Applied Radiology

Diagnosis

Cerebral coccidioidomycosis complicated by basal ganglia lacunar infarction

Findings

The post-contrast axial T1-weighted MR image reveals extensive enhancement throughout the basal cisterns (figure 1). Diffusion-weighted MR imaging shows a punctate focus of restricted diffusion within the left basal ganglia (figure 2), consistent with acute lacunar infarction.

Discussion

Meningeal enhancement can occur with any process that irritates the meninges. Leptomeningeal enhancement involves the pia and inner layer of the arachnoid, while pachymeningeal enhancement affects the dura and outer layer of the arachnoid.1 While carcinomatous involvement of the meninges usually affects the outer dura mater, resulting in a thickened nodular appearance, infectious meningitis most commonly causes abnormal leptomeningeal enhancement.1 Reactive meningitis due to trauma and instrumentation usually results in pachymeningeal enhancement presumably related to irritating subarachnoid blood.1

Leptomeningeal spread of tumor via the cerebrospinal fluid (CSF) is commonly associated with hydrocephalus and cranial nerve palsies. Deposition of tumor clls occurs in areas of relative CSF stasis, such as the cerebellopontine angles, suprasellar cistern, and lateral recesses of the fourth ventricle.2 Later in the course of the disease, subarachnoid enhancement may become confluent.

Infectious meningitis may be bacterial, granulomatous (tuberculous), viral, or fungal. Infectious involvement of the meninges is usually the result of hematogenous spread, but may be secondary to direct extension from mastoiditis or sinusitis. Fungi that have yeast forms small enough to spread hematogenously into the meningeal microcirculation include Blastomyces, Candida, Coccidiodes, Cryptococcus, Histoplasma, Paracoccidioides, and Torulopsis species.3 Coccidiodes immitis is a saprophytic dimorphic yeast residing in the topsoil of the southwestern United States, northern Mexico, and Central and South America. The high incidence of infection in the arid region of the San Joaquin Valley gives rise to the term "valley fever," where affected individuals present with erythema nodosum, upper respiratory symptoms, and transitory pulmonary infiltrates on radiography. Human inoculation occurs when there is perturbation of the soil with release of the spores and inhalation. Coccidioidomycosis is the result of inhalation of the arthrospore that causes the pulmonary infection. These develop into spherules that rupture and release endospores. These endospores spread hematog-enously and result in disseminated disease. Risk factors for infection include pregnancy, advanced age, and an immunosuppressed state. Affected patients may be asymptomatic or develop an upper respiratory infection (60%); the remaining 40% develop a lower respiratory tract infection. Fungemia occurs in approximately 7% of patients, but only 1% of nonimmunocompromised, nondiabetic patients develop significant extrapulmonary disease.4

The most common manifestation of CNS involvement is meningitis. Chronic meningeal involvement is particularly difficult to eradicate, and untreated coccidioidal meningitis is nearly uniformly fatal within 2 years.4,5 Dense arachnoid fibrosis causes communicating hydrocephalus by occluding the basal cisterns. Further scarring may obstruct the fourth ventricle producing a noncommunicating hydrocephalus and is responsible for most of the morbidity related to coccidioidomycosis.4 Other sequelae include abscess or granuloma formation and vascular complications, such as deep white matter infarcts and vasculitis.3,5

Acute infection may manifest as areas of focal or nodular enhancement in the basal cisterns, representing focal collections of the organism with surrounding inflammation. In more chronic cases, patients develop diffuse confluent leptomeningeal enhancement in the basal cisterns, sylvian fissures, craniocervical junction, and pericallosal regions. Parenchymal coccidioidal brain abscesses are uncommon, and occur more often in patients with coexisting HIV infection.6

Morbidity and mortality are related to uncontrollable hydrocephalus, progressive bulbar palsies, infarction, and spinal cord involvement. There is progressive diminution of enhancement with treatment.

Deep white-matter infarcts related to coccidioidal meningitis occur less commonly than in the setting of tuberculous and other fungal meningitides. Small perforating arteries supplying the basal ganglia and periventricular white matter are usually involved. Leptomeningeal carcinomatosis may also cause infarction of small penetrating arteries. Aneurysm formation, subarachnoid hemorrhage, and anterior spinal artery occlusion related to coccidioidal meningitis have been reported.3,6

Specific patterns of meningeal enhancement can suggest an infectious etiology. Coccidioidomycosis and other fungal infections have become more common outside endemic areas; however, the clinical diagnosis can be difficult because the fungus is cultured in only 25% to 50% of cases. Confirming the specific diagnosis requires a high index of suspicion, and the typical pattern of diffuse leptomeningeal enhancement (seen later in the course of the disease) may suggest the diagnosis; definitive diagnosis may require a brain biopsy.

1. Sze G, Soletsky S, Bronen R, et al: MR imaging of the cranial meninges with emphasis on contrast enhancement and meningeal carcinomatosis. Am J Neurorad 10:965-975, 1989.

2. Collie DA, Brush JP, Lammie GA, et al: Imaging features of leptomeningeal metastases. Clin Radiol 54:765-771, 1999.

3. Go JL, Kim PE, Ahmadi J, et al: Fungal infections of the central nervous system. Neuroimag Clin North Am 10:409-425, 2000.

4. Romeo JH, Rice LB, McQuarrie IG: Hydrocephalus in coccidioidal meningitis: Case report and review of the literature. Neurosurgery 47:773-777, 2000.

5. Erly WK, Bellon RJ, Seeger JF, et al: MR Imaging of acute coccidioidal meningitis. Am J Neurorad 20:509-514, 1999.

6. Wrobel CJ, Meyer S, Johnson RH, et al: MR Findings in acute and chronic coccidioidomycosis meningitis. Am J Neurorad 13:1241-1245, 1992.