Aseptic meningitis

Aseptic meningitis is a condition in which the layers lining of the brain, or meninges, become inflamed and a pyogenic bacterial source is not to blame. Meningitis is diagnosed on a history of characteristic symptoms and certain examination findings (e.g. Kernig's sign). Investigations should show an increase in the number of leukocytes present in the cerebrospinal fluid (CSF), obtained via lumbar puncture, (normal being fewer than five visible per microscopic high power field).

The term aseptic is frequently a misnomer, implying a lack of infection. On the contrary, many cases of aseptic meningitis represent infection with viruses or mycobacteria that cannot be detected with routine methods. While the advent of polymerase chain reaction has increased the ability of clinicians to detect viruses such as enterovirus, cytomegalovirus, and herpes virus in the CSF, many viruses can still escape detection. Additionally, mycobacteria frequently require special stains and culture methods that make their detection difficult. When CSF findings are consistent with meningitis, and microbiologic testing is unrevealing, clinicians typically assign the diagnosis of aseptic meningitis—making it a relative diagnosis of exclusion.

Aseptic meningitis can result from non-infectious causes; it is a relatively infrequent side effect of medications, and can be an early finding in autoimmune disease.

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Classification

There is no formal classification system. It is usually by the causative organism if identified.

Symptoms and Signs

These are varied, depending on the causative organism. There are usually non-specific constitutional symptoms lasting for hours or days. These are then followed by meningitis, characterised by headache, stiff neck, fever, photophobia, drowsiness, and myalgia. A rash may be present, which could suggest a particular virus - for example, varicella zoster. However, a non-blanching purpuric rash is not associated with meningitis and suggests systemic bacterial infection.

Complications

Seizures, encephalitis and cognitive problems can develop, although rarely.

Cause/Etiology

The cause can be infectious or non-infectious.

Infectious

Viruses

• HSV 1 and 2
• HIV
• Enteroviruses
• Varicella zoster
• Epstein-Barr virus
• CMV
• Lymphocytic choriomeningitis virus
• Polio Virus

Bacteria

• Partially treated meningitis
• Endocarditis
• Mycoplasma
• Mycobacterium tuberculosis
• Borrelia burgdorferi
• Treponema pallidum
• Brucella

Fungi

• Cryptococcus neoformans
• Blastomyces dermatitidis

Parasites

• Toxoplasma gondii

Non-infectious

Drugs

• NSAIDs
• Amoxicillin
• Azathioprine
• Methotrexate
• Intravenous immunoglobulin
• Isoniazid
• Allopurinol

Systemic Diseases

• Sarcoidosis
• Meningioma
• SLE
• Wegener's
• Vasculitis
• Behcet's disease

Miscellaneous

• Migraine
• Arachnoiditis

Diagnosis

Usually the history and examination will arouse suspicion. Confirmation is mainly through CSF findings:

• Less than 500 mononuclear cells/mm³ (pleocytosis) should develop with 8-48 hours
• Normal glucose
• Elevated pressure
• Elevated protein
• No findings which suggest another diagnosis - e.g. negative bacteria antigen tests, no lactate
• PCR may identify a causative organism

Viruses may be cultured from swabs of other areas, such as the throat.

Blood tests are rarely helpful in establishing the diagnosis (but may be of use to establish baseline chemistry). Imaging is useful in excluding other diagnoses, or identifying other features of infection by an organsim - for example, a chest X-ray may be useful if tuberculosis is suspected.

Pathophysiology

Invasion into or past the meninges by a pathogen can set up a local inflammatory response. The clinical signs are due to this meningeal irritation - for example, Kernig's sign is due to pain produced by stretching of the inflamed meninges.

Treatment/Management

Anti-pathogenic

If the causative organism has been identified and has a specific therapy, this should be started.

Bacteria

Even though true aseptic meningitis cannot be caused by pyogenic bacteria, broad-spectrum antibiotic cover should be started as the consequences of misdiagnosing a bacterial meningitis are dire, and relatively easily avoided. For non-pyogenic bacteria, local sensitivities should be taken into account, but generally broad-spectrum is best. Some bacteria are normally sensitive to certain drugs - for example, rifampicin is good for Brucella.

Viruses

HSV, varicella and CMV have a specific antiviral therapy; most other viruses do not. For HSV the treatment of choice is acyclovir^[1]

Fungi

Amphotericin B and fluconazole are the best antifungals in most situations.

Supportive

This will be the majority of the treatment. Fluids, analgesia and antiemetics should cover most cases. Antipyretics should be used judiciously - fever can be a natural response. Steroids are not recommended unless raised intracranial pressure occurs. Phenytoin and other anticonvulsants can be used is seizures occur, but prophylaxis is not recommended.

Prognosis

In immunocompetent individuals, the disease is usually mild and self-limiting. Full recovery 5-14 days afterwards is normal.

Prevention/Screening

Vaccines are available for some organisms that cause aseptic meningitis. Good infection control in hospital, as ever, is recommended. If the causative organism is contagious, steps may need to be taken to isolate the individual and protect the community.

Epidemiology

Aspetic meningitis is relatively common, with an incidence of around 10/100,000. The male:female ratio is around 1:1.

References