Brain Abscess:

Ahmed Mohamed Sleem

SUMMARYBrain abscess is a localized suppurative process within thebrain parynchyma caused by wide variety of bacteria, fungi,parasites and protozoa.Brain abscess appear to be increasing in frequency becausethe growing number of modern and multiple techniques ofdetection of the intracranial lesions and the growing number ofimmunocompromised patients caused by opportunistic infections.The pathogenesis of solitary brain abscess differs frommultiple brain abscesses, it is thought that solitary lesions are oftenthe result of an infected parameningeal focus as paranasal sinuses,middle ear or thrombosed venous sinuses and from directincubation of the brain in trauma and postoperatively, incontrastmultiple brain abscesses are more common with systemicinfections which are often spread haematogenously, frontal,ethmoidal, sphenoidal sinusitis can cause brain abscess byretrograde thrombophlebitis via valveless veins, however theincidence of brain abscess secondary to, infected parameningealfocus appear to be decreased with more effective treatment withmodern era. of antibiotics.101Trauma is a well known predisposing factor for thedevelopment of brain abscess, in orderto prevent brain abscess as aresult of penetrating scalp injuries, perfect debridmentof necroticbrain tissue, bone fragments, and other foreign bodies must bedone soon after injury, the intracranial abscesses caused byhaematogenous spread of infection from distant septic foci such aschronic pulmonary suppuration distant osteomyelitis, bacterialendocarditis, congenital heart disease, pulmonary arteriovenous.malformation. with right to left shunt, and infected prostheticdevices represent the most frequent primary causes of brainabscess caused by haematogenous spread, however abdominal orpelvic sepsis are very occasionally source of brain abscess.The organisms responsible for intracranial infection in theimmuno-compromised host often differ from organisms nonecommonly recovered from brain abscesses, there is an increase inso called opportunistic organisms ”fungi, parasites and unusualbacteria” that are normally of low virulence and pathogenicity forhumans, the central nervous system is vulnerable to destruction byinfectious processes and is incapable of mounting a significantdefense, brain abscess caus~d by pyogenic bacterial infection wasdivided into four stages based on histological criteria, Ist stage”early cerebritis 1-4 days” is the initial response with localized102inflammatory infiltrate with polymorphonuclear leukocytes,lymphocytes and plasma cells which migrate from the peripheralblood circulation, begining in the first day and increasingdramatically by day 3, infl~matory cells appear in the adventitialsheaths of the blood vessels which surround the developing area ofinfected necrosis, the 2nd stage ”late crebritis days 4-9” the mostsignificant histoloigical changes occur during this time, thenecrotic centre enlarges because of an increase in the cellulardebris and the formation of pus caused by release of enzymes frominflammatory cells at the periphery of necrotic centre, the brainabscess reach the largest size during this stage, the 3rd stage an”early capsule formation days 10-13” in this stage the necroticcentre start to decrease in size and development of the capsule andadditional new blood vessels from out side the developing capsulewith reactive astrocytes begin to increase in the surrounding brain.from above the fully developed brain abscess have 5 distincthistological zones which are, zone ”1” well formed necrotic centre,zone ”2” peripheral zone of inflammatory cells, zone ”3” densecollagen capsule, zone ”4” a layer of neovascular developmentoutside the capsule and zone ”5” reactive astrocytosis, gliosis andcerebral edema external to the capsule.103The majority of brain abscesses occur in the first two decadesof life because of the congenital heart disease, middle ear and sinusinfections are the most common predisposing factors in this agegroup, in one study 72% of cases between 5-15 years with slightlymale predominant, male to femal ”2: 1”.The presentation of patients with brain abscess vary littlewhether there are solitary or multiple lesions, the most commonclinical symptoms and signs which include seizures, altered mentalfunction, focal neurological deficits or signs of increasedintracranial pressure indicate the presence of an intracranial masslesion, about half of the pt. have a low grade fever.In the following conditions one should be alert to thepossibility of brain abscess, purulent infection especially sinusitis,mastoiditis, chronic otitis media, penetrating scalp injures, nasal oroccipital dermal sinus tract and cyanotic heart disease.Successful management of patients with brain abscessrequire, early detection, correct time of treatment, detection ofpredisposing factors and proper follow up.The following neuroradiological studies are used to be donewhen brain abscess is suspected.104l-Plan X-ray skull: the common finding in relation to Plan X-rayskull are, fractures, osteomyelitis, bone defect, gas formation andshift of calcified pineal body.2-Computed tomography: with the advent ofC.T. it has becomepossible to both localize and determine the stage of evolution ofthe abscess formation.In a fully developed brain abscess on C.T. scan the capsule isrepresented by a ring of contrast enhancement surrounded by focalbrain edema, a typical ringenhancementusually appear in pyogenicbacterial brain abscess, nodular enhancement associated with asolid granuloma or a small brain abscess. Low density lesions withvarying degrees of contrast enhancement corresponding tomeningoencephalitis or encephalitis.3.Magnetic Resonance Imaging M.R.I.. is superior to C.T.scanbecause it allows earlier detection of the disease, M.R.l. is usefulin delineating small multicentric lesions which might otherwisebe missed by C.T., it is more sensetive for detection of crebritisstage, the edema of inflammation is seen as high signal intensityon T2 weighted images, however Tl weighted M.R.I. shows thecapsule an isointense ring separating the central low intensityabscess cavity and surrounding edema, on the T2 weightedimages the pus of the central abscess cavity is seen as very highlOSintensity separated from the surrounding edema by thehypointense capsule.Routine laboratory tests are usually not helpful in diagnosinga brain abscess, E.S.R. is elevated in 75 to 90% of cases, W.B.C.usually abnormal in 60 to 70 percent of cases.The management strategies for patients with brain abscessdepend on several important factors. The clinical circumstances ofthe pt. illness, the location of the lesion and the number oflesions.The aim of treatment of brain abscess is based on eliminating theinfectious process while reducing the mass effect caused both bynecrosis of brain tissue and by the surrounding cerebral edema, theinfection process is eliminated in the majority of cases by surgicalinterference whether aspiration or excision and by medicaltreatment in the form of antibiotic therapy. Surgical managementconsists of excision or .aspiration, aspiration is recommended indeeply seated solitary or multiple lesions, bad general condition ofthe patient, lesions in critical area of the brain and abscess incerebritis stag/There is four methods of aspiration, free hand aspirationwhich recently has little place after the development of stereotactictechnique and ultrasound guided aspiration which is ,as found to106be a simple, quick and low cost method that give reliable results,during the last decade a few reports were published thatrecommended stereotactic techniques for abscess aspiration, thestereotactic technique seems to have poor mortality rates rangingfrom 0 to 23%. Stereotactic endoscopy combines the best of bothpossible worlds, the accuracy of stereotactic guidance and theability to see the brain.Excision means craniotomy with excision of the entireabscess is used for, cerebellar abscesses, large superficial wellencapsulated, solitary abscess, traumatic brain abscess with foreignmaterial ”bone fragments, metalic fragments or other debris” thatmust be removed to prevent recurrence and fungal brain abscessbecause the resistance of many fungi to antibiotic treatment.Although the definitive treatment of brain abscess ISoperative, it has been suggested that selected patients who are notin imminent danger from increased intracranial pressure mayqualify for a trial of non operative therapy however the increasingnumber of reported nonopertive cures of brain abscesses should notbe interpreted as an indication that brain abscesses are anonoperative disease, because the primary advantage ofnonoperative treatment of brain abscesses has been that it avoidsthe risks of operation and anesthesia. However modern stereotactic107aspiration procedures guided by computed tomography orultrasound imaging can be performed under local anesthesia so therole of nonoperative treatment is too limited. Follow up of thepatient, with serial C.T. scan in the postoperative period should beobtained until resolution of the abscess is documented, in case ofaspiration C.T. scan is don 1 to 2 days after aspiration, innonoperative treatment C.T. scan is done weekly. However ringenhancement disapper 4 weeks after completion of antibiotictherapy, urgent C.T. scan must be done if the patient deterioratesat any time of the course of managemerit.