تويت
Microbiology of the Nervous System
Richard T. Johnson
General Concepts
The anatomy of the brain and meninges determines the special character of central nervous system (CNS) infections. Epidural abscesses remain localized, whereas subdural abscesses spread over a hemisphere. Subarachnoid space infections spread widely over the brain and spinal cord. The blood-brain barrier formed by the tight junctions between cells of the cerebral capillaries, choroid plexus, and arachnoid largely prevents macromolecules from entering the brain parenchyma. As a result, immunoglobulins and immune-competent cells are scarce in the brain except at foci of inflammation. The space between cells in the brain parenchyma is too small to permit passage even of a virus. However, tetanus toxin and some viruses travel through the CNS by axoplasmic flow.
Meningitis
Etiology
Major bacterial causes are Haemophilus influenzae, Streptococcus pneumoniae, and Neisseria meningitidis. Major viral causes are enteroviruses, mumps virus and lymphocytic choriomeningitis virus.
Pathogenesis
Most agents invade from blood. Bacteria grow rapidly in cerebrospinal fluid; viruses infect meningeal and ependymal cells.
Clinical Manifestations
Headache, fever and stiff neck are the symptoms of meningitis. Untreated bacterial meningitis is usually fatal; viral meningitis is benign. Cerebrospinal fluid findings are critical in differential diagnosis.
Treatment
Antibiotics are used to treat bacterial and fungal meningitis. Viral meningitis is treated symptomatically.
Brain Abscess
Etiology
Brain abscesses often exhibit a mixed flora of aerobic and anaerobic bacteria. Fungi are uncommon.
Pathogenesis
Abscesses begin when bacteria seed sites of necrosis, caused usually by infarction.
Clinical Manifestations
Headache, focal signs and seizures indicate a brain abscess. There are also characteristic computed tomography (CT) and magnetic resonance image (MRI) findings.
Treatment
Treatment consists of surgical drainage and appropriate antibiotics.
Encephalitis
Etiology
Many viruses cause mild meningoencephalitis; herpes simplex viruses and arboviruses are the major causes of potentially fatal disease.
Pathogenesis
Herpes simplex virus causes acute diffuse encephalitis in neonates. Herpes simplex type 1 causes focal temporal and frontal encephalitis in children and adults probably owing to invasion along olfactory or sensory nerves in the immune host. Arboviruses invade from the blood and cause diffuse predominantly neuronal infection. Rabies invades along peripheral nerves.
Clinical Manifestations
Encephalitis causes headache, fever, CNS depression, seizures, and mononuclear cells in cerebrospinal fluid. Focal temporal lobe signs occur in herpes simplex virus encephalitis.
Treatment
Acyclovir is used to treat herpes simplex encephalitis. Some arboviruses can be prevented by mosquito control or vaccines.
Slow and Chronic CNS Infections
Spirochetes
Untreated syphilis and Lyme disease can cause varied later CNS disease.
Retroviruses
Human immunodeficiency virus can cause acute and progressive CNS disease. HTLV-I causes chronic spastic paraparesis in a small number of infected persons.
Conventional Viruses
Persistent measles and rubella virus infections can cause subacute encephalitis with dementia. JC virus, a papovavirus, can cause progressive demyelinating disease in immunodeficient patients.
Unconventional Agents
Kuru and Creutzfeldt-Jakob disease are chronic noninflammatory, degenerative diseases of the brain that are caused by unconventional agents called prions.
Parasites
Parasites may cause acute meningitis or encephalitis, chronic encephalopathy, and cerebral granulomas. Neurocysticercosis is the most common parasitic neurologic disease.
Richard T. Johnson
General Concepts
The anatomy of the brain and meninges determines the special character of central nervous system (CNS) infections. Epidural abscesses remain localized, whereas subdural abscesses spread over a hemisphere. Subarachnoid space infections spread widely over the brain and spinal cord. The blood-brain barrier formed by the tight junctions between cells of the cerebral capillaries, choroid plexus, and arachnoid largely prevents macromolecules from entering the brain parenchyma. As a result, immunoglobulins and immune-competent cells are scarce in the brain except at foci of inflammation. The space between cells in the brain parenchyma is too small to permit passage even of a virus. However, tetanus toxin and some viruses travel through the CNS by axoplasmic flow.
Meningitis
Etiology
Major bacterial causes are Haemophilus influenzae, Streptococcus pneumoniae, and Neisseria meningitidis. Major viral causes are enteroviruses, mumps virus and lymphocytic choriomeningitis virus.
Pathogenesis
Most agents invade from blood. Bacteria grow rapidly in cerebrospinal fluid; viruses infect meningeal and ependymal cells.
Clinical Manifestations
Headache, fever and stiff neck are the symptoms of meningitis. Untreated bacterial meningitis is usually fatal; viral meningitis is benign. Cerebrospinal fluid findings are critical in differential diagnosis.
Treatment
Antibiotics are used to treat bacterial and fungal meningitis. Viral meningitis is treated symptomatically.
Brain Abscess
Etiology
Brain abscesses often exhibit a mixed flora of aerobic and anaerobic bacteria. Fungi are uncommon.
Pathogenesis
Abscesses begin when bacteria seed sites of necrosis, caused usually by infarction.
Clinical Manifestations
Headache, focal signs and seizures indicate a brain abscess. There are also characteristic computed tomography (CT) and magnetic resonance image (MRI) findings.
Treatment
Treatment consists of surgical drainage and appropriate antibiotics.
Encephalitis
Etiology
Many viruses cause mild meningoencephalitis; herpes simplex viruses and arboviruses are the major causes of potentially fatal disease.
Pathogenesis
Herpes simplex virus causes acute diffuse encephalitis in neonates. Herpes simplex type 1 causes focal temporal and frontal encephalitis in children and adults probably owing to invasion along olfactory or sensory nerves in the immune host. Arboviruses invade from the blood and cause diffuse predominantly neuronal infection. Rabies invades along peripheral nerves.
Clinical Manifestations
Encephalitis causes headache, fever, CNS depression, seizures, and mononuclear cells in cerebrospinal fluid. Focal temporal lobe signs occur in herpes simplex virus encephalitis.
Treatment
Acyclovir is used to treat herpes simplex encephalitis. Some arboviruses can be prevented by mosquito control or vaccines.
Slow and Chronic CNS Infections
Spirochetes
Untreated syphilis and Lyme disease can cause varied later CNS disease.
Retroviruses
Human immunodeficiency virus can cause acute and progressive CNS disease. HTLV-I causes chronic spastic paraparesis in a small number of infected persons.
Conventional Viruses
Persistent measles and rubella virus infections can cause subacute encephalitis with dementia. JC virus, a papovavirus, can cause progressive demyelinating disease in immunodeficient patients.
Unconventional Agents
Kuru and Creutzfeldt-Jakob disease are chronic noninflammatory, degenerative diseases of the brain that are caused by unconventional agents called prions.
Parasites
Parasites may cause acute meningitis or encephalitis, chronic encephalopathy, and cerebral granulomas. Neurocysticercosis is the most common parasitic neurologic disease.
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