Bacterial meningitis is the infection of the arachnoid membrane, subarachnoid space, and cerebrospinal fluid by bacteria

A total of c.3 million cases of meningitis worldwide

In Australia, menigitis incidence is about 9 cases per 100 000 person-years

Bacterial meningitis is diagnosed about half as much as viral

Incidence ranges from 65 cases/100,000 people (Africa) to 2/100000 (US/Canada)

Overall mean worldwide mortality is 16% but ranges from 2% – 33% depending on country

The subarachnoid space is bounded externally by the arachnoid membrane and internally by the pia, and dips into the brain along blood vessels in the perivascular (Virchow-Robin) spaces.

It extends from the optic chiasm to the cauda equina and surrounds the brain and spinal cord completely.

REF

Autopsy of pneumococcal meningitis

IMAGE CREDIT

Head and neck infections (e.g. sinusitis, mastoiditis, otitis media)

Head and neck surgery or prostheses (e.g. cochlear implants, VP shunt, ICP monitor, EVD, craniectomy)

CSF leak (e.g. base of skull fracture)

Extremes of age (e.g. pneumococcus and listeria)

Comorbidities (e.g. liver and renal failure)

Immunosuppression (e.g. HIV, functional asplenia, splenectomy, hypogammaglobulinemia, complement deficiency, steroids, diabetes mellitus)

Malnutrition

Low socioeconomic status and overcrowding

Exposure to others with infective source (esp neisseria meningitidis)

Meningitis is infection of the arachnoid membrane, subarachnoid space, and cerebrospinal fluid. Encephalitis is infection of the brain tissue. The presence or absence of normal brain function is the important distinguishing feature. The term ‘meningoencephalitis’ recognizes overlap between the two.

Most viruses are capable of causing either meningitis or encephalitis, but in general, a given virus will tend to cause one or the other (e.g HSV 1 – encephalitis, HSV 2 – meningitis)

Most common causes of aseptic meningitis include enteroviruses, HSV, HIV, West Nile virus, VZV, mumps, and lymphocytic choriomeningitis virus (LCMV).

Viral encephalitis can be either primary or postinfectious. HSV-1 is the most commonly identified cause of sporadic encephalitis in Australia. West Nile has emerged as the most common cause of viral encephalitis in the United States.

Most common causes are primary viral infection or immune mediated; this section just covers primary viral infection.

Viral meningitis is among the most common CNS infections, which accounts for the majority of cases of total viral CNS infections.
Encephalitis and myelitis, on the other hand, are much less common
The estimated annual incidence of all viral causes of encephalitis worldwide is

• 10 per 100,000 in children,
• 2 per 100,000 for adults and
• 6 per 100,000 for all ages

The mortality from viral encephalitis ranges from 3.8% to 7.4%
Causative virus only detected in minority of cases (40% in one study)

IMAGE MRI HSV:

HSV MRI Axial DWI

IMAGE CREDIT
Contrast-enhanced MRI is sensitive for early HSV encephalitis, showing edema in the orbitofrontal and temporal areas, which HSV typically infects.
MRI shows demyelination in progressive multifocal leukoencephalopathy and may show basal ganglia and thalamic abnormalities in West Nile and eastern equine encephalitis.

Children 5 times more likely to get than adults
Travel to area with high endemic rates

A detailed history is key, including:

> Sexual (e.g. HIV)

> Travel (e.g. West Nile, St Louis, Western and Eastern Equine, Japanese Encephalitis)

> Exposure history to insects (e.g. tick-borne, Nipah, Hendra, Zika)

> Exposure History to animals (e.g. rabies)

No consensus on definition.

CDC definition:
EITHER

1. organisms are cultured from the cerebrospinal fluid (CSF)

OR

1. the patient has at least one sign or symptom of ventriculitis including:

• fever (> 38°C),
• headache
• stiff neck/meningeal signs
• cranial nerve signs
• irritability with no other recognized cause

AND at least ONE of the following:
(a) increased white cells + elevated protein + decreased glucose in CSF,
(b) organisms seen on Gram stain of CSF,
(c) organisms cultured from blood,
(d) positive laboratory test of CSF, blood, or urine,
(e) diagnostic single antibody titer (IgM) or four-fold increase in paired sera (IgG) for the pathogen,
and if the diagnosis is made antemortem, the physician institutes appropriate antimicrobial therapy

Common organisms:

• Staphylococcal aureus
• Coagulase negative staphylococci (CoNS)
• E.coli
• Pseudomonas
• Klebsiella
• Acinetobacter

Common organisms: Staphylococcal aureus and Coagulase negative staphylococci (CoNS)

Early infection with skin flora is the most common type of cerebrospinal fluid shunt infection; about half of all shunt infections are due to CoNS and about a third of cases are due to Staphylococcus aureus.

Diphtheroids (such as Cutibacterium [formerly Propionibacterium] acnes and Corynebacterium jeikeium) may also be pathogenic.

Later shunt infections, from distal end contamination (months after placement), are more likely to be caused by  streptococci, gram-negative bacteria (including Pseudomonas aeruginosa), anaerobes, mycobacteria, and fungi.

What is a VP shunt?

• Staph. epidermidis (coagulase-negative staph): 60–75% of infections (most common)
• Staph. aureus
• Gram-negative bacilli: 6–20% (may come from intestinal perforation)
• Candida species

Presentation:

• Non-specific syndrome: fever, nausea, headache, lethargy, anorexia, irritability.
• May also present as shunt malfunction
• Erythema and tenderness along shunt tubing may occur.
• Distal infection of ventriculoperitoneal shunts may mimic acute abdomen.
• S. epidermidis infections tend to be indolent (smoldering).
• GNB infections usually cause more severe illness.
• Abdominal findings are more common; main clinical manifestation is fever, usually intermittent and low grade.

Incidence 0.3 per 100 000
70% Male
Mean age 34

Cerebral abscesses result from pathogens growing within the brain parenchyma.

Initial parenchymal infection is known as cerebritis, which may progress into a cerebral abscess.

Historically direct extension from sinus or scalp infections was the most common source.

Now haematological spread is the most common source.

Direct introduction by trauma or surgery is rare.

Cerebral infection is commonly divided into four stages with distinct imaging and histopathologic features:

1. Early cerebritis (Day 1-3: a focal infection without a capsule or pus formation,can resolve or develop into frank abscess)
2. Late cerebritis (Day 4-9)
3. Early abscess/encapsulation (Day 10-14)
4. Late abscess/encapsulation (> Day 14: collagen capsule, necrotic centre, gliosis around capsule)

The direct spread of organisms from a contiguous site (e.g. from otitis media, mastoiditis) usually causes a single brain abscess.

Brain abscesses associated with bacteremia usually result in multiple abscesses that are most commonly located in the distribution of the middle cerebral artery.

Endocarditis and IV drug use

Pulmonary abscess

Pulmonary AVMs and AV fistulas

Congenital cyanotic heart disease

Immune compromise (especially HIV)

Chronic sinusitis/otitis

Dental procedures

Symptoms are similar to any other mass lesions but tend to progress rapidly:

• Headache
• Nausea / vomiting
• Lethargy
• Hemiparesis and seizures in 30-50% cases

Abscesses may rupture into the ventricular system, which results in a sudden and dramatic worsening of the clinical presentation and is a bad prognostic sign.

CT

Need pre- and post-contrast scans, unless going straight to MRI regardless of the CT findings. Typical appearances include:

• Outer hypodense and inner hyperdense rim (double rim sign) in most cases
• Ring of iso- or hyperdense tissue, typically of uniform thickness
• Central low attenuation (fluid/pus)
• Surrounding low density (vasogenic oedema)
• Ventriculitis may be present, seen as enhancement of the ependyma
• Obstructive hydrocephalus will commonly be seen when intraventricular spread has occurred

MRI

MRI is more sensitive than CT. Although peripherally-enhancing lesions may be non-specific by imaging, diffusion-weighted sequences (less commonly MR spectroscopy) showing central diffusion restriction are critical for suggesting the diagnosis of a cerebral abscess.

T1

• central low intensity (hyperintense to CSF)
• peripheral low intensity (vasogenic oedema)
• ring enhancement
• ventriculitis may be present, in which case hydrocephalus will commonly also be seen

T2/FLAIR

• central high intensity (hypointense to CSF, does not attenuate on FLAIR)
• peripheral high intensity (vasogenic oedema)
• the abscess capsule may be visible as an intermediate to slightly low signal thin rim

DWI/ADC

• high DWI signal is usually present centrally
• represents true restricted diffusion.
• peripheral or patchy restricted diffusion may also be seen; this finding is however not as constant as one may think, with up to half of the rim-enhancing lesions demonstrating some restriction not proving to be abscesses.
• in some immunocompromised states, central content may not diffusion restrict

SWI

• low-intensity rim
  • complete in 75%
  • smooth in 90%
  • mostly overlaps with contrast-enhancing rim
• dual rim sign: a hyperintense line located inside the low-intensity rim

MR perfusion: rCBV is reduced in the surrounding oedema cf. to both normal white matter and tumour oedema seen in high-grade gliomas

MR spectroscopy: elevated peaks are seen corresponding to lipids/lactate, succinate, acetate, and amino acids (alanine, valine, leucine, and isoleucine)

REF

Wide variety of organisms cause brain abscess.

The pathogens involved differ depending upon the site of the primary infection.

Sources associated with specific microorganisms:

> Paranasal sinuses – Streptococcus spp (especially S. milleri), Haemophilus spp, Bacteroides spp, Fusobacterium spp

> Odontogenic sources – Streptococcus spp, Bacteroides spp, Prevotella spp, Fusobacterium spp, Haemophilus spp

> Otogenic sources – Enterobacteriaceae, Streptococcus spp, Pseudomonas aeruginosa, Bacteroides spp

> Lungs – Streptococcus spp, Fusobacterium spp, Actinomyces spp

> Urinary tract – Pseudomonas aeruginosa, Enterobacter spp

> Penetrating head trauma – Staphylococcus aureus, Enterobacter spp, Clostridium spp

> Neurosurgical procedures – Staphylococcus spp, Streptococcus spp, Pseudomonas aeruginosa, Enterobacter spp

> Endocarditis – Viridans streptococci, S. aureus

> Congenital cardiac malformations (especially right-to-left shunts) – Streptococcus spp

> Immunocompetent hosts – usually bacterial species

> Immunocompromised – broad array including fungi

The immunocompromised patient is susceptible to a host of other organisms including:

• Toxoplasma gondii
• Listeria monocytogenes
• Nocardia asteroides
• Candida albicans
• Mycobacterium spp.
• Aspergillus fumigatus
• Fungi that cause mucormycosis

In terms of how common each organism is:

• Streptococcus sp: 35-50%, especially S. pneumoniae
• Sterile: 25%
• Mixed: variable, 10-90% of cases depending on source
• Staphylococcus aureus and epidermidis: following neurosurgery
• Gram-negative species more common in infants
• Listeria in pregnant women and older patients
• Group B Streptococcus (GBS) and E. coli in neonates

REF

A suppurative infection in the subdural space.

This has no anatomic barrier so can spread over the convexity and into the interhemispheric fissure.

Infection may spread to the subdural space from air sinuses or from the middle ear.
The subdural space is traversed by bridging arteries and veins but has no vascular network of its own.
Therefore, antibiotics have no access to this space.

The usual portal of entry is from the exterior.

If infection arises from ears or paranasal sinuses, likely to be streptococci and/or other anaerobes such as Cutibacterium (formerly Propionibacterium) and Peptostreptococcus species.

If infection follows neurosurgery, likely to be staphylococci, especially Staphylococcus aureus or gram-negative bacteria.

Infection can also spread inward from osteomyelitis of the skull or be introduced by fetal monitoring probes applied to the skull during birth.

Less common than cerebral abscess (ratio of abscess:empyema is ≈ 5:1).

Male:female ratio is 3:1.

Location: 70–80% are over the convexity, 10–20% are parafalcine.

Causes

Paranasal sinusitis (especially frontal): 70%

Otitis (usually chronic otitis media): 15%

Post surgical (neuro or ENT):  4%

Trauma: 3%

Meningitis (more common in peds): 2%

Others including unrelated infections: 6%

Suspect if meningismus and unilateral hemisphere dysfunction.

Marked tenderness to percussion or pressure over affected air sinuses is common

In order or frequency, symptoms are:

• Fever
• Headache
• Meningismus
• Hemiparesis
• Altered mental status
• Seizures
• Sinus tenderness, swelling or inflammation
• Nausea and/or vomiting
• Homonymous hemianopsia
• Speech difficulty
• Papilledema

CT with contrast

IV contrast is usually helpful.

Crescentic or lenticular extracerebral lesion with dense enhancement of medial membrane

As with any subdural collection, there is inward displacement of the gray-white interface, ventricular compression and possible effacement of basal cisterns

Unenhanced CT may miss small lesions. Lesion is hypodense (but denser than CSF)

MRI

Low signal on T1WI, high signal on T2WI

Here is the (less common) parafalcine SDE:

T1 MRI SDE

T2 MRI SDE

REFERENCE

If associated with sinusitis, often caused by aerobic and anaerobic streptococci

Following trauma or neurosurgical procedures, staphylococci and Gram-negative species predominate (Note: S. aureus rare)

Sterile cultures occur in up to 40%

Bugs in order of frequency:

• Aerobic streptococcus
• Staphylococci
• Microaerophilic and anaerobic strep
• Aerobic Gram-negative rods e.g. Escherichia coli, Klebsiella pneumoniae, Serratia marcescens.
• Other anaerobes

REFERENCE

An epidural space filled with adipose tissue exists normally around the spinal cord.
A spinal epidural abscess arises when organisms from osteomyelitis or tuberculosis of the vertebral column spread to this space.
There is no epidural space normally in the cranium.
However, a cranial epidural abscess may develop when bacteria colonize a traumatic epidural hematoma, or when infection from air sinuses extends in the plane between the dura and bone.

Incidence: 0.2–1.2 per 10,000 hospital admissions annually

Average age: 58

Thoracic level is the most common site (≈ 50%), followed by lumbar (35%) then cervical (15%)

Often associated with vertebral osteomyelitis and intervertebral discitis.

Causes

No source can be identified in up to 50% of patients

Hematogenous spread is the most common source (26–50% of cases) either to the epidural space

or to the vertebra with extension to epidural space. Sources include:

• Skin infections (most common): furuncle may be found in 15% of cases
• Parenteral injections, especially with IV drug abuse
• Bacterial endocarditis
• UTI
• Respiratory infection (including otitis media, sinusitis, or pneumonia)
• Pharyngeal or dental abscess

Direct extension from:

• Decubitus ulcer
• Psoas abscess
• Penetrating trauma, including: abdominal wounds, neck wounds, GSW
• Pharyngeal infections
• Mediastinitis
• Pyelonephritis with perinephric abscess
• Dermal sinus

Following spinal procedures (e.g. epidurals or LP’s) or operations (lumbar discectomy)

After recent back trauma

REFERENCE

All forms of immunosuppression, most commonly:

• Diabetes
• IV drug abuse
• Chronic renal failure
• Alcoholism

Usually presents with excruciating pain localized over spine with tenderness to percussion.

Radicular symptoms follow with subsequent distal cord findings, often beginning with bowel/bladder disturbance, abdominal distension, weakness progressing to para- and quadriplegia.

Average time is 3 days from back pain to root symptoms; 4.5 days from root pain to weakness; 24 hrs from weakness to paraplegia.

Fever, sweats or rigors are common, but are not always present

A furuncle (skin boil) somewhere on the body may be identified in 15%.

Patients may be encephalopathic

Meningismus with a positive Kernig sign may occur.

Patients with postoperative SEA may demonstrate surprisingly few signs or symptoms aside from local pain

REF

MRI with Gadolinium is best

Also look for associated osteomyelitis and discitis

2 patterns:

1. Phlegmonous stage of infection results in homogeneous enhancement of the abnormal area which correlate to granulomatous-thickened tissue with embedded micro-abscess without a significant pus collection
2. Liquid abscess surrounded by inflammatory tissue which shows varying degree of peripheral enhancement with gadolinium
   
   

   MRI T2

MRI T1 + C

REFERENCE

Staph. aureus: the most common organism (cultured in > 50%)

Streptococcus: second most common

E.coli

Pseudomonas aeruginosa

Serratia marcescens

Enterobacter

Chronic infections:

• TB, usually associated with vertebral osteomyelitis,
• Brucellosis
• Fungal: cryptococcosis, aspergillosis
• Parasitic: Echinococcus

REFERENCE