Subdural empyema pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: João André Alves Silva, M.D. [2]

Overview

Subdural empyema, also referred to as subdural abscess, pachymeningitis interna and circumscript meningitis, is a life-threatening infection. It consists of a localised collection of purulent material, usually unilateral, between the dura mater and the arachnoid mater and accounts for about 15-22% of the reported focal intracranial infections The empyema may develop intracranially (about 95%) or in the spinal canal (about 5%), and in both cases, it constitutes a medical and neurosurgical emergency. Bacterial or occasionally fungal infection of the skull bones or air sinuses can spread to the subdural space producing a subdural empyema. The underlying arachnoid and subarachnoid spaces are usually unaffected, but a large subdural empyema may produce a mass effect. Further, a thrombophlebitis may develop in the bridging veins that cross the subdural space resulting in venous occlusion and infarction of the brain. In children, subdural empyema most often happens as a complication of meningitis while in adults it usually occurs as a complication of sinusitis, otitis media, mastoiditis trauma or as a complication of neurological procedures.

If diagnosis and treatment are prompt, complete recovery is usual.

Pathophysiology

A localised collection of pus between the dura mater and the arachnoid mater. It's a rare infection that accounts for about 15-25% of focal CNS infections and may occur in the intracranial space or in the spinal canal, being that the intracranial type is fairly more common that the spinal subdural empyema. Since the etiologic agents, the course of the disease and the treatment of this two types of empyema are different, they should be approached in separate ways.

Intracranial Subdural Empyema

Usually unilateral, it may involve the base of the brain, its convexity, the inter-hemispheric fissure along the falx cerebri or the posterior fossa. The anatomy of the meningeal membranes dictate to the course and characteristics of the disease. The dura mater and the arachnoid mater which define the initial limits of the empyema, are joined only at the base of the brain, along the falx cerebri and at the tentorium cerebelli, being elsewhere held against each other, by the pressure of the brain and cerebrospinal fluid. The virtual space between these two meningeal membranes makes the perfect way for the infection to oread along the cerebral hemisphere, inter-hemispheric fissure and posterior cranial fossa. The infection's main origin depends on the age of the individual. In younger children, the empyema most commonly results from a complication of purulent meningitis while in older children and adults, it most commonly results from complication of sinusitis (more common), otitis media or mastoiditis. In case of sinusitis the frontal sinus is the most common culprit, followed by the ethmoidal, sphenoidal and maxillary sinuses. The infection may then spread in two ways:

1. hematogenous route via retrograde infection, from thrombophlebitis of mucosal veins, which drain the sinuses.
2. contiguous infection via bone erosion and Haversian canals in bone, as a complication of osteomyelitis. In case of infection of the mastoid or middle ear it spreads by eroding the tegmen tympani, while if its an infection of the frontal air sinus, the erosion usually occurs in its posterior wall.

Other possible sources of infection may include:

• neurosurgical procedures
• head trauma (In this setting subdural empyema may not develop immediately, taking up to months or years to develop)
• in rare occasions it may develop after bacteremic seeding of an existing subdural hematoma In this situation, a relatively indolent process may evolve into rapidly progressive one.

The subdural empyema causes an inflammatory reaction in the subdural space which may be accompanied by CSF pleocytosis and encephalitis The inward venous extension of the infection may lead to hemorrhagic infarction or superficial abscess Following, cerebral edema and hydrocephalus may develop, which combined with the empyema creates a mass effect that increases intracranial pressure leading to transtentorial herniation, brainstem compression and death.

Spinal Subdural Empyema

This type of empyema is rare, compared to the intracranial type. The sources of infection can be:

• hematogenous (most common)
• osteomyelitis
• meningitis