Brain abscess

A brain abscess is a walled-off pocket of pus inside brain tissue. The microbes reach it from a nearby focus in the skull, arrive through the bloodstream, or are carried in by a wound. The immune system cannot destroy them, so it builds a wall around them instead, sealing them off from the rest of the brain. What forms is a cavity of pus that presses outwards and keeps growing. This is rare, but time works against the person: the tissue around the abscess swells, squeezes the blood vessels, and part of the damage becomes permanent.

Where the pus in the brain comes from

The brain is better protected than any other organ, so infection does not simply drift into it. There are three ways past that protection.

• From a neighbouring focus. A long-running discharging middle ear infection, infection of the bone behind the ear, sinusitis, a pocket of pus at the root of a tooth. From there the microbes travel in a straight line, through thinned bone or along the veins that connect the face with the inside of the skull.
• Through the blood. The source can sit anywhere: infection of the inner lining of the heart, widened bronchi that discharge constantly, an abscess in the lung, a purulent process in the abdomen or pelvis.
• Through a wound. An open skull fracture, a penetrating injury and, less often, a complication of brain surgery.

In some people the source is never identified, and that does not mean the search was careless.

One cause is talked about least and is worth the most. Normally all venous blood passes through the lungs, and the lungs act as a filter: small clots and stray bacteria are trapped there and never reach the brain. If there is a direct connection between venous and arterial blood, that filter is bypassed. This happens with cyanotic congenital heart defects and with arteriovenous connections in the lungs, and those connections are often part of an inherited condition, Rendu-Osler disease, in which the person and their relatives have frequent nosebleeds and small red vascular spots on the lips, tongue and fingertips. So a brain abscess in someone with healthy teeth, no ear infection and no head injury is a reason to look for such a connection rather than to call it bad luck.

A separate group carries extra risk: people whose defences are weakened by HIV, by chemotherapy, by the drugs taken after a transplant, or by poorly controlled diabetes. In them the culprit is often not an ordinary bacterium but a fungus or a parasite, and the treatment is entirely different.

What the person notices

The main trap is that the expected combination of high fever, headache and a weak arm is usually incomplete: the full set appears in fewer than a third of cases, and in roughly half of people the temperature stays normal. No fever rules nothing out.

• headache, the most constant sign: it builds over days, usually sits in one half of the head, does not lift with the usual painkillers, and worsens in the morning and when lying down;
• nausea and vomiting unrelated to meals;
• drowsiness, slowed thinking, confusion, unusual irritability or changes in behaviour;
• weakness or numbness of the arm and leg on one side, a drooping face, slurred speech, difficulty finding words;
• double vision, blurring, loss of part of the visual field;
• a seizure;
• a stiff neck and discomfort in bright light.

In a baby the picture is thinner: listlessness, refusing feeds, an unfamiliar cry, a bulging fontanelle, vomiting without diarrhoea.

When it cannot wait until morning

Call an ambulance (112 across much of Europe, or your local emergency number) if any one of these appears:

• weakness down one side of the body, a drooping face or slurred speech;
• a first-ever seizure;
• the person is hard to rouse or does not know where they are;
• the headache suddenly intensifies and comes with vomiting;
• high fever together with a stiff neck and discomfort in bright light.

Anyone already treated for an abscess should know one further warning sign: if the headache returns and builds after several quiet weeks, a second abscess may have formed. More dangerous still is rupture of the pus into the brain's ventricles, which develops over hours and needs intensive care, so a sudden deterioration during recovery is never watched at home.

How the focus is found

The main test is MRI with contrast. The diffusion sequence matters most: pus is thick, water barely moves inside it, and the focus lights up quite differently from a tumour that is breaking down. That often settles the question before any operation. CT is faster and more widely available, so emergency departments start there: the abscess shows as a rounded lesion with a bright ring around the rim.

Blood cultures are taken before the first dose of antibiotic, because afterwards they are frequently negative. The most valuable material comes from the abscess itself during needle sampling, and it is cultured in oxygen-free conditions as well, since a large share of these organisms will not grow otherwise.

At the same time the original focus is hunted down: imaging of the sinuses and the temporal bone, a dental check, and an echocardiogram if the heart valves are suspected.

A lumbar puncture is not performed when an abscess is suspected. While a mass sits inside the skull, drawing fluid from the spinal canal can shift the brain downwards and cause herniation. It also yields very little useful information.

How it is treated

Antibiotics are given into a vein and the course runs for weeks, not days: the drug has to cross a dense capsule. Treatment starts before culture results, with a combination covering both ordinary bacteria and those that live without oxygen, usually a cephalosporin together with metronidazole; if a resistant staphylococcus is suspected, a drug active against it is added. If a fungus is found, the regimen is completely different. The exact combination and duration are set by the doctor from the culture and the follow-up scans.

Surgery is not always needed. A small, deep abscess is sometimes managed with drugs alone. A large one is drained: under navigation a small hole is drilled in the skull and the pus is aspirated with a needle, which also supplies the sample for culture. If the cavity has several chambers, responds poorly, or contains a fragment, the skull is opened and the abscess removed together with its capsule.

Swelling is controlled when it threatens to compress the brain, and anti-seizure medication is prescribed to anyone who has already had a fit. One part of the plan is not optional: clearing the original focus, whether that means taking out the diseased tooth or operating on the sinus or the ear. Without it the story repeats itself.

What comes afterwards

The hospital stay lasts weeks and recovery at home takes months. How fully speech, movement and attention return depends above all on how early treatment began.

The commonest late consequence is seizures: the scar left where the abscess sat can become an epileptic focus, sometimes a year later or more. Neurological follow-up and repeat imaging are therefore needed, and returning to driving is decided individually, since the rules differ from country to country and must be discussed with the treating doctor. Contact sports are avoided after a craniotomy.

Online consultation

An online appointment is no substitute for an ambulance: weakness down one side, seizures or confusion mean calling the emergency number, not writing a message. A doctor at a distance is useful for something else: working through a prolonged headache that is behaving oddly, judging whether urgent imaging is warranted, explaining the discharge summary and the medication after hospital, and suggesting what to ask the neurosurgeon and when the next scan is due.

This material is for information only and does not replace medical advice.