Pyogens, Mycobacteria, and Fungus

CHAPTER 41 Pyogens, Mycobacteria, and Fungus



Neurosyphilis is a chronic infection caused by the spirochete Treponema pallidum. When untreated, the disease progresses through three clinical stages designated primary, secondary, and tertiary syphilis. Alternate names include neurolues, syphilitic gumma, tabes dorsalis, and general paresis.


In meningovascular syphilis the leptomeninges show thickening and perivascular lymphocytic infiltrates. The intracranial vessels show evidence of vasculitis that leads to stenoses and occlusions with ischemia and infarction. In one series, 23% of patients with syphilis showed evidence of cerebral infarctions.2 In a second series, 43% of patients had cerebral infarction.3 The infarctions primarily involved the basal ganglia, the middle cerebral artery territory, and branches of the basilar artery.4 Large arteries show concentric or asymmetric stenoses. These may manifest as segmental constriction and occlusion of the supraclinoid carotid arteries.5 Small arteries show focal stenosis and aneurysmal dilatations.

Syphilitic gummas represent a parenchymal form of neurosyphilis that results from cell-mediated immune response to T. pallidum.



Meningovascular syphilis causes cerebral infarctions in 43% of patients.3 Acute infarctions appear as zones of restricted diffusion with high signal intensity on diffusion-weighted imaging (DWI) and low apparent diffusion coefficient (ADC) values. Subacute and chronic infarctions appear as high signal lesions on T2-weighted (T2W) and fluid-attenuated inversion recovery (FLAIR) images. MR angiography shows the narrowing or occlusions of the vessels. Nonspecific focal or diffuse enhancement observed on enhanced MR images cannot distinguish meningovascular syphilis from meningitis of other origin.

In gummatous neurolues, gummas are usually hyperdense on CT and hyperintense on T2W MR images. On contrast-enhanced MRI, cortically located ring- or nodular-enhancing lesions with adjacent meningeal enhancement should raise suspicion of a possible syphilitic gumma.7

In general paresis, T2W and FLAIR images show cerebral atrophy and multiple high signal intensity lesions.


Nocardiosis is a bacterial infection caused by weakly grampositive, filamentous Nocardia species.


Nocardia is a genus of gram-positive, rod-shaped bacteria. N. asteroides is responsible for approximately 80% of CNS nocardial infections.9 Other pathogenic species include N. farcinica, N. brasiliensis, N. transvalensis, and N. otitidis-caviarum. Nocardia usually establishes a primary infection in the lung and spreads hematogenously to the CNS in 15% to 44% of cases. In one study of 30 Nocardia infections in HIV-positive patients, 73% of patients had pulmonary disease.8 Solitary brain abscess is the most common form of CNS infection with Nocardia, but multiple lesions are found in about 38% of cases.



The MR appearance of a nocardial abscess is similar to that of other pyogenic brain abscesses. The necrotic center is hyperintense on T2W imaging and hypointense on T1-weighted (T1W) imaging. The capsule has high T1 signal and low T2 signal and shows smooth peripheral enhancement on contrast-enhanced images. Nocardial CNS infection may also manifest as subependymal nodules and meningitis.11 In one study, 92% of AIDS patients with nocardial CNS infection had imaging evidence of associated meningeal disease.11 Of the nine patients, hydrocephalus was present in five, subependymal nodules were evident in five, and clinical evidence of meningitis occurred in three. Histologic examination of the small subependymal nodules revealed inflammatory cells consistent with ventriculitis/ependymitis and developing subependymal abscesses.11 Nocardial abscesses may assume a characteristic “budding” appearance in which multiple, closely spaced, hematogenously disseminated abscesses conglomerate to create a budding appearance as they enlarge.12 Diffusion-weighted images can show homogeneous hyperintensity within the lesion (Fig. 41-2). In limited case material of active nocardiosis, MR spectroscopy showed a rise in the mean choline/creatine ratio and a slight reduction in the mean Nacetyl-aspartate (NAA)/creatine ratio, with lactate and amino acid peaks consistent with bacterial abscess.13


Tuberculosis is a bacterial disease caused by organisms of the Mycobacterium tuberculosis complex. Most tuberculous infections of the CNS are caused by M. tuberculosis. Less frequently, other mycobacteria may be involved. Other terms associated with this disease include tuberculous meningitis, tuberculous granuloma, and tuberculoma.

Clinical Presentation


The infectious etiology of tuberculosis was definitively proven by Robert Koch, who discovered the tubercle bacillus in 1882.

CNS tuberculosis most commonly results from hematogenous spread of the infection from an outside focus. Tuberculous meningitis may also result from outward extension and rupture of a subpial or subependymal focus (“Rich focus”) into the subarachnoid space. Rich and McCordock suggested that CNS tuberculosis develops in two stages:

Tuberculous meningitis is characterized by thick gelatinous exudates, which favor the meninges at the base of the brain. Tuberculous inflammation (vasculitis) of the vessels that traverse the subarachnoid space causes narrowing, occlusions of the vessels, and subsequent infarctions, usually affecting the middle cerebral artery territory and the small perforating arteries that supply the basal ganglia.17 The meningeal involvement impairs cerebrospinal fluid flow and resorption, so communicating hydrocephalus is the most common complication of meningeal tuberculosis.18 Villoria and colleagues report hydrocephalus in 51% of 35 patients with AIDS-related CNS tuberculosis.19

Parenchymal forms of the tuberculous infection include tuberculomas, tuberculous abscesses, and focal tuberculous cerebritis.

Tuberculous granulomas (tuberculomas) result from the hematogenous spread of infection or from the extension of meningitis into the parenchyma. Tuberculomas may be solitary or multiple, can occur anywhere in the brain, but are found predominantly in the supratentorial compartment.19

Tuberculous abscess is a true pyogenic lesion.

Focal tuberculous cerebritis is a rare form of tuberculosis.

Calvarial tuberculosis is another rare manifestation of extrapulmonary tuberculosis and usually occurs by hematogenous spread from the lungs. Because of the relatively low percentage of cancellous tissue in the bones of the cranium, calvarial tuberculosis is a rare condition. In some cases, extensive extradural granulations may cause thrombosis of the venous structures.

Jan 22, 2016 | Posted by in NEUROLOGICAL IMAGING | Comments Off on Pyogens, Mycobacteria, and Fungus

Full access? Get Clinical Tree

Get Clinical Tree app for offline access