Residency · Residency · Neurosurgery

Central Nervous System Infections: Abscess and Empyema

Introduction

Brain abscess and subdural or epidural empyema are life-threatening CNS infections that frequently require neurosurgical intervention. Despite advances in antimicrobial therapy and imaging, these infections carry significant morbidity and mortality, particularly when diagnosis is delayed. Neurosurgeons must understand the pathogenesis, microbiology, imaging characteristics, and surgical management of these conditions.

Brain Abscess

Epidemiology and Pathogenesis

Brain abscess has an incidence of 1 to 2 per 100,000 population per year, with higher rates in immunocompromised patients. Infection reaches the brain through several routes. Contiguous spread accounts for 40 to 50 percent of cases and includes extension from sinusitis (typically causing frontal lobe abscess), otitis media or mastoiditis (temporal lobe or cerebellum), and dental infections (frontal lobe). Hematogenous spread occurs in 25 to 30 percent, arising from endocarditis, pulmonary infections, or skin infections, and often produces multiple abscesses at the gray-white matter junction. Post-traumatic and post-surgical abscesses account for 10 to 15 percent, resulting from penetrating trauma or craniotomy. In 15 to 20 percent of cases, no identifiable source is found.

Microbiology

Streptococci, particularly viridans group and S. anginosus, are the most common organisms overall. Staphylococcus aureus, including MRSA, predominates in post-traumatic and post-surgical infections. Anaerobes such as Bacteroides, Prevotella, and Fusobacterium are often found in mixed infections arising from sinogenic or odontogenic sources. Gram-negative bacilli including Proteus, Klebsiella, and Pseudomonas are associated with otogenic and post-surgical sources. In immunocompromised patients, the microbiology shifts to include Toxoplasma gondii in HIV/AIDS, Aspergillus and Nocardia in transplant recipients and patients on chronic steroids, and Mycobacterium tuberculosis. Polymicrobial infections are common, occurring in 30 to 60 percent of cases.

Stages of Development

Brain abscess evolves through four stages. Early cerebritis (days 1 to 3) presents as a poorly defined area of inflammation. Late cerebritis (days 4 to 9) features a developing necrotic center. Early capsule formation (days 10 to 14) produces the characteristic ring-enhancing lesion on imaging. Late capsule formation (beyond 14 days) features a well-formed collagen capsule, which is notably thinner on the ventricular side, predisposing to intraventricular rupture.

Clinical Presentation

The classic triad of headache, fever, and focal neurological deficit is present in only about 20 percent of patients. Headache is the most common individual symptom, occurring in approximately 70 percent. Fever is present in about 50 percent, focal deficits in 50 percent, seizures in 25 to 30 percent, and nausea, vomiting, and altered consciousness in varying proportions. Rapid deterioration should raise concern for abscess rupture into the ventricular system, causing ventriculitis, which carries a mortality rate exceeding 80 percent.

Imaging

MRI with contrast is the modality of choice. The abscess shows ring enhancement with a thin, uniform, smooth capsule. Diffusion restriction within the abscess cavity, appearing bright on DWI and dark on ADC, is the key differentiating feature from necrotic tumor. Surrounding vasogenic edema and mass effect are typical. The capsule is characteristically thinner on the medial (ventricular) side. CT with contrast shows a ring-enhancing lesion with surrounding edema but has lower sensitivity than MRI. MR spectroscopy reveals elevated amino acids, lactate, succinate, and acetate within the abscess.

Management

Stereotactic or image-guided aspiration is the preferred initial surgical approach for most abscesses larger than 2.5 cm, providing material for culture while reducing mass effect. Craniotomy with excision is reserved for multiloculated abscesses, fungal abscesses that respond poorly to aspiration, abscesses in accessible locations, or cases where aspiration has failed. Medical therapy alone may be considered for abscesses smaller than 2.5 cm in the cerebritis stage, multiple small abscesses, or deep-seated inaccessible locations, with close imaging follow-up required. Empiric antibiotics should include a third-generation cephalosporin such as ceftriaxone plus metronidazole, with vancomycin added if post-surgical or MRSA risk is present. The antibiotic course is 6 to 8 weeks of IV therapy, with longer durations for immunocompromised patients. Seizure prophylaxis is recommended given the approximately 30 percent seizure risk. Serial MRI every 1 to 2 weeks is performed initially to monitor treatment response.

Subdural Empyema

Pathogenesis

Subdural empyema is a collection of pus between the dura and arachnoid membranes. The most common source is paranasal sinusitis, especially frontal sinusitis in adolescents and young adults. Other sources include otitis media, post-craniotomy infection, and meningitis, particularly in infants. The infection can spread rapidly over the cerebral convexity and into the interhemispheric fissure. It is more common in males (3:1 ratio) with peak incidence in the second and third decades of life.

Clinical Presentation

Patients typically present with rapid onset of headache, fever, and neurological deterioration. Meningeal signs including neck stiffness and Kernig and Brudzinski signs are common. Focal deficits such as hemiparesis and seizures, often focal, develop as the infection progresses. Without treatment, subdural empyema can progress rapidly to cerebral herniation, and its mortality is higher than that of brain abscess when treatment is delayed.

Imaging

MRI with contrast is the preferred study, showing a crescent-shaped extra-axial collection with peripheral enhancement. Diffusion restriction distinguishes empyema from sterile effusion. CT with contrast shows a hypodense crescentic collection with rim enhancement, though findings may be subtle. Imaging should include evaluation for paranasal sinus or mastoid opacification as the source.

Surgical Management

Subdural empyema is a surgical emergency. Craniotomy with evacuation is the gold standard, as it provides wider exposure for thorough irrigation and drainage compared to burr holes. Burr holes may be appropriate for early, localized collections but carry a higher recurrence rate. Concurrent source control, including sinus drainage or mastoidectomy, may be necessary. IV antibiotics are continued for 4 to 6 weeks after surgical drainage.

Epidural Abscess (Intracranial)

Intracranial epidural abscess is a collection of pus between the skull and dura. It is less common than subdural empyema and is often associated with osteomyelitis of the skull or post-craniotomy infection. Dural attachment limits its spread, so it tends to be more localized. Management consists of surgical drainage via craniotomy combined with IV antibiotics, and infected bone should be removed if osteomyelitis is present.

Spinal Epidural Abscess

Spinal epidural abscess has an incidence of 2 to 25 per 100,000 hospital admissions and is increasing due to IV drug use and spinal procedures. The classic triad of back pain, fever, and neurological deficit is present in only about 15 percent of cases. Staphylococcus aureus is the most common organism, accounting for more than 60 percent of cases, with gram-negative bacilli as another important cause. MRI with contrast is the diagnostic study of choice, revealing a ring-enhancing epidural collection, most commonly in the thoracic and lumbar spine. Surgical decompression via laminectomy with drainage is a surgical emergency when neurological deficit is present or progressing. Medical management alone may be appropriate for patients without neurological deficit, provided close monitoring and serial imaging are maintained. Antibiotics are continued for 6 to 8 weeks.

Clinical Pearls

Diffusion restriction on MRI (DWI) is the key differentiating feature between brain abscess and ring-enhancing necrotic tumor. The brain abscess capsule is thinner on the ventricular side, predisposing to rupture into the ventricle with catastrophic ventriculitis. Subdural empyema is a surgical emergency, and craniotomy with drainage is preferred over burr holes for definitive treatment. Empiric antibiotic coverage for brain abscess should include a third-generation cephalosporin, metronidazole, and vancomycin pending culture results. Spinal epidural abscess with neurological deficit requires emergent surgical decompression, as delays in surgery directly correlate with worse outcomes.

References

1. Brouwer MC, Coutinho JM, van de Beek D. Clinical characteristics and outcome of brain abscess: Systematic review and meta-analysis. Neurology. 2014;82(9):806-813.
2. Patel K, Clifford DB. Bacterial brain abscess. Neurohospitalist. 2014;4(4):196-204.
3. Darouiche RO. Spinal epidural abscess. N Engl J Med. 2006;355(19):2012-2020.
4. French H, Schaefer N, Keijzers G, et al. Intracranial subdural empyema: A 10-year case series. Ochsner J. 2014;14(2):188-194.

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