Etiology, Epidemiology, and Pathogenesis

Meningitis is usually an acute process that is generally caused by either bacteria or viruses. Meningoencephalitis is predominantly caused by viruses, mostly enteroviruses.

Because enteroviruses are the most common cause of viral meningitis and meningoencephalitis, the incidence of these infections peaks in the summer and fall and wanes in the winter. Herpes family viruses such as herpes simplex virus (HSV) and varicella zoster (VZV) also cause meningoencephalitis, as can a number of other pathogens (Box 92-1).

Historically, the most common causes of bacterial meningitis beyond the neonatal period were Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae type b (Hib). Fortunately, there has been a dramatic reduction in the incidence of Hib infections after the introduction of the conjugate Hib vaccine in 1987. Additionally, there has been a reduction in the incidence of meningitis caused by S. pneumoniae since the introduction of the heptavalent pneumococcal conjugate vaccine (PCV7); unfortunately, there are serotypes that are not contained in that vaccine that have the ability to cause disease in humans, so the effect of the PCV7has not been as dramatic as the Hib vaccine. The currently licensed vaccine for N. meningitidis covers serogroups A, C, W135, and Y. It does not elicit a protective response against serogroup B, which continues to cause both sporadic disease and outbreaks, mostly on college campuses and military bases.

All three of these bacteria colonize the upper respiratory tract (nasopharynx and oropharynx). From there, they are able to invade through the mucosal epithelium, usually when that surface is inflamed because of a viral infection, and enter the bloodstream. The bacteria circulate in the bloodstream and gain access to the CNS via the choroid plexus. The bacteria readily replicate in the subarachnoid space within the cerebrospinal fluid (CSF) because this area is normally sequestered from the immune system. In response to bacterial replication, white blood cells (WBCs) migrate to the CSF, and the ensuing inflammatory response leads to some of the signs and symptoms of meningitis.

In addition to hematogenous seeding of the CSF, bacteria can also gain access to the CNS via direct extension. Congenital malformations, such as dermoid sinuses, or traumatic injuries, including basilar skull fractures and penetrating trauma, can allow bacteria to enter the CSF. Infections of the sinuses, mastoid air cells, or middle ear can also act as portals of entry (Figure 92-1).

Figure 92-1 Bacterial meningitis.

Other causes of acute meningitis include Borrelia burgdorferi, the etiologic agent of Lyme disease; gram-negative bacilli such as Citrobacter, Salmonella, and Pseudomonas spp.; and fungal pathogens.

Clinical Presentation

Neonates and infants with meningitis often present with nonspecific signs and symptoms. The most common parental complaint is fever, and the infants frequently have a history of irritability and poor feeding. They may also develop vomiting and a bulging fontanelle. In more severe cases, infants may present with lethargy or after having a seizure and may have focal neurologic deficits on examination. Although children with meningitis can present in many ways, toddlers and school-aged children are more likely to present with the “classic” signs and symptoms. These include high fever, photophobia, headache, neck stiffness, and vomiting. Of these, fever and headache are the most common. On physical examination, these patients may have nuchal rigidity as well as Kernig’s or Brudzinski’s signs (Figure 92-2). Children with disseminated N. meningitidis infections can have a petechial or purpuric rash. Infants with enteroviral meningitis often have an erythematous, macular rash. Cranial nerve palsies, especially of cranial nerve 7, and papilledema are frequently found in patients with Lyme meningitis but are unusual in other forms of meningitis.

Figure 92-2 Kernig’s sign and Brudzinski’s neck sign.

Differential Diagnosis

The differential diagnosis of meningitis and meningoencephalitis depends on which signs and symptoms are prominent because the presentation of these infections is so varied.

The combination of fever and irritability in an infant or toddler has a very broad differential diagnosis, which includes many infectious and noninfectious etiologies. This is a situation in which a detailed medical history and thorough physical examination are critical in narrowing the differential diagnosis. There will almost always be additional information that helps to focus the differential. A history of vomiting or diarrhea points toward gastroenteritis. Tachypnea or focal findings on lung auscultation are a sign of pneumonia, either viral or bacterial. Torticollis or other unusual positioning of the head and neck or a history of drooling are concerning for a focal infection in the neck or upper thorax such as a retropharyngeal or peritonsillar abscess. Another consideration in this situation, especially for toddlers with low-grade fevers rather than high fevers, is a foreign body in the upper airway, esophagus, or trachea. Cardiac and metabolic abnormalities are also important considerations in neonates and infants who present with fever, irritability, poor feeding, and vomiting.

Neck stiffness or pain on neck flexion may also be caused by a number of processes. These include peritonsillar or retropharyngeal abscesses, cervical lymphadenitis, muscle strain or spasms, cervical epidural infections, and upper lobe pneumonia.

Seizures as an isolated symptom have many possible causes. These include epilepsy, brain tumors, CNS infections, or metabolic abnormalities (especially hypoglycemia). They may also occur secondarily to trauma.

As patient age increases and they are better able to verbalize their symptoms, the differential diagnosis becomes more focused. However, adolescents who present with fever and altered mental status may not be able to communicate with the medical team to describe their symptoms. The differential diagnosis of fever and altered mental status in an older child includes CNS infections, ingestions or drug use, and metabolic abnormalities including diabetic ketoacidosis.

Diagnostic Approach

The most important diagnostic procedure in the evaluation of a child with suspected bacterial meningitis is a lumbar puncture (LP; see Figure 92-1). An opening pressure should be measured at the time of the LP, and fluid should be sent for at least cell count with differential, gram stain and culture, protein, and glucose. If viral meningitis or meningoencephalitis is suspected, polymerase chain reaction tests for enteroviruses and HSV are useful. Viral culture is an additional way to make the diagnosis of viral meningitis, although many laboratories no longer perform this test. Various CNS infections have typical CSF parameters that can aid the clinician while the results of the CSF culture are pending (Table 92-1). If there is concern for elevated intracranial pressure (ICP) before performing the LP based on clinical signs such as papilledema or focal neurologic findings, it is reasonable to perform either a computed tomography (CT) scan or magnetic resonance imaging (MRI) study. These studies allow the clinician to determine whether there is a space-occupying lesion, such as a brain abscess or brain tumor, or dilatation of the ventricles. These findings indicate that there is a risk of brain herniation if a lumbar puncture were performed.

Blood tests can yield additional information in children with meningitis. Although blood cultures are rarely positive, it is useful to draw a blood culture prior to the administration of antibiotics, especially if there may be a delay in performing the LP. A complete blood count (CBC) occasionally has an elevated WBC count. This is a nonspecific finding, and the remainder of the CBC is usually normal. The results of a basic metabolic panel is rarely abnormal, although some patients develop the syndrome of inappropriate antidiuretic hormone secretion and subsequently have hyponatremia. Inflammatory markers such as an erythrocyte sedimentation rate and C-reactive protein are rarely helpful and do not reliably differentiate between bacterial meningitis and other causes of meningitis. If Lyme meningitis is suspected, tests for serologic evidence of Lyme exposure are useful as long as there is a confirmatory test included in the laboratory analysis.

Management and Therapy

Patients with bacterial meningitis can be critically ill and may require admission to an intensive care unit for management of their airway and circulation. In addition to respiratory and cardiovascular support, antibiotics are the most important intervention in the treatment of patients with bacterial meningitis. In patients in whom the risk of bacterial meningitis is high, antibiotics should not be withheld pending the results of diagnostic procedures or radiographic scans. Empiric antibiotic selection should be made based on local resistance patterns, and the final treatment decision should be made based on the CSF culture and sensitivity reports. In general, the combination of an intravenous (IV) third-generation cephalosporin such as ceftriaxone or cefotaxime plus vancomycin offers excellent activity against the most common pathogens. If gram-negative bacilli are seen on the CSF Gram stain, some experts recommend broader gram-negative coverage with meropenem and consideration of adding an aminoglycoside while awaiting the results of the CSF culture. The recommended duration of therapy varies according to the causative pathogen. In general, N. meningitidis meningitis is treated for 7 days, Hib meningitis for 7 to 10 days, and pneumococcal meningitis for 14 days.

The treatment of enteroviral meningitis consists of supportive care of the patient. If HSV disease is suspected, IV acyclovir is the drug of choice and is administered in three doses of 20 mg/kg/m² per day for 3 weeks. For Lyme meningitis, the current recommendation is 14 to 28 days of ceftriaxone.

The role of dexamethasone in the treatment of children with bacterial meningitis continues to be controversial. Studies of patients with Hib meningitis found that children treated with dexamethasone had decreased morbidity and mortality. This finding has not been reproduced for other causes of bacterial meningitis. Randomized trials have found a suggestion that dexamethasone therapy 1 hour before antibiotic therapy is associated with less hearing loss in children with S. pneumoniae meningitis, but this finding did not reach statistical significance.

The most common sequela of bacterial meningitis is sensorineural hearing loss, and all children with meningitis are recommended to have a hearing test after completion of therapy. Most patients with enteroviral meningitis recover with no long-term sequelae. Neonates with HSV meningitis have high rate of morbidity and mortality, and older children with HSV meningitis often have long-term neurologic deficits.

Etiology, Epidemiology, and Pathogenesis

Many microbes can form a focal suppurative infection of the CNS, including fungi, parasites, and bacteria, with bacteria being most common. Bacterial brain abscesses are often polymicrobial, and the causative organisms vary by age and pathogenesis. Overall, streptococci are most common, followed by staphylococci and then gram-negative organisms.

Organisms gain access to the brain parenchyma via a number of different mechanisms. These include hematogenous seeding, direct extension from infections of the middle ear or sinuses, or by inoculation after a surgical procedure or penetrating trauma.

Neonates with no predisposing factors can develop brain abscesses after an episode of bacteremia. These infections are usually caused by Enterobacteriaceae such as Escherichia coli or Citrobacter spp. Infants with cyanotic congenital heart disease are at especially high risk of developing brain abscesses because part of their venous blood flow bypasses the lungs. Infected emboli in the venous system have direct entry into the arterial blood flow and from there can travel to the brain. One large case series of children with brain abscesses found that 25% of the patients had congenital heart disease as a risk factor. Another study estimated that up to 6% of people with cyanotic congenital heart disease develop a brain abscess over the course of their lives.

Older children are at risk for brain abscesses from direct extension of other infections, specifically sinusitis or otitis media with mastoiditis. Brain abscesses that arise secondary to these infections are often polymicrobial and may include both aerobic and anaerobic organisms. The most commonly isolated organism from these infections is Streptococcus intermedius, which is part of the Streptococcus milleri group. Other commonly isolated organisms include Streptococcus viridans and Staphylococcus aureus. Many additional pathogens have been reported as causative agents of brain abscesses (Box 92-2).

Recently, fungal brain infections have become more common in pediatric patients. Almost all patients have some type of immune dysfunction. Many of the patients are on immunosuppressive medications to prevent rejection of a solid organ transplant or are undergoing treatment of a malignancy and had long periods of neutropenia before development of the fungal infection. Primary immunodeficiencies or infection with HIV have not been reported as common risk factors for fungal brain abscesses in children.

Clinical Presentation

Children with brain abscesses can have clinical presentation that is similar to patients with meningitis. They often present with fever and headache and may complain of nausea or vomiting. Sometimes they present to medical attention after a seizure or with focal neurologic abnormalities. If the abscess is in a location that affects CSF flow, leading to obstructive hydrocephalus, patients can present with signs of increased ICP, including vital sign abnormalities (bradycardia, hypertension, and abnormal respirations, or Cushing’s triad) or papilledema. The clinical presentation of patients with brain abscess differs from the presentation of patients with meningitis because it often lacks nuchal rigidity and Kernig’s and Brudzinski’s signs.

Diagnostic Approach

The most important diagnostic procedure is brain imaging with contrast, either a CT or MRI (Figure 92-3). The advantages of CT scans are that they are quicker than MRIs and usually do not require sedation for the patient. On CT scans, abscesses appear as rim-enhancing, space-occupying lesions, often with surrounding cerebral edema. After an abscess is found, the patient should be evaluated by a neurosurgeon for drainage of the abscess. Samples should be sent from the operating room to a microbiology laboratory for Gram stain and aerobic and anaerobic culture.

Figure 92-3 Parameningeal infections.

Management and Therapy

Surgery and antibiotics are the mainstays of therapy for brain abscesses. If a neurosurgeon is not immediately available, antibiotics should not be held awaiting surgery for clinical specimens to be sent for culture. Empiric antibiotic therapy should be broad and include activity against anaerobes, streptococci, S. aureus, and gram-negative organisms. One possible regimen is vancomycin, a third-generation cephalosporin such as cefotaxime, and metronidazole. β-Lactam and β-lactamase inhibitor combinations or clindamycin should not be used because the β-lactamase inhibitors and clindamycin do not achieve adequate concentrations in the CNS. For a broader spectrum of gram-negative activity, carbapenems such as imipenem or meropenem can be used to replace the third-generation cephalosporin and metronidazole. If the patient shows signs of increased ICP, therapy to reduce this pressure may be necessary before drainage of the abscess.

Most of the time, cultures sent from the abscess reveal the causative organisms, but antibiotics given before the drainage can affect culture results. These infections usually require a prolonged course of IV antibiotics. The decision to stop antibiotics is guided by the patient’s clinical response as well as the results of follow-up imaging. Long-term sequelae are difficult to predict and depend on a number of factors including patient comorbidities, location of the abscess, and clinical factors such as increased ICP, seizures, and number of neurosurgical procedures.

Etiology, Epidemiology, and Pathogenesis

Subdural empyema is most commonly associated with infection of the paranasal sinuses and mostly affects children in their second decade of life. It is the most common suppurative complication of sinusitis and affects boys more often than girls (the reported male : female ratio ranges from 1.3 : 1 to 4.5 : 1). It can occur via two mechanisms: either direct extension of the infection, usually from the frontal sinus, which passes through the dura into the subdural space, or via thrombophlebitis of the valveless diploic veins. Subdural empyema can also occur after penetrating trauma, after neurosurgery, or as a superinfection of subdural effusions after meningitis in infants.

The microbiology of subdural empyema is determined by the mechanism of the infection. Subdural empyemas which are a complication of paranasal sinusitis are most likely to grow S. intermedius. Often, these infections are polymicrobial and include anaerobes such as Fusobacterium or Prevotella spp. Some data suggest that these anaerobes increase the virulence of S. intermedius, which may explain the frequent polymicrobial nature of these infections. Other flora, which commonly cause bacterial sinusitis, including S. pneumoniae, Moraxella catarrhalis, and nontypable H. influenzae, are not common causes of subdural empyema. S. aureus is a common cause of subdural empyema after penetrating trauma or neurosurgery. Historically, Hib was a common cause of subdural empyema infecting subdural effusions complicating Hib meningitis, but since the introduction of the Hib vaccine, this entity is now rarely seen.

Clinical Presentation

Patients with subdural empyema can present with a spectrum of symptoms. Patients may be asymptomatic; a subdural empyema can be an incidental finding in patients with bacterial sinusitis. The most frequent symptoms associated with subdural empyema are fever, headache, and vomiting. Because vomiting is an uncommon presenting feature of sinusitis, the combination of sinusitis, fever, and vomiting should raise concern for intracranial spread of the infection. Other presenting signs include focal neurologic deficits, seizures, and decreased sensorium. The severity of presentation is usually associated with the extent of the infection and the degree of elevated ICP.

Diagnostic Approach

The most important diagnostic procedure when subdural empyema is suspected is neuroimaging, preferably with an MRI with gadolinium. A CT with contrast may not identify a small subdural collection or may underestimate the extent of involvement of a subdural empyema. An MRI with contrast offers excellent visualization of the subdural space and allows the clinician to differentiate between blood and purulent fluid. It often contains enough information about localization and the presence of loculations to allow neurosurgeons to decide whether a burr hole is adequate or whether a craniotomy is needed to drain the fluid.

Routine blood work, including CBCs and inflammatory markers, does not give any significant information to aid in the diagnosis of subdural empyema but may be important for overall management of the patient. If antibiotics have not been administered, it is useful to obtain a blood culture. Although bacteremia is uncommon in patients with subdural empyema, it occurs in some of the other processes that are part of the differential diagnosis.

If neuroimaging reveals subdural empyema or signs of elevated ICP or if there is concern about elevated ICP based on the clinical examination findings, an LP should not be performed until the elevated ICP is addressed. An LP is usually not diagnostic of subdural empyema because the infection is sequestered within part of the subdural space. Unlike meningitis, the pathogen does not freely circulate throughout the CSF. If an LP is performed, the CSF often has a mild pleocytosis, but the CSF culture rarely grows an organism.

Management and Therapy

After subdural empyema is diagnosed, the most important step in management is neurosurgical consultation. Most patients with this infection require either a burr hole or craniotomy, although there are reports of some patients with small subdural empyemas being managed with medical therapy alone. The fluid drained by the neurosurgeon should be sent for Gram stain, routine culture, and anaerobic culture.

Empiric antibiotic regimens for subdural empyema are the same as the regimens for brain abscesses and should include broad coverage of aerobic and anaerobic streptococci, other oropharyngeal anaerobes, and gram-negative organisms. Antibiotics should be given immediately and should not be delayed awaiting surgical intervention. Patients with subdural empyema require intensive care management even if they do not require surgery. Before surgery, they can have vital sign instability if there is significant elevation of ICP, and they can develop rapid changes in their neurologic function. Postoperatively, they require close monitoring of their cardiorespiratory and neurologic status. The duration of antibiotics depends on their clinical course but usually lasts at least 4 to 6 weeks. Many patients experience some long-term sequelae from these infections, but death is rare.

Etiology, Epidemiology, and Pathogenesis

Epidural abscesses can occur either within the cranium or within the spinal canal (see Figure 92-3). The pathogenesis and infectious organisms associated with these infections vary according to the site of infection.

Overall, epidural abscesses are quite rare, and most clinical information about pediatric epidural abscesses consists of case reports and case series. Most intracranial epidural abscesses occur as a complication of sinusitis. The published case series have found an increased incidence during adolescence, which is thought to be attributable to the rapid growth of the sinuses during that time and the increased vascularity of the diploic veins during adolescence. These case series have also described a male predominance, although the etiology is unclear. The frontal sinuses are usually the primary site of infection. It has been hypothesized that epidural abscesses are seen more often in conjunction with frontal sinusitis because of a larger potential space between the frontal sinuses and the cranial vault compared with other locations, where the dura is more adherent to the cranium. Less commonly, epidural abscesses can occur as a complication of otitis media and mastoiditis.

The microbiology of epidural abscesses that are caused by sinusitis is similar to the microbiology of subdural empyema because they have the same primary source of infection. Organisms of the S. milleri group, including S. intermedius, are the most commonly identified pathogens. These infections can be polymicrobial and include anaerobic bacteria.

Spinal epidural abscesses are less common than intracranial epidural abscesses with an estimated incidence of 0.6 per 10,000 admissions in one large case series. In pediatric patients, the pathophysiology is usually hematogenous seeding of the epidural space with subsequent response by the immune system, leading to abscess formation. These abscesses can occur anywhere from the cervical spine down to the sacrum. Whereas most pediatric patients with spinal epidural abscesses do not have any comorbidities, the majority of adults who develop these infections have an underlying condition that puts them at risk. These conditions include particular disease states (diabetes mellitus, HIV infection), spinal abnormalities (spinal surgery, trauma, degenerative joint disease), or a source of infection (urinary tract infection, epidural anesthesia, IV drug use).

S. aureus is the most common pathogen associated with pediatric spinal epidural abscesses. Many times there is also evidence of osteomyelitis, discitis, or paraspinal pyomyositis. These infections are very rarely polymicrobial, and anaerobes have not been described as part of this syndrome. Other pathogens that have been reported to cause spinal epidural abscesses include Pseudomonas aeruginosa, S. pneumoniae, Salmonella spp. (in children with sickle cell disease), E. coli, Fusobacterium spp., and S. viridans. Children receiving chemotherapy for malignancies or with primary immunodeficiencies have been reported to have infections caused by unusual pathogens, including Aspergillus flavus, Mycobacterium bovis, and Candida tropicalis.

Clinical Presentation

The clinical presentation of patients with epidural abscesses depends on the anatomic location of the infection. Patients with epidural abscesses arising from the frontal sinuses typically present with intermittent headaches, which can last for a few days to a few weeks. They may also have fevers, nausea, and vomiting. Occasionally, patients with these infections develop cerebritis and may develop personality changes or other neurologic symptoms. Epidural abscesses can also result from a complication of mastoiditis. In these situations, patients may present with headaches, nausea, and vomiting in addition to the signs and symptoms of mastoiditis. Finally, patients may have epidural abscesses along the spinal cord. The symptoms of this infection are attributable to inflammation of or mass effect on the spinal nerve roots arising in the affected area. Patients with cervical epidural abscesses may present with neck stiffness or focal arm pain or weakness. Patients with epidural abscesses in the lumbosacral region may present with back pain, gait abnormalities, changes in bowel or bladder function, or other symptoms relating to the lower extremities.

Diagnostic Approach

If an epidural abscess is suspected, the first diagnostic step is an MRI with gadolinium or a CT with contrast of the suspected area. Noncontrast studies are less useful because contrast improves the sensitivity of the imaging study and displays a better outline of the abscess. For spinal lesions, MRI is the preferred modality because of its superiority in identifying bone and soft tissue inflammation compared with CT. After an abscess has been identified, a neurosurgeon should be consulted to discuss drainage of the lesion. If sinusitis is also identified on the imaging study, an otolaryngologist should be consulted to discuss whether drainage of the sinuses would also be helpful. Some spinal epidural abscesses may be drained by an interventional radiologist under ultrasound or CT guidance. Occasionally, small abscesses do not require drainage and can be treated with antibiotics alone. If an abscess is drained, samples should be sent for Gram stain and aerobic and anaerobic culture.

Management and Therapy

For small epidural abscesses or those that are difficult to approach surgically, medical therapy with antibiotics is the initial step in management. The location of the infection guides empiric antibiotic therapy. The microbiology of epidural abscesses arising from sinusitis or mastoiditis is similar to the microbiology of subdural empyemas and brain abscesses. The combination of a third-generation cephalosporin, vancomycin, and metronidazole offers broad activity with good CNS penetration in the event that there is a subdural component to the infection. Antibiotic therapy can be narrowed based on the results of cultures obtained from the epidural collection. These infections are usually treated for a minimum of 3 weeks, but the total duration depends on the size of the abscess, whether it was drained, and the patient’s clinical response. In most cases, there are no long-term sequelae associated with these infections, but the risk of morbidity is increased if there is associated subdural empyema or if the patient presented with focal neurologic signs.

Empiric antibiotic selection for patients with spinal epidural abscesses without any underlying conditions (sickle cell disease, immunodeficiency, or malignancy) should focus on S. aureus. Recently, community-acquired methicillin-resistant S. aureus (CA-MRSA) has become a common pediatric pathogen, so this must be taken into account when deciding on empiric antibiotics. Clindamycin has good activity against most CA-MRSA isolates and is a good empiric choice. In patients with predisposing conditions, empiric coverage may need to be broader and include gram-negative organisms. The only data about duration of therapy are cases series, and most authors recommend at least 6 weeks of therapy. Factors to take into consideration include the size of the abscess, whether it was drained, whether the patient has a predisposing medication condition, and the patient’s response to therapy. Most patients recover fully unless they had focal neurologic findings at the time of presentation.