AETIOLOGY, PATHOLOGY AND MANAGEMENT OF CEREBRAL ABSCESS
DR SATYAKAM PHUKAN
INDEPENDENT SURGICAL PRACTITIONER
GUWAHATI, ASSAM (INDIA)
with attachment to WINTROBE HOSPITAL, GUWAHATI, ASSAM (INDIA)
Cerebral or brain abscess is a localized focus of suppuration in the brain parenchyma produced by pyogenic bacteria, fungi , protozoa or even by helminths .
A numbers of bacteria causes abscess in the brain . Streptococci including S.millieri (member of Viridans group) other Viridans and non-hemolytic Streptococci, Enterococci, B-hemolytic Streptococci , Pepto-streptococci are most commonly isolated organisms.
Next in the order of frequency comes , Bacteroides , Enterobacteriaceae (Proteus, Escherichia coli , Klebsiella) and Staphylococcus aureus, Pneumococci, Meningococci , and Haemophilus influenzae rarely are responsible.
Anaerobic actinomyces, Veillonellae, Fusobacteria, acid fast bacilli Nocardia and Mycobacterium tuberculosis, and also Clostridium welchi are known too cause cerebral abscess.
Staphylococcal abscesses are usually a consequence of penetrating head injury or bacteraemia.
Enteric organisms are associated with ear infections. Anaerobic streptococci mostly metastasizes from lung.
Fungi are known to cause cerebral abscess and cases are increasing in patient on immunosuppressive therapy . Crytococcus is the most common fungal infection of brain. Coccidioidomycosis occurs in south-west U.S.A. Mexico and central America. Histoplasma, Blastomycosis, Candida and Aspergillus attacks immunosuppressed patients, the latter two being the most commonly involved. Phycomycetes affects predominantly those with uncontrolled diabetic ketoacidosis or other acidotic states and in leukaemia.
Protozoa and helminths are other aetiological agents responsible for brain abscess.
Entamoeba histolytica and Toxoplasma gondii are two of the commonest protozoa causing abscess in the brain.
Among the helminths Taenia solium a cestode and trematode Paragonimus westermanii and Schistosoma Japonicum can cause cerebral abscess.
Cerebral abscess may be caused by direct extension of adjacent infection, blood born or metastatic and locally implanted due to head injury.
Infection in an adjacent area may extend to brain directly, through the way of venous channels or through the bony wall, former being the commoner of the two.
Abscess so caused are either otogenic or rhinogenic in origin. Otogenic abscess generally occur in the temporal lobe. Extension occurs through the roof of tympanum or mastoid antrum frequently preceded by extradural abscess, chronic ottitis media is the commonest cause.
Extension of infection from mastoiditis or labyrinthitis can give rise to cerebellar abscess.
Rhinogenic abscess originates in frontal or ethoidal sinus infection, which spreads to the frontal lobe brain.
Primary foci of infection in metastatic brain abscess lie commonly in the lungs and heart, but may also come from skin, bone or teeth or any other site in the body. It occurs frequently in congenital heart disease with right to left shunts (e.g. tetralogy of Fallot’s) and arteriovenous vascular abnormalities of lung as in familial telengiectasia (Osler-Rendu-Weber-syndrome).
Infection very often travels from a bronchiectasis, empyema thoracis, lung abscess or a broncho-plueral fistula to cause abscess in the brain.
Metastastic abscesses are predominantly seen at the junction of gray and white matter. Frontal lobe is the most frequent site of metastatic brain abscess, cerebellum is involved rarely.
Fungal abscesses are generally caused by metastasis from a primary site in the body, but local trauma may also give rise to implantation type of abscess especially in case of Candida.
Protozoal and helminthic abscess are almost as a rule produced by spread through blood stream.
Trauma caused to head can implant organism into brain, giving rise to a implantation type of abscess. Such abscess may follow soon after the trauma or there may be long period of latency. Cases have been reported when brain abscess have occurred more then 40 years after sustaining injury.
Abscess formation is same in the brain as in any other argans, with the formation of a fibrous capsule around a focus of necrotizing cerebritis. It is one of the few instances in which fibrosis occurs with collagen production in the brain. Astrocytes do not produce collagen,it is derived from blood vessels that proliferate around the margin of the necrotic brain. This exhuberent neo vascularisation is responsible for vasogenic oedema so characteristic around brain abscess. A zone of gliosis lies outside the fibrous capsule.
The development of cerebral abscess is arbitrarily divided into three stages, First is focal acute inflammation without pus formation. Pus appears but is not well defined in the second stage. A definite wall is formed in the third stages. In rapidly fatal cases, the condition remains one of spreading suppurative encephalitis.
Microscopically, the inner layer consist of pus cells surrounded by a layer of granulation tissue containing blood vessels and the fibrous tissue derived from it. Fat granule cells, plasma cells and neutrophil leucocytes are plentiful in the middle layer. There is usually an inflammatory reaction in the overlying meninges.
Suppurative thrombophlebitis is apparent if the abscess has spread through venous channels.
The shape of the abscess is irregular as the thickness of the capsule is not uniform. Loculations may occur and in areas where capsule remains thin daughter abscess may form due leakage of infected materials.
The fungal cerebral abscess have certain special characteristic features, the same holds true also holds the protozoal and helminthic abscess.
Cryptococcus neoformans produces small gelatinous cystic abscesses predominantly in superficial cerebral and cerebellar cortices and in the basal ganglia. It evokes minimal inflammatory reaction, and oedema or gliosis are insignificant. Cysts contain masses of yeasts and multinucleate giant cells, small numbers of lymphocytes and plasma cells may be present at the periphery.
Perivascular inflammation is striking in Histoplasma and Blastomycosis abscesses.
Candida produces multiple micro-abscesses, larger ones are uncommon.
Aspergillus causes focal or multifocal cerebrities with neurotrophic exudation.
Abscess caused by Entamoeba histolytica are usually single, localized predominantly in the rich vascularised cortical grey matter or basal ganglia. In early stages it appears as a soft red focus surrounded by petechiae and as a rule cavitates. Occasionally multiple abscesses have been found. The contents of the cavity are yellow-green, its walls are irregular and there is no evidence of encapsulation.
Microscopically, the abscess wall has an inner zone of necrotic tissue and a broad outer zone of congestion and vascular proliferation. The surrounding brain is the site of glial proliferation and is infiltrated by lymphocytes, plasma cells and scanty polymorphs around bloode vessels. Trophozoites can be identified in the necrotic wall of the abscess.
Toxoplasma gondii causes abscess which are grouped into three morphological types. First type is a necrotizing abscess consisting of poorly circumscribed areas of necrosis which includes the blood vessels, with variable amounts of hemorrhage, intimal proliferation and thrombosis. Organism are invariable present. The second type is a organising abscess with a central areas of coagulative necrosis surrounded by macrophages, with organisms present in a minority of cases. The third types is a chronic abscess consisting of well demarcated cystic spaces containing macrophages and on rare occasions micro-organisms.
Among the helminths causing abscess, Taenia soluim gives rise to multiple cysts in brain and meninges. Paragonimus westermanii gives rise to a granulomatous mass containing necrotic tissue, giant cells, parasites and ova.
ABSCESSES AT UNCOMMON SITES
Abscess may occur inside a brain tumour , usually causing necrosis and pyogenesis in the core.
Pituitary gland may have an abscess causing complete necrosis of the gland
Brain stem abscess can be heamatogenous or post –traumatic following whiplash injury of upper cervical spine or due to osteomyelitis of odontoid process.
Thalamic abscess which are very rare resemble a fast growing thalamic tumour.
The management of cerebral abscess entails proper diagnosis and treatment .
It is based on clinical diagnosis and investigations.
Clinical diagnosis is extracted from :-
Evidence of past or present infection .
Focal neurological symptoms and signs.
Symptoms and sign of raised intracranial pressure .
Evidence of infection in the areas adjacent to brain are looked for , as also in other sites from which there is likelihood of metastases to brain. Even an acute history of upper respiratory tract infection, such as a heavy cold without positive radiological evidence of sinusitis may be relevant . General feature of infection may be few or absent. Thus fever is frequently absent as is tachycardia , but is a dominant feature. If there is meningitis accompanying . Severe rise of temperature occurs if an abscess ruptures into a ventricle indicating sure catastrophy
There is usually loss of appetite , exhaustion and a furred tongue.
Focal neurological symptoms and signs depend upon the site of the lesion. The patient of cerebral abscess frequently appear in psychiatric clinic presenting diverse psychiatric and neurological signs and symptoms.
Frontal lobe abscess may present with apathy, having a superficial impression of depression (indeed many patients treated for depression were later found to have frontal lobe abscess ), with occasional angry outbursts, indifferences, discrepent motor and verbal behaviour , if the pathology lies in the frontal convexity . There may be disinhibited impulsive behaviour with emotional lability , poor judgement and insight , if the lesion lies in the orbito – frontal region . There may be contralateral or bilateral hemiplegia depending on the sites , and spastic pseudo- bulbar palsy may also be found.
Temporal lobe abscess may present with dysphasia , psychomotor epilepsy, auditory and visual hallucinations and hyper- sexuality. Interestingly enough seizures are not commonly seen , as one might suspect in cases of temporal lobe abscess. There may be homonymous upper quadrantopia, dysphasia and contralateral hemiparesis,
Cerebeller abscess often present with sub-occipital headache , which may radiate down the neck and be associated with neck stiffness, head may be flexed to the side of the lesion or retracted. Signs vary in severity. There may be phasic nystagmus, most marked on looking toward the side of the lesion , hypotonia and inco-ordination in the limbs on the affected side. Past pointing outwards may be demonstrated with the affected hand and there may be a tendency to deviate or fall to the side of the lesion while walking.
Occipital lobe abscess presents with visual hallucination and cortical blindness with denial .
Brain stem abscess will show diminished consciousness with fluctuating attention which may progress to unconsciousness. . Larger lesion of brain stem will show spontaneous extensor spasm of all four limbs, ophistotonus , rapid pulse, shallow and inadequate respiration, small pupils, pyrexia and sweating. Any stimuli will tend to increase the tendency to extension.
Symptoms and signs of raised intracranial pressure.
The symptoms of raised intracranial pressure include headache of a persisting type , deterioration in the level of consciousness, vomiting and failing vision . on examination a depressed level of consciousness , papilloedema , slowing pulse and rising blood pressure are the signs that can be elicited.
The investigation must be done with outmost urgency, and it must be remembered that lumbar puncture is strictly contra–indicated even in the presence of concomitant meningitis because of the risk of coning .
Plain skull radiography
This is useful as it may show a midline shift if the pineal gland is calcified .
CT scan is the best investigation if available . It is quick, noninvasive and extremely accurate in localizing abscesses. Brain abscess is recognized by its distinctive capsule a smooth ring like structure of uniform thickness recognized only after contrast enhancement . The lack of capsular enhancement in the initial 24 to 48 hours when only focal “ cerebritis” is presence may lead to an abscess being missed if the index of clinical suspicion is not high enough to repeat the examination daily . Tubercular and fungal abscesses show irregular marginal enhancement after contrast.
For brain scan radioactive Technetium is injected intravenously and , brain is scanned by a gamma camera . It is very valuable as an investigation in supratentorial abscess . A “doughnut“ sign consisting of an area of decreased uptake surrounded by increase uptake has been reported. It is however not specific for abscess.
Electro- encephalography is useful but is of lesser help in posterior fossa abscess.
Arteriography is still recommended as an important investigation , but is not without risk and its accuracy can be doubtful . In this examination a “ring “sign occurs because of concentration of contrast media in new vascular structure . The sign for this reason is more likely to occur in a chronic abscess. A cortical “Ripple “ sign or pattern due to alteration of normal sulci and gyri is seen in the intermediated phase in quality films . It also shows areas of intracerebral shifts and avascularity.
Ventriculogram is not in use to the nowadays due to the advent of more better investigation. This investigation is very much informative in cases of the rare thalamic abscess , showing concave filling defect with or without elevation of the floor of the lateral ventricle.
Pyography is a pre-operatively performed investigation wherin contrast is a in stilled into abscess cavity at the time of aspiration . Colloidal barium is the medium generally used.
MRI (Magnetic resonance imaging ) is a good investigation and has become available at many centres around the world . Toxoplasma abscess is best delineated by M.R.I. Below are two MRI images of pyogenic brain abscess.
Ultrasonography can be useful for investigation of infants before closure of the fontanellae .
Routine examination of blood in cases of brain abscess show W.B.C. count above 10,000 in only 50% and above 20,000 in 10% only. E.S.R. is raised in the majority.
The treatment is based on the following principle .
Control of infection by appropriate drugs.
Surgical management of the abscess.
Eradication of the primary focus .
Aftercare for sequelae like seizures, hemiparesis, dysphasia etc.
Control of infection by appropriate drugs
Antimicrobials or antihelminthics that readily cross the blood brain barrier receives greter preference in the treatment with systemic drugs . where facilities exists , aspiration of the pus by C.T. guided percutaneous stereotactic aspiration and examination by both microscopy and culture will help in confirming the diagnosis and in administering the specific drug.
Surgical management of cerebral abscess
Treatment of the brain abscess takes priority over the treatment of the primary focus because of the threat to life from increase intracranial pressure and possibilities of ventricular or sub-arachnoid rupture . Surgical treatment must be under taken at the earliest possible moment when the history ,clinical sign and investigation if available indicate well formed abscess. This has to be done as an emergency procedure.
But when diagnosis is in doubt or when there is possibility of the lesion still in the stage of ”cerebritis” it is preferable to wait for a few hour or a day or two .
Aspiration of the abscess
Abscess can be approached easily and accurately by using CT guided stereo –tactic percutaneous operative technique. The abscess is aspirate, which serves both curative as well as daignotic purpose .Appropriate antimicrobial drugs can be instilled in to abscess cavity .
Pyography can be done along with the curative procedure .CT guided streotactic percutaneous operation because of the convenience and low risk has proved to be a useful treatment.
Aspiration and catheter drainage
A burn hole is a pre- requisite for performing this procedure . 12-16 French rubber cather is used and the cavity is generally irrigated using appropriate anti-microbial solution.
Excision of abscess
The indication for excision are persistence of infection , failure of aspiration , persistence of sign and symptoms of increase intra cranial pressure inspite of adequate treatment .
Excision after one initial aspiration has been advocate by many neuro –surgeons.
In supra –tentorial abscess, a suitable osteoplastic flap is turned with reduction of intra –cranial pressure by intra–venous mannitol, started simultaneously with the incision . This relaxes the dura. Adhesions between dura and brain or any fibrous cord often seen in traumatic abscesses are carefully looked for . Incisions in the cortex are made avoiding important vessels .
Capsule is meticulously dissected using soaked cotton pledgets caring to look for lobulations . Special care is needed or deeper abscess in order to avoid opening the ventricular wall . Spillage of pus if any is removed at once . Dura is not closed but left open if brain continues to be tense due to extensive oedma.
For excision of cerebellar abscess, a vertical para –median incision is used . Some surgeons recommend a wide decompression of the posterior fossa after excision of cerebellar abscess to prevent medullary compression which is the commonest cause of death.
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