense ring with ring enhancement, and a mural nodule suggestive of scolex are diagnostic. MRI features vary according to the stage. Sometimes intraventricular cysts are difficult to visualize on plain T1-and T2-weighted images. Magnetic resonance CSF flow studies may demonstrate flow obstruction at the site of the cyst. MRI can accurately visualize all types of NCC (Figs. 8.12 and 8.13). Currently, most centers use an enzyme-linked immunoelectrotransfer blot (EITB) with purified glycoprotein antigens (Western blot), which can be done in serum samples or in CSF.49

Fig. 8.11 Cysticercal larva in the brainstem. (H&E ×100.)

Treatment

The most common surgical indication in NCC is hydrocephalus.49 Both obstructive and communicating hydrocephalus can occur. Neuroendoscopic surgery is an effective treatment modality for patients with intraventricular NCC.50 A large cyst of more than 2 cm in the cerebellar hemisphere may warrant surgical excision. Antihelminthic agents are the main stay of definitive treatment when it is required. But controversy exists as to whether antiparasitic treatment of cysticercosis is necessary in most cases. The indiscriminate use of cysticidal agents in patients with a heavy parasitic load may cause lysis of the cysts, resulting in brain swelling and coma; in patients with intraventricular cysticercosis cysticidal agents may lead to hydrocephalus.

Albendazole is the most commonly used and time-tested drug for cysticercosis; it is prescribed at 15 mg/kg body weight per day in divided doses for 30 days. Albendazole is cost-effective and devoid of major side effects. It has a higher parasiticidal effect than praziquantel.51 Praziquantel dosage is 50 mg/kg/day orally divided three times a day for 2 weeks. It has to be administered under cover of steroids if the lesions are in the brainstem or orbit to avoid a flare up of edema due to the release of toxic products from the cyst contents.

Corticosteroids are the main form of therapy for cysticercotic encephalitis, angiitis, and chronic meningitis that causes progressive entrapment of cranial nerves.52 Patients with only calcification should not receive cysticidal drugs because the lesions represent dead parasites.49 New therapies with albendazole and praziquantel have reduced the duration of the anticysticercal therapy to 8-and 1-day courses, respectively.31,53

Antiepileptic drugs (AEDs) are recommended to treat acute symptomatic seizures and can be withdrawn once the follow-up scans show resolution of the lesion.54 In most cases, the seizures can be managed with a single medication.

Hydatid Disease

Hydatid disease is a parasitic infestation by a tapeworm of the genus Echinococcus. The liver is the most common site for a hydatid cyst (65–75%), whereas hydatidosis of the brain accounts for 1 to 3% of all hydatid lesions.55 The infection spreads to the human brain primarily via the hematogenous route or by metastatic spread when a cyst ruptures in the heart or lung. Usually the hydatid cysts in the brain are single and most commonly present with features of raised ICP. About 50 to 75% of intracranial hydatid cysts are seen in children.56 They are mostly located in the frontal and parietal regions.57 The cysts tend to occur in the distribution of the middle cerebral artery.

Fig. 8.12 Axial T2-weighted MRI scans and sagittal sections showing multiple cysticerci in the parenchyma, including the brainstem and cerebellum.

A posterior fossa hydatid cyst is very rare.58 It has been demonstrated experimentally that it takes 5 to 16 months for the cyst to grow 1 cm in diameter in the brain.59 CT shows a well-defined round or oval cystic structure of CSF density. Rim enhancement is infrequent, but intravascular contrast material may minimally increase the attenuation value of the cystic fluid. There is no surrounding edema, but a considerable mass effect and hydrocephalus may be present. Calcification occurs in less than 1% of the brain hydatid cysts. A typical germinal layer and scolex are diagnostic pathology features (Fig. 8.14). Positive Casoni's test is diagnostic, but a negative test does not rule out the diagnosis.60 MRI is superior to CT for evaluation of brain hydatid disease. In MRI, usually an intraaxial fluid-containing structure is seen with no associated edema or enhancement, unless cystic rupture and leakage occur and induce an inflammatory reaction. A diagnostic feature of cerebral hydatid disease that has been described is the low signal intensity of the cyst wall in T2-weighted images.61

Surgical Treatment

Surgical excision of the hydatid cyst is the treatment of choice if it occurs in the posterior fossa. Intact delivery of the hydatid cyst should be the goal in all cases. Care must be taken in opening the dura to avoid premature rupture of the cyst. The pia-arachnoid is opened at the periphery of the cyst, and the cyst is allowed to deliver itself by the pulsations of the brain. The cyst can be delivered by lowering the head of the operating table and instilling warm saline between the cyst and the surrounding brain.62

A recurrent hydatid cyst is far more complicated to excise due to the inflammatory adhesions between the surrounding neurovascular structures and the basal dura. Medical management with albendazole is necessary in instances where complete excision is not possible and when spillage of contents occurs. Albendazole results in disappearance of up to 48% of cysts and a substantial reduction in size of the cysts in another 28%.60

Fig. 8.13 Axial T1-weighted MRI scans and sagittal sections showing multiple cysticerci in the parenchyma including the brainstem and cerebellum. The classic "starry sky" appearance.

Fig. 8.14 Germinal layer of hydatid cyst with two scolices. (H&E ×100.)

Mycotic Aneurysms

Mycotic aneurysm are rare neurovascular lesions, constituting approximately 2 to 5% of total intracranial aneurysms.63 They are more commonly located on the middle cerebral artery (MCA) or its distal branches. Mycotic aneurysms involving the posterior circulation are rare.64 The causative organisms could be either bacterial or fungal. In the series published by Chun et al,65 the majority of the organisms were either streptococcus or staphylococcus. Fungal organisms such as aspergillus, candida, and phacomycetes may also cause mycotic aneurysms. Aspergillus spp. have an amigo-invasive nature due to the ability to produce the enzyme elastase. The elastin present in arterial walls and anatomic barriers offers resistance to bacterial invasion but cannot protect against fungi that produce elastase.43

Fungal mycotic aneurysms involve large intracranial vessels, in contrast to bacterial aneurysms. Though the exact incidence of fungal mycotic aneurysms is not known, usually they occur in approximately 50% of cases involving large vessels of the posterior fossa.66 The infectious emboli lodge in distal cerebral arteries and occlude the blood flow, and the resultant intense inflammation in the vessel wall weakens it.65

Clinical Manifestations

The clinical manifestations depend on the mode of presentation. A ruptured mycotic aneurysm usually presents with a hematoma, and the resultant mass effect might lead to neurologic deterioration or cerebellar signs and symptoms.

The gold standard for the diagnosis of peripherally located mycotic aneurysms is digital subtraction angiography. Serial angiograms may be necessary during the treatment process to monitor the efficacy of treatment.67

Treatment

Treatment of the underlying predisposing infection is an important component of therapy,68 and in a majority of the cases it is endocarditis.65 The aims of treatment include treating the underlying infectious process responsible for mycotic aneurysms and obliteration of the aneurysm. The management of unruptured mycotic aneurysms depends on their size and location, and on the risk of bleeding.68 Antibiotic treatment needs to continue for 4 to 6 weeks or until the cultures are consistently sterile.

Indications for Surgery

The indications for surgery are hematoma with mass effect and failure to respond to antimicrobial therapy. One of the problems in the obliteration of the aneurysm by surgery is the friable nature of the aneurysm wall and the parent artery. Prior antibiotic therapy might make the aneurysm less friable and facilitate surgical clipping. Mycotic aneurysms that are peripherally located can be excised.

The mycotic aneurysm on the proximal vessels that is otherwise surgically difficult to access may be treated conservatively with antibiotics. The use of stereotactic, angiographic guidance for localization and clipping of a small, distal intracranial bacterial aneurysm may be quite useful for surgically treating mycotic and other peripheral aneurysms.69 If the mycotic aneurysm involves the proximal intracranial vertebral artery, excision of the aneurysm with end-to-end anastomoses or extracranial-intracranial bypass may be attempted. In patients, harboring multiple mycotic aneurysms involving the posterior circulation, treatment strategy includes obliteration of the lesion most likely to have bled, as well as any easily accessible aneurysm where obliteration will not unnecessarily increase the risk of surgical complications.

Various reports have shown the safely and efficacy of endovascular treatment of cerebral mycotic aneurysms.70,71 The number of cases in the above series are small, and much larger studies are needed to confirm the role of endovascular treatment.

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