Hyperbaric oxygen therapy of intracranial abscesses refers to the following disorders: cerebral abscess, subdural empyema, and epidural empyema. It is defined as a focal, encapsulated infection of cerebral parenchyma and is caused by a wide array of microorganisms such as bacteria, mycobacteria, protozoa, fungi or helminths. It is a relatively rare condition, with the incidence estimated at 0.3 to 1.3 per 100,000. However, this number is believed to be elevated in particular high-risk groups; for example HIV/AIDS patients.
There are multiple mechanisms responsible for the development of intracranial abscesses. The infectious organism can invade the brain by either direct spread, which accounts for 20% to 60% of the cases. This is typically caused by a contiguous infection such as sinusitis, otitis, mastoiditis, or dental infection. Brain abscesses can also arise from hematogenous seeding or cranial trauma, which typically manifest as multiple abscesses. Streptococcus and Staphylococcus are the most frequent causes of brain abscesses, with Viridans streptococci and Staphylococcus aureus being the most common. Anaerobes are also a common constituent of brain abscesses, which originate from the normal oral flora. A patient’s immune status is also important when considering the cause of the infection. Bacterial abscesses are typically seen in immunocompetent individuals, while immunocompromised patients can be infected with a wide array of organisms, including fungi.
Although it is a rare condition, mortality remains at a high rate for patients with brain abscesses. However, one systematic review and meta-analysis demonstrated that the prognosis of patients with brain abscesses has significantly improved. This study showed that over past 6 decades, case fatality rate decreased from 40% to 10%, and the rate of patients with full recovery increased from 33% to 70%. Another study, a report in 289 patients with pyogenic brain abscess treated between 1999 and 2006, showed a mortality rate as low as 2.7%.
Several factors have been credited towards these improved outcomes. Introduction of CT imaging has been critical in improving treatment results for brain abscesses. One retrospective study demonstrated that intracranial abscess mortality rate dropped from 40% to 20% within the first decade after the invention of CT imaging. This invention has allowed clinicians to make a faster diagnosis and facilitated less invasive, more precise neurosurgical procedures such as stereotactic aspiration of abscesses. Improvements in neurologic surgical technique and antimicrobial therapy, which are the mainstay of therapy, have also been instrumental in these improved outcomes.
According to the Undersea and Hyperbaric Medical Society, adjunct hyperbaric oxygen therapy should be considered for treatment of intracranial abscesses in the following situations:
The only certain contraindication to hyperbaric oxygen therapy is an untreated pneumothorax. Relative contraindications include claustrophobia, obstructive lung disease, asymptomatic pulmonary blebs on chest radiograph, upper respiratory or sinus infections, recent ear or thoracic surgery, and uncontrolled therapy. However, these relative contraindications should not automatically deter clinicians from considering hyperbaric oxygen therapy.
According to the Undersea and Hyperbaric Medical society, hyperbaric oxygen treatment should be given at a pressure of 2.0 to 2.5, with oxygen administration for a total duration of 60 to 90 minutes per treatment. Depending on the patient’s clinical condition, hyperbaric oxygen therapy can be administered 1 or 2 times daily. There are no guidelines for the optimal number of treatments needed for intracranial abscesses. However, in the largest series of ICA patients treated with hyperbaric oxygen therapy, the average number of sessions was 14 in the absence of osteomyelitis. Radiologic findings and a patient’s clinical response should be used to help determine the duration of hyperbaric oxygen therapy needed, thus requiring each patient’s therapy to be individualized.
Middle ear barotrauma is the most common complication of hyperbaric oxygen therapy. This trauma is regularly seen in patients undergoing multiple treatments. Sinus barotrauma is the second most encountered complication with hyperbaric therapy. This condition typically occurs in patients with an upper respiratory infection or allergic rhinitis.
Patient’s can also experience pulmonary complications as a result of hyperbaric therapy. Pulmonary oxygen toxicity, which manifests as chest tightness, can occur in patients receiving multiple treatments. Pulmonary barotrauma is a serious, but unusually complication that can occur during hyperbaric therapy.
Central nervous system (CNS) oxygen toxicity is a rare and severe complication that manifests as a seizure. One retrospective study showed that out of 62,614 hyperbaric oxygen sessions with 2334 patients, only one patient experienced a seizure directly related to oxygen toxicity. The risk of this complication increases with hyperbaric exposure greater than 90 to 120 minutes and at any pressure greater than 2.8 ATA. This complication can be managed by decreasing the oxygen concentration of therapy and by administering anticonvulsive medication.
Particular circumstances and complications remain and represent major therapeutic problems. This, along with improved mortality rates, has caused clinicians to search for more conservative approaches to therapy. There also remain areas of improvement necessary for the outcomes of treatment. One study revealed that despite proper medical and surgical treatment, neurologic sequelae had been reported in as many as 44% of patients. More so, half of these reported sequelae were severe enough to alter social or professional activities.
Adjunctive hyperbaric oxygen therapy has been postulated to add additional benefit to the management of intracranial abscess therapy, which can be explained by the physiologic effects that hyperbaric therapy provides. Multiple factors jeopardize a cure by antibiotics alone. First, antibiotics do not readily penetrate the blood-brain barrier, ischemic tissues, and abscesses. Furthermore, the local environment of an intracranial abscess may be too hypoxic and acidic for the proper effect of the antibiotics. The hypoxic environment seen in abscesses can also have a significant impact on immune function. The low oxygen tension in a bacterial abscess impairs white blood cell phagocytic capacity and reduces the antimicrobial effect of antibiotics.
Fortunately, these physiologic factors are directly addressed with hyperbaric oxygen therapy. Elevated partial pressures of oxygen can correct tissue hypoxia and exert bacteriostatic and bactericidal effects, particularly with anaerobic organisms. Hyperbaric oxygen therapy has also been shown to enhance the effect of antibiotic agents and the neutrophil-mediated phagocytosis of infecting organisms. Lastly, hyperbaric therapy exerts vasoconstrictive and dose-dependent anti-inflammatory benefits, which reduces focal brain swelling, resulting in a reduced intracranial pressure.
There is limited outcome data on the efficacy of adjuvant hyperbaric therapy for the treatment of intracranial abscesses. However, one recent retrospective cohort provided beneficial data about the utility of adjuvant hyperbaric therapy. In this study, the control group received surgical aspiration or excision in addition to antibiotics. The experimental group received additional adjuvant hyperbaric therapy. Data showed better outcomes for the patients receiving hyperbaric therapy. These patients had lower morbidity and a lower rate of reoperation to resolve the abscess completely. Furthermore, 80% of the patients receiving adjuvant hyperbaric therapy returned to their pre-morbid states, which is significantly higher than the 40% seen the non-hyperbaric group.
Hyperbaric therapy has provided clinicians with an effective, non-invasive therapeutic option for patients with intracranial abscesses. While the data provided is very encouraging, only a limited number of cases have been presented. Prospective, controlled studies are unlikely to be done since this condition is so rare. The limited data available has demonstrated the utility of hyperbaric therapy in the treatment of an intracranial abscess and resulted in its use in particular clinical situations.