CNS Infections — Meningitis, Encephalitis, Brain Abscess, TB, Cryptococcal, Neurocysticercosis

Overview & Definition

CNS infections are defined by two questions: where the organism sits, and what it is. Where — the subarachnoid space (meningitis), the brain parenchyma (encephalitis, abscess), both (meningoencephalitis), or an extra-axial pocket (subdural empyema, epidural abscess). What — a pneumococcus, a herpesvirus, a mycobacterium, a cryptococcus, or a tapeworm larva. The antibiotic ladder, the duration, and the adjuncts all pivot on that second answer, so name it early.[1][17]

Name the syndromes precisely — examiners reward the distinction. Meningitis inflames the meninges and subarachnoid space but spares the parenchyma, so fever, headache and meningismus dominate. Encephalitis inflames the parenchyma, so altered mental status, seizures and focal signs dominate; when both patterns coexist, call it meningoencephalitis. A brain abscess is a focal infection of the brain that begins as a localised area of cerebritis — it is both an infection and a mass lesion, which is exactly why you image before you put a needle near the theca. Aseptic meningitis means CSF pleocytosis with negative routine cultures — usually viral, but also TB, fungal, drug-induced, or partially-treated bacterial disease.[1][16]

The one rule that overrides everything else: antibiotics within one hour; dexamethasone before or with the first dose. The ESCMID guideline advises starting empiric treatment within one hour of arrival in all suspected meningitis cases, with antibiotic choice differentiated by age, risk factors and local pneumococcal resistance — and dexamethasone, the only proven adjunctive treatment, started together with the antibiotics. Say the mantra to yourself on the way to the patient: antibiotics within one hour; dexamethasone before or with the first dose.[3]

Meet the patient

A 22-year-old university student is brought to your emergency department at 3 a.m. Twelve hours ago she was well; now she has a fever of 39°C, a severe generalised headache she cannot lift her head for, a stiff neck, and photophobia so bad she will not open her eyes. She is confused, GCS 13, and there is no rash yet. Two questions will decide her night, and her brain: is this bacterial meningitis? and what do you do in the first hour? Get both right and she may walk out of hospital; get either wrong and she may not.[2]

Classification

Classify by syndrome, organism and tempo — in that order. Syndrome tells you where, organism tells you what, and tempo tells you how fast it is killing the patient. Together they fix the empiric antibiotic at the bedside.[1]

The classification is not academic — it decides your first hour. The acute bacterial syndrome gets antibiotics and dexamethasone now; a focal deficit or papilloedema gets imaging before any needle (think abscess or mass); an immunocompromised host with subacute headache gets TB, cryptococcal and toxoplasmic pathways added from the moment they arrive.[3][16]

Epidemiology & Risk Factors

The organism-by-age ladder is the single most exam-tested fact, because it sets the empiric antibiotic. Conjugate vaccination has rewritten the ladder: the incidence of bacterial meningitis has fallen since pneumococcal and meningococcal conjugate vaccine introduction, leaving Streptococcus pneumoniae (51 percent of adult episodes) and Neisseria meningitidis (37 percent) as the dominant community pathogens in the Dutch nationwide cohort. In the same cohort the overall mortality was 21 percent — 30 percent for pneumococcal versus 7 percent for meningococcal disease.[1][2]

Risk factors reflect the routes in. Contiguous spread after otitis, sinusitis, neurosurgery or cranial trauma, and haematogenous dissemination, explain most brain abscesses; in immunocompetent patients bacteria cause over 95 percent of them. Immune compromise widens the differential: advanced HIV carries cryptococcal meningitis and toxoplasmic encephalitis, and HIV co-infection raises tuberculous meningitis mortality to around 50 percent.[16][17][21]

Know your catchment, because regional epidemiology is examined deliberately. Cryptococcosis still accounts for one in five AIDS-related deaths globally, with mortality near 50 percent in low-resource settings — the burden sits in sub-Saharan Africa. Tuberculous meningitis exceeds 100,000 estimated new cases per year worldwide.[17][21]

Pathophysiology

Most bacterial meningitis is haematogenous, and CSF is an immunological sanctuary. The organism colonises the nasopharynx, gains bacteraemia and crosses the blood–brain barrier. Experimental and genetic association studies have mapped the pathogenesis: once organisms reach the subarachnoid space they multiply nearly unchecked, and the inflammatory response — not just the organism — drives vasogenic oedema, obstructed CSF outflow and subarachnoid vasculitis.[1][17]

This cascade is also the rationale for steroids. Antibiotics kill bacteria but transiently worsen inflammation — which is why dexamethasone, given just before or with the first antibiotic dose, blunts the inflammatory surge, reduces hearing loss and, in pneumococcal disease, reduces mortality.[7][8]

HSV encephalitis is a different beast. Caused by herpes simplex virus type 1, and more rarely HSV-2, it remains the most important cause of fatal sporadic encephalitis in man; even with aciclovir treatment, nearly a third of patients may die or suffer significant morbidity.[13]

A brain abscess begins as cerebritis. A focal infection of the brain that begins as a localised area of cerebritis, it matures into a walled collection; it behaves as a mass and may rupture into the ventricle.[16]

TB meningitis ends in a basal exudate. Antibiotic regimens are based on those used to treat pulmonary tuberculosis, which probably results in suboptimal drug levels in the cerebrospinal fluid owing to poor blood–brain barrier penetrance; the basal exudate blocks CSF pathways — hydrocephalus complicates about two-thirds of cases and independently worsens prognosis.[17][20]

Cryptococcus kills the immunodeficient host. Cryptococcus neoformans and Cryptococcus gattii species complexes cause meningoencephalitis with high fatality rates, particularly in persons with deficient T cell-mediated immunity, most commonly affecting people living with HIV.[21]

Clinical Presentation

Acute bacterial meningitis

The triad is fever, neck stiffness and altered mental status — but do not wait for all three. In the nationwide Dutch cohort of 696 adult episodes, the classic triad was present in only 44 percent; however, 95 percent had at least two of the four symptoms of headache, fever, neck stiffness and altered mental status. On admission, 14 percent were comatose and 33 percent had focal neurologic abnormalities. The classic trap is waiting for the full set — almost all patients have at least two of the four.[2]

Meningococcal disease has its own signature. Invasive meningococcal disease usually manifests as meningitis or septicaemia — or both — and can be severe and life-threatening; asymptomatic carriers can transmit the organism to others. Escalate the moment the diagnosis crosses your mind.[27][29]

Viral encephalitis

Fever with a change in behaviour or consciousness is encephalitis until CSF PCR and imaging prove otherwise. In the prospective English series, herpes simplex virus was the commonest identified infectious cause (19 percent of cases), followed by varicella zoster virus (5 percent) and Mycobacterium tuberculosis (5 percent); 37 percent had no cause identified and 21 percent had acute immune-mediated encephalitis. In herpes simplex encephalitis, aciclovir delay of more than two days worsens outcome — treat first and localise second.[13][14]

Brain abscess

Brain abscess hides behind headache more than fever. The triad of headache, fever and focal neurological deficit is complete in only about 20 percent of patients on admission, so the diagnosis hangs on imaging, not the bedside. Contrast MRI is the reference standard; ask about otitis, sinusitis, dental sepsis, neurosurgery and cranial trauma: the source points to the bug.[16]

Tuberculous meningitis

TB meningitis declares itself slowly, then all at once. Tuberculous meningitis is the most lethal and disabling form of tuberculosis; diagnosis is often delayed by the insensitive and lengthy culture technique required for confirmation. Cranial nerve palsies, visual impairment, seizures and hemiparesis complicate the course, and hydrocephalus — present in about 65 percent at presentation in one prospective cohort — is significantly associated with mortality and poor outcome.[17][20]

Cryptococcal meningitis

In advanced HIV, cryptococcal meningitis is as lethal as any ICU disease. Mortality remains around 50 percent in low-resource settings, and cryptococcosis still accounts for one in five AIDS-related deaths globally. Serum cryptococcal antigen screening with pre-emptive azole therapy for antigenaemia is well established — the diagnosis should be made before the brainstem is under pressure.[21]

Neurocysticercosis

New-onset seizures in a patient from an endemic region is neurocysticercosis until proven otherwise. Neurocysticercosis — infection of the nervous system by the cystic larvae of Taenia solium — is a frequent cause of seizure disorders and is endemic or presumed endemic in many low-income countries; CT and MRI have substantially improved knowledge of the disease course.[23]

Atypical presentations

Examiners test the atypical presentations deliberately, because that is where patients die. The elderly may have little or no fever and present as confusion or falls. The immunocompromised present subtly, with atypical organisms and muted signs. Individual clinical items have low accuracy — pooled sensitivity for headache is only about 50 percent — so do not anchor on any single finding. Burn this in: no fever does not mean no meningitis.[1][5]

Differential Diagnosis

The acute febrile headache has several mimics, and several of them are emergencies too. Subarachnoid haemorrhage gives thunderclap headache and meningismus but rarely fever at onset — CT and xanthochromia settle it. Encephalitis overlaps with meningitis but is ruled by altered mental status and seizures. Brain abscess adds a focal deficit and is excluded only by imaging — which is why any focal sign forces imaging before LP. Drug-induced meningitis (NSAIDs, co-trimoxazole, IVIG) and autoimmune encephalitis complete the modern list — immune-mediated disease was 21 percent of the English encephalitis cohort.[14]

The ring-enhancing lesion is its own exam topic. Brain abscess is the emergency; imaging with contrast — preferentially MRI — is the reference standard for diagnosis and should be followed by stereotactic aspiration of at least one lesion before the start of any antimicrobials.[16]

Clinical & Bedside Assessment

Run the bedside exam through five questions, fast. Is the patient septic or shocked? Airway, breathing, circulation — meningococcaemia can kill within hours. Is there meningeal irritation? Neck stiffness, Kernig and Brudzinski signs: individual items of the history and examination have low accuracy, but the absence of fever, neck stiffness and altered mental status effectively eliminates meningitis (sensitivity 99 to 100 percent for the presence of at least one). Is there a rash? Inspect the whole skin. Is there a focal deficit or depressed consciousness? These predict an abnormal CT and mandate imaging before LP. What is the access and disposition? ICU if comatose or shocked.[4][5]

Draw a bedside capillary glucose the moment you suspect meningitis. CSF glucose is interpreted against the concurrent blood glucose: among adults with CSF leucocytosis and a negative Gram stain, a CSF glucose under 1.9 mmol/L is one of the features that flags bacterial rather than viral disease.[6]

Investigations

Lumbar puncture and CSF analysis

CSF is the diagnostic cornerstone — but it comes second, never first. Empiric treatment starts within one hour of arrival; LP follows, only when safe. Clinical characteristics and laboratory parameters are of limited diagnostic accuracy on their own, which is why CSF analysis remains the principal contributor to the final diagnosis.[3]

Read the CSF like the lab does. Among adults presenting with CSF leucocytosis and a negative Gram stain, six features separate bacterial from viral meningitis or encephalitis: serum leukocyte count over 10.0 × 10⁹/L, CSF leukocyte count over 2000 per mm³, granulocyte count over 1180 per mm³, protein over 2.2 g/L, glucose under 1.9 mmol/L, and fever on admission. None present means low risk for bacterial meningitis; any present means high risk. In viral meningitis or encephalitis the Gram stain is negative and the pleocytosis lymphocytic — send CSF PCR for HSV, which is confirmatory when positive; the timing and frequency of repeat CSF PCR when an early result is negative remain matters of debate.[6][13]

TB and cryptococcal CSF each have a fingerprint. Tuberculous meningitis: diagnosis is often delayed by the insensitive and lengthy culture technique; the next-generation GeneXpert MTB/RIF Ultra assay on CSF offers diagnostic sensitivity of approximately 70 percent — a negative result cannot rule out the disease. A CSF protein over 2.5 g/L and a total cell count over 100 per cu mm were associated with hydrocephalus, which itself predicts poor outcome. Cryptococcal meningitis: serum cryptococcal antigen screening is well established, and short courses of amphotericin-based therapy combined with flucytosine are the preferred induction.[18][20][21]

Blood and other tests

Bloods confirm and risk-stratify. Draw blood cultures before antibiotics if you can — and use the serum leukocyte count as part of the CSF risk score: over 10.0 × 10⁹/L is one of the bacterial-flagging features. Send an HIV test in any atypical, subacute or immunosuppressed presentation, and add serum cryptococcal antigen screening in advanced HIV.[6][21]

Imaging

Contrast MRI is the reference standard for abscess, and CT clears the patient for LP. In brain abscess, brain imaging with contrast — preferentially MRI — is the reference standard for diagnosis and should be followed by stereotactic aspiration of at least one lesion before the start of any antimicrobials. In the Mayo series, MRI was performed in 93.1 percent of patients.[15][16]

TB and neurocysticercosis have imaging signatures worth memorising. In tuberculous meningitis, basal exudates, tuberculoma and infarcts on neuroimaging were significantly associated with hydrocephalus. In neurocysticercosis, CT and MRI have substantially improved knowledge of the disease course.[20][23]

EEG

Cerebrospinal fluid PCR for HSV is the diagnostic mainstay, and its timing matters. Controversies include the timing and frequency of cerebrospinal fluid examinations for the polymerase chain reaction detection of HSV — an early negative result with high suspicion does not close the case. EEG remains useful to catch non-convulsive status epilepticus, which can masquerade as reduced consciousness in any CNS infection.[13]

The one-hour rule

The first hour decides who lives and who is left disabled. Run four time-critical tasks in parallel, and let nothing — not CT, not LP — push antibiotics past the one-hour mark.[3][7]

1. Airway, breathing, circulation — oxygen, IV access, fluids and vasopressors if hypotensive or shocked; intubate if the airway or consciousness demands it.
2. Blood cultures, then empiric IV antibiotics within one hour — do not wait for LP or CT. The ESCMID guideline advises empiric treatment within one hour of arrival in all suspected meningitis cases, with the choice of antibiotics differentiated according to the patient's age, risk factors, and local resistance rates of pneumococci.[3]
3. Dexamethasone 10 mg every six hours for four days, before or with the first antibiotic dose — in the landmark trial it was given 15 to 20 minutes before or with the first antibiotic dose, reduced the risk of an unfavorable outcome (relative risk 0.59) and of death (relative risk 0.48).[7]
4. Lumbar puncture after stabilisation, if no red flag mandates imaging first. CT before LP if age 60 or over, immunocompromise, CNS disease history, seizure within a week, abnormal consciousness, or focal neurologic abnormality.[4]

Exam application bank (NEET-PG / INICET)

One-line answer

CNS infections span the meninges, the parenchyma, or both — and bacterial meningitis is a time-critical emergency. Fever, headache, neck stiffness and altered mental status (triad in 44 percent; at least two of the four in 95 percent) demand empiric IV antibiotics within one hour, with dexamethasone 10 mg every six hours for four days begun before or with the first dose. CSF analysis separates bacterial (neutrophilic, low glucose, high protein; with a negative Gram stain, cell counts, protein and glucose thresholds flag bacterial disease) from viral (lymphocytic, negative Gram stain) patterns. HSV encephalitis needs IV aciclovir 10 mg/kg every eight hours, started before CSF PCR returns. Brain abscess needs contrast MRI, stereotactic aspiration before antibiotics, and a prolonged third-generation cephalosporin plus metronidazole. TB meningitis needs antituberculosis drugs plus dexamethasone; cryptococcal meningitis needs short-course amphotericin-based induction with flucytosine; neurocysticercosis needs antiparasitic therapy with corticosteroids for viable cysts.[2][3][6][7][11][16][19][21][24]

Worked stems (answer without another resource)

Stem 1 — Classic presentation. Map the symptoms to the mechanism, name the first investigation, and give the first treatment step with dose and route.[2][3]

Stem 2 — Unstable or complicated. List the red flags that force immediate resuscitation, theatre, ICU, an antidote or reperfusion — and state what you do in the first 15 minutes.[3]

Stem 3 — Atypical group. Elderly, pregnant, child or immunocompromised: explain how the presentation and the treatment thresholds change.[4][21]

Stem 4 — Differential trap. Name the three closest mimics and one discriminator for each.[14]

Stem 5 — Disposition. Who goes home with safety-netting, who is admitted, who needs HDU, ICU or theatre, and what follow-up is mandatory.[3]

Rapid viva checklist

1. Definition and classification
2. Pathophysiology chain
3. Bedside signs and criteria
4. Score with exact components (MRC grade for TB meningitis)
5. Emergency bundle
6. Definitive therapy with doses
7. Complications of disease and of treatment
8. Special populations
9. Guideline or trial name if classic
10. Three exam traps

Coverage self-check

If you cannot answer any stem above from this page alone, re-read the matching section — the page is built to be self-sufficient for final-prof and NEET-PG or INICET questions on CNS Infections — Meningitis, Encephalitis, Brain Abscess, TB, Cryptococcal, Neurocysticercosis.[1]

For suspected viral encephalitis, add IV aciclovir 10 mg/kg every eight hours, started empirically and early while CSF PCR and imaging are awaited — because outcome in herpes simplex encephalitis is time-dependent, and delays beyond 48 hours after hospital admission are associated with a worse prognosis.[10][11]

For brain abscess, the priority is to image (contrast MRI, the reference standard), then achieve stereotactic aspiration of at least one lesion before the start of any antimicrobials, and treat with a prolonged course — a six-week combination of a third-generation cephalosporin and metronidazole will cure most community-acquired cases.[16]

Management — Definitive & Stepwise

Acute bacterial meningitis — empiric and targeted therapy

Pick empiric therapy by age, immune status and setting. The ESCMID guideline advises empiric treatment within one hour of arrival in all suspected cases, with antibiotic choice differentiated according to the patient's age, risk factors, and local resistance rates of pneumococci; dexamethasone is the only proven adjunctive treatment and should be started together with the antibiotics.[3]

De-escalate by organism, and know the steroid evidence cold — it is a viva staple. The de Gans and van de Beek trial (NEJM 2002) gave dexamethasone 10 mg every six hours for four days, 15 to 20 minutes before or with the first antibiotic dose, and cut the risk of an unfavorable outcome (relative risk 0.59) and death (relative risk 0.48); among pneumococcal cases, unfavorable outcomes were 26 percent on dexamethasone versus 52 percent on placebo. The Cochrane review (Brouwer 2015) confirms reduced severe hearing loss (RR 0.67) and reduced mortality in pneumococcal meningitis (RR 0.84), with benefit in high-income countries and none in low-income countries. The individual-patient-data meta-analysis (van de Beek, Lancet Neurology 2010) is the caution: no significant reduction in death or neurological disability overall and no significant effect in any prespecified subgroup, though hearing loss among survivors was reduced (OR 0.77). That controversy is worth knowing and worth naming.[7][8][9]

Viral encephalitis (HSV)

Start aciclovir before the PCR returns — always. Give IV aciclovir 10 mg/kg every eight hours — the regimen of the randomised Swedish multicentre trial — for the trial duration of ten days; the American collaborative trial used 30 mg/kg/day for ten days. The Swedish trial cut mortality to 19 percent versus 50 percent on vidarabine, with 56 percent versus 13 percent returning to normal life; the American trial reported 28 percent versus 54 percent mortality and established acyclovir as the treatment of choice. Delaying aciclovir by more than two days worsens outcome, and even with treatment nearly a third of patients may die or suffer significant morbidity — that is why the threshold to treat is deliberately low.[11][12][13]

Brain abscess

Treat brain abscess with drainage, prolonged antibiotics and source control. Empiric treatment should cover oral streptococci (including the milleri group), methicillin-susceptible staphylococci, anaerobes and Enterobacteriaceae; a six-week combination of a third-generation cephalosporin and metronidazole will cure most community-acquired brain abscesses in immunocompetent patients. Stereotactic aspiration is both diagnostic and therapeutic, and should precede antimicrobials. A prompt combined surgical and medical approach matters: in the Mayo series, patients managed with medical therapy alone had higher mortality than those receiving combined therapy (21.4 percent versus 6 percent), and the median duration of antimicrobial therapy was 42 days.[15][16]

Tuberculous meningitis

TB meningitis is antituberculosis therapy plus steroids, started early. Antibiotic regimens derive from those used for pulmonary tuberculosis and probably achieve suboptimal CSF levels — intensified and higher-dose regimens are under trial, with small trials suggesting benefit from increased rifampicin doses or the addition of linezolid or fluoroquinolones. Adjunctive dexamethasone improves survival: in the randomised Vietnamese trial of 545 patients over 14 years of age, dexamethasone reduced the risk of death (relative risk 0.69, 95 percent CI 0.52 to 0.92), though it probably does not prevent severe disability. Hydrocephalus complicates about two-thirds of cases and is significantly associated with mortality and poor outcome — neurosurgical management may be required.[17][18][19][20]

Cryptococcal meningitis

Cryptococcal meningitis runs in phases — induction first, and mortality depends on getting it right. Short courses (seven days or less) of amphotericin-based therapy combined with flucytosine are currently the preferred options for induction therapy. The AMBITION-cm trial (NEJM 2022) randomised 844 HIV-positive adults to a single high dose of liposomal amphotericin B 10 mg/kg on day 1 plus 14 days of flucytosine 100 mg/kg/day and fluconazole 1200 mg/day, versus the WHO-recommended amphotericin B deoxycholate 1 mg/kg/day plus flucytosine 100 mg/kg/day for 7 days followed by fluconazole 1200 mg/day for 7 days: ten-week mortality was 24.8 percent versus 28.7 percent (noninferior), with fewer grade 3 or 4 adverse events (50.0 percent versus 62.3 percent). Serum cryptococcal antigen screening with pre-emptive azole therapy for antigenaemia is well established.[21][22]

Neurocysticercosis

Treat neurocysticercosis by subtype — viable cysts get drugs, and the choice is individualised. Treatment is individualised according to the involvement of parenchymal or extraparenchymal spaces, the number and form of parasites, and the extent of degeneration and associated inflammation. For viable parenchymal cysticerci with seizures, the randomised trial gave albendazole 800 mg per day with dexamethasone 6 mg per day for 10 days: seizures were reduced by 46 percent overall (not statistically significant), seizures with generalisation were significantly reduced by 67 percent, and more intracranial cystic lesions resolved than with placebo. Treatment can induce seizures, hydrocephalus and infarcts — the reason corticosteroids accompany antiparasitic therapy. The 2017 IDSA/ASTMH clinical practice guidelines (White et al.) codify diagnosis and treatment.[23][24][25]

Specific Subtypes & Scenarios

Meningococcal disease and its prevention

Meningococcus spreads by respiratory droplets — so prophylax the close contacts. N. meningitidis is transmitted person-to-person via respiratory droplets and oropharyngeal secretions; asymptomatic carriers can transmit the bacteria to others, and six serogroups (A, B, C, W, X and Y) account for most cases. Antibiotic prophylaxis for close contacts of infected persons is critical to preventing secondary cases — give it per current public-health guidance, promptly.[27]

HSV encephalitis — the temporal-lobe emergency

Outcome turns on how early aciclovir starts. Herpes simplex encephalitis, caused by HSV-1 and more rarely HSV-2, remains the most important cause of fatal sporadic encephalitis in man; even with acyclovir treatment, nearly a third of patients may die or suffer significant morbidity, and delay beyond two days makes it worse — so treat empirically and treat now.[11][13]

Subdural empyema and epidural abscess

Subdural empyema and epidural abscess are surgical emergencies. They arise by direct spread from otitis, sinusitis or cranial sources, and — like brain abscess — demand imaging, urgent neurosurgical drainage and prolonged antibiotics: the same combined surgical-and-medical principle that cut mortality from 21.4 to 6 percent in the abscess series applies.[15][16]

Toxoplasmosis in HIV

Toxoplasmic encephalitis was one of the first opportunistic infections described in HIV-infected patients, and treatment is comparatively successful. Randomised trials compare pyrimethamine plus sulfadiazine with pyrimethamine plus clindamycin, and with trimethoprim-sulfamethoxazole; the available evidence fails to identify any one superior regimen, and trimethoprim-sulfamethoxazole is an effective alternative where pyrimethamine plus sulfadiazine are not available. Management follows current HIV opportunistic-infection guidance.[26]

Complications & Pitfalls

Complications span the acute phase and the rest of the patient's life. Hearing loss is the classic sequela — the reason dexamethasone is given and the reason follow-up of surviving patients should include evaluation for hearing loss. Expect seizures, hydrocephalus (about two-thirds of tuberculous meningitis), focal deficits and stroke, cranial nerve palsies and cognitive impairment. In cryptococcal meningitis mortality remains near 50 percent in low-resource settings.[3][8][20][21]

Each subtype carries its own catastrophe. In brain abscess it is the mass lesion itself and rupture into the ventricular system. In tuberculous meningitis it is hydrocephalus and infarction, both tied to the basal exudate. In cryptococcal meningitis it is death from uncontrolled disease — one in five AIDS-related deaths globally.[16][20][21]

The classic pitfalls are procedural and cognitive, and they recur. Ordering CT before LP in everyone — clinical features can identify who needs it. Delaying antibiotics for CT or LP against the one-hour rule. Assuming the absence of fever, neck stiffness and altered mental status still leaves room for meningitis — their combined absence effectively eliminates it, but any one present keeps it live. And stopping aciclovir on a single early negative PCR — the timing and frequency of repeat CSF PCR remains debated, and delay beyond two days costs outcome.[4][5][13]

Prognosis & Disposition

Know the numbers — they decide disposition and they come up in exams. Adult bacterial meningitis killed 21 percent in the Dutch nationwide cohort — 30 percent for pneumococcal and 7 percent for meningococcal disease — with an unfavorable outcome in 34 percent and focal neurologic abnormalities in 33 percent on admission. Herpes simplex encephalitis on aciclovir killed 28 percent in the 1986 trial (versus 54 percent on vidarabine); the Swedish trial showed 19 percent versus 50 percent. Tuberculous meningitis kills or disables more than half of those affected, with mortality approaching 50 percent in HIV co-infection. Cryptococcal meningitis kills about half of patients in low-resource settings. Brain abscess managed with medical therapy alone carried 21.4 percent mortality versus 6 percent with combined medical and surgical treatment.[2][11][12][17][19][21][15]

Disposition follows GCS, time-to-antibiotics, the organism, immune status, seizures and comorbidity. Admit to ICU if comatose, intubated or in septic shock; otherwise a neurology or infectious diseases ward with close observation. Follow-up of surviving patients should include evaluation for hearing loss.[2][3]

Special Populations

Antibiotic choice is differentiated by age, risk factors and local resistance — the neonate, the elderly and the immunocompromised sit on different rungs of that ladder. The ESCMID framework applies at every age; consult the current guideline and local policy for agent and dose selection in neonates, whose organisms and regimens differ from adults.[3]

In advanced HIV with T-cell deficiency, run a different differential from the start. Cryptococcus (serum antigen screening is established, with pre-emptive azole therapy for antigenaemia), toxoplasmic encephalitis (pyrimethamine-based regimens), and tuberculous meningitis (mortality approaching 50 percent with co-infection) lead the list.[17][21][26]

Evidence, Guidelines & Regional Differences

A handful of landmark studies anchor the field — name them. de Gans and van de Beek (NEJM 2002) established adjunctive dexamethasone in adults with bacterial meningitis (unfavorable outcome RR 0.59; death RR 0.48). The Cochrane review (Brouwer 2015) confirmed hearing-loss benefit and pneumococcal mortality benefit in high-income countries, while the individual-patient-data meta-analysis (van de Beek, Lancet Neurology 2010) found no significant overall mortality or disability benefit — the modern controversy. The ESCMID guideline (2016), summarised by van Ettekoven (2017), anchors the one-hour rule and age-and-resistance-differentiated empiric therapy; the IDSA practice guidelines (Tunkel, 2004) remain the classic US reference; and the van de Beek Nature Reviews Disease Primers (2016) synthesises current understanding.[1][3][7][8][9][28]

In viral encephalitis the evidence is older but decisive. The Swedish randomised trial (Sköldenberg, Lancet 1984; aciclovir 10 mg/kg eight-hourly) and the American collaborative trial (Whitley, NEJM 1986; acyclovir 30 mg/kg/day) put aciclovir far ahead of vidarabine, and the UK ABN/British Infection Association guidelines (Solomon, 2012) operationalised early empiric aciclovir. The Granerod encephalitis study (Lancet Infectious Diseases 2010) defined the contemporary aetiological spectrum: HSV 19 percent, VZV 5 percent, TB 5 percent, immune-mediated 21 percent, unknown 37 percent.[10][11][12][14]

TB and cryptococcal disease have modern, practice-changing trials. In TB meningitis, the Thwaites trial (NEJM 2004) proved dexamethasone improves survival (RR of death 0.69), the Wilkinson primer (2017) and the Huynh review (2022) define the modern agenda — Xpert Ultra at approximately 70 percent CSF sensitivity, intensified regimens in trial — and the Raut cohort (2013) tied hydrocephalus to prognosis. In cryptococcal meningitis, the AMBITION-cm trial (NEJM 2022) established single-dose liposomal amphotericin B with flucytosine and fluconazole as noninferior and safer, and the Tugume primer (2023) integrates short-course amphotericin induction into the modern paradigm.[17][18][19][20][21][22]

For neurocysticercosis, the 2017 IDSA/ASTMH guidelines (White et al.) draw the line, and the Garcia trial (NEJM 2004) supplies the randomised evidence for albendazole 800 mg per day with dexamethasone 6 mg per day for 10 days in viable parenchymal disease.[24][25]

Regional deltas are real and examined. Meningococcal serogroup distribution drives vaccine strategy — six serogroups (A, B, C, W, X, Y) account for most disease, and outbreaks cluster around large gatherings; the cryptococcal burden sits in sub-Saharan Africa (one in five AIDS deaths, near 50 percent mortality in low-resource settings); and tuberculous meningitis exceeds 100,000 estimated cases per year, concentrated in TB-endemic regions.[17][21][27]

Ward-round test

These are the questions a consultant fires on a ward round. Cover the answer, then reveal.[1]

A 60-year-old with fever, headache and confusion arrives in ED. What empiric therapy, and what imaging decision does his age force?[3][4]

Fever, agitation and aphasia for two days; MRI shows temporal-lobe signal change. First drug, at what dose, and why now?[10][11]

A ring-enhancing lesion with fever and a focal deficit. What is the sequence?[16]

CSF shows 2500 leukocytes (granulocyte-predominant), glucose 1.2 mmol/L, protein 2.5 g/L, Gram stain negative. What is the pattern?[6]

Exam Pearls