INFECTIONS OF THE CENTRAL NERVOUS SYSTEM
Infection of the central nervous system (CNS) can be viral, bacterial, fungal, or parasitic in origin. Infectious microorganisms most often enter the CNS by direct penetration after trauma or by travelling in the bloodstream. People who are immunocompromised from conditions such as AIDS, cancer, steroid use, diabetes or alcoholism may be at risk for opportunistic infections which would not ordinarily affect persons with normal immune defenses. There are three major categories of CNS infections treated by a neuro-ICU:
» Brain Abscess
What is encephalitis?
Encephalitis is defined as inflammation of the brain due to an infection. This inflammation is commonly the result of a viral infection. Viruses can gain access to the central nervous system (CNS) through the blood or by traveling within nerve cells (neurons). The neuro-ICU deals primarily with acute viral encephalitis. In the United States the most common cause of acute viral encephalitis is the herpes simplex virus. There are approximately 20,000 cases of encephalitis in America each year.
What are the characteristics of herpes simplex encephalitis?
Herpes Simplex Encephalitis is the most common form of acute viral encephalitis, accounting for 10-20% of all cases of encephalitis in the United States. There are two types of herpes simplex viruses, HSV-1 and HSV-2. In adults, Herpes Simplex Encephalitis is caused by HSV-1, which gains access to the brain by traveling backwards along facial sensory nerves which have been infected.
Herpes Simplex Encephalitis is clinically characterized by headaches and fever in its early stages. Seizures are commonly present in the early stages of the disease. In addition, because of the viruses' localization to the temporal lobe of the brain, hallucinations, behavioral abnormalities, and personality changes are present in 90% of patients early on. Among other clinical manifestations are: memory impairment, an inability to understand words or to speak fluently (aphasia), and loss of movement most commonly occurring in the face or arm.
With progression of the disease the frontotemporal region of the brain (that area between the frontal and temporal lobes) can swell with fluid, a condition known as cerebral edema. This edema can cause a dangerous increase in the pressure within the skull (increased intracranial pressure).
What is the prognosis of Herpes Simplex Encephalitis?
Antiviral therapy is critical to effect a favorable prognosis for Herpes Simplex Encephalitis. In the absence of such therapy the mortality rate is 70%. Delayed treatment, an age of the patient over 30, and coma are all associated with a poorer prognosis.
How is Herpes Simplex Encephalitis diagnosed?
Diagnosis of this disease is particularly difficult. It is estimated that only half of the patients suspected to have Herpes Simplex Encephalitis actually have the disorder. Herpes Simplex Encephalitis can be confused with bacterial infections, tumors, vascular disease, and other viral infections.
The disease is diagnosed by positive results from a number of tests. One such test is the analysis of cerebrospinal fluid or CSF, a fluid which circulates around the brain and serves to cushion it from injury. Although this test is considered only moderately effective, recent advances focused on the identification of HSV DNA by using the polymerase chain reaction have made this test more sensitive and specific. Another test employs the analysis of brain waves via an electroencephalogram or EEG. This device can detect abnormalities which are commonly associated with the disease. CT and MRI scanning can detect early signs of edema in the frontal and temporal lobes.
Nonetheless, the most accurate and the only definitive way of diagnosing Herpes Simplex Encephalitis is via brain biopsy. In this technique a small specimen of the brain is neurosurgically removed and analyzed. Without biopsy the accuracy of diagnosis is only about 45%.
How is Herpes Simplex Encephalitis treated in a neuro-ICU?
Treatment of this disorder involves the use of the antiviral drug acyclovir. Other, antiviral agents include the drug vidarabine and Xascarnet. The sooner acyclovir treatment is instituted the better the prognosis of the patient. As a result, it is imperative that this treatment be instituted promptly.
The neuro-ICU also effectively treats frontotemporal edema and/or increased intracranial pressure associated with more severe cases of the illness. To monitor for brain swelling, an intracranial pressure monitor can be inserted. If ICP is elevated efforts are made to reduce it.
What is meningitis?
In order to better understand meningitis one must first have knowledge about the structures surrounding the brain. Beneath the inner surface of the skull the brain is surrounded by a membranous covering known as the meninges. A fluid known as cerebrospinal fluid (CSF) circulates around the brain and serves to cushion the brain against injury. Meningitis is an inflammation of the meninges due to infection. It occurs when a foreign pathogen invades the subarachnoid space and populates the CSF. The foreign microorganisms can either be bacteria or viruses. Accordingly, meningitis can be classified as either bacterial or viral. Since bacterial infection is much more serious, a neuro-ICU is specialized towards the treatment of this type of meningitis.
What are the causes of bacterial meningitis?
Generally, bacterial meningitis is more dangerous than the viral form and can constitute a medical emergency. Two of the major forms of bacteria which cause bacterial meningitis are Streptococcus pneumoniae and Neisseria meningitidis. Therefore, bacterial meningitis usually occurs in either a pneumococcal or a meningococcal form. Pneumococcal meningitis is typically observed in adults. It can arise following brain trauma, and is predisposed by sickle cell anemia, alcoholism, and diabetes. Meningococcal meningitis most often occurs in children, adolescents, and young adults.
Whatever the type of meningitis, a critical tool for diagnosis of the disease is lumbar puncture (or spinal tap). This procedure involves the extraction of a sample of cerebrospinal fluid surrounding the spinal cord in the subarachnoid space. The sample is obtained by the insertion of a needle into a region of the lower back. The CSF sample is required both to establish a diagnosis of bacterial meningitis and to determine the identity of the invading bacteria, a determination which is essential in treating the disease.
What are the clinical features of bacterial meningitis?
Bacterial meningitis presents as an acute disease. It is characterized by three main features: headache, high persistent fever, and neck stiffness (an inability to bow the head, known as nuchal rigidity). Bacterial meningitis can also be accompanied by a variety of other symptoms including rashes, nausea, lethargy, and general malaise. In addition, seizures occur in about 20% of patients and coma occurs in 5-10% of patients. The latter development is associated with a particularly poor prognosis.
What are the complications of bacterial meningitis?
Patients with meningococcal meningitis can experience extremely low blood pressure resulting in shock, a condition known as the Waterhouse-Friderichsen syndrome. In addition, bacterial pathogens can increase the amount of fluid leaving blood vessels to enter the tissue of the brain. This condition destroys the so-called blood-brain barrier and results in a swelling of the brain with fluid, a condition known as cerebral edema. This can translate into a dangerous increase in the pressure within the skull (the intracranial pressure or ICP). Both of these complications are life-threatening and mandate treatment in an intensive care unit.
What is the prognosis for bacterial meningitis?
It is absolutely crucial that treatment for bacterial meningitis begin as soon as possible. Prognosis thus directly depends on the speed with which therapy is initiated. It is also dependent on the identity of the invading bacteria as well as the age and medical status of the patient. With rapid and effective treatment mortality rates vary between 5-25%. If treatment is delayed long-term sequelae such as deafness and intellectual deterioration can occur. If treatment is further prolonged the outcome is often death. Therefore, the earlier a diagnosis is made and treatment instituted, the greater the chance of survival without neurological disabilities.
How is bacterial meningitis treated in a neuro-ICU?
The first step towards treatment is to obtain a CSF sample via lumbar puncture and to then initiate antibiotic therapy. The antibiotic regimen should be specific to the type of bacteria causing the meningitis. Generally, however, a regimen of ampicillin and a third-generation cephalosporins such as ceftriaxone is effective for initial treatment.
In addition to treating the infection proper, measures are taken to reverse the complications of bacterial meningitis, Dehydration and/or shock is treated by blood volume expansion and the use of pressors to increase blood pressure. Cerebral edema and increased intracranial pressure are associated with a depressed level of consciousness, severe headache, and projectile vomiting. They are treated by employing techniques used to measure intracranial pressure by ICP monitoring. Once ICP is correctly monitored efforts are made to reduce it. Seizures are treated by the use of rapid acting anticonvulsants such as diazepam and lorazepam. Lorazepam infusion is commonly followed-up with a longer lasting anticonvulsant known as phenytoin (Dilantin).
What is a brain abscess?
A brain abscess is a circumscribed region of infection within the substance of the brain. The abscess is initially characterized by an area of necrotic (dead) brain tissue surrounded by a zone of cerebritis (local inflammation of brain cells). As the abscess develops the necrotic area becomes filled with pus and a ring of cells surrounds the area. A mature abscess is characterized by a necrotic puss-filled region of brain tissue, surrounded by an area of cerebritis.
What causes a brain abscess to form?
A brain abscess forms as the result of the spread into brain tissue of an infection elsewhere. There are three possible origins of this infection:
- An abscess most commonly arises via the direct extension into the skull of a local infection in the paranasal sinuses or in the middle ear.
- Microorganisms can also be spread by the blood during a systemic infection. In this case bacteria are carried to the site of abscess from a distant source, typically the skin, lungs, mouth, or heart valves. Under these circumstances there is not a solitary abscess but rather multiple abscesses in the brain.
- Lastly, a brain abscess can result from head trauma. An infection can arise from a wound penetrating the skull. In this case inoculation with bacteria occurs from infected bone fragments or debris from the penetrating instrument.
What are the symptoms of a brain abscess?
Symptoms resulting from a brain abscess depend on the size and the location of the infection. Only 50% of patients with a brain abscess present with a fever and, when present, fever is often low-grade. A brain abscess can also present with symptoms typical of any space-occupying mass within the substance of the brain (a focal neurological deficit. The commonly observed deficits include weakness on one side of the body (hemiparesis), impaired speech production (dysphasia), visual field deficits, and an inability to smoothly coordinate muscle movements, such as during walking (ataxia).
What is the prognosis of a brain abscess?
Generally, the prognosis associated with brain abscess is dependent on the rapidity and efficacy of treatment, the age and medical condition of the patient, and the size and location of the abscess. In recent decades brain imaging techniques (such as MR and CT scanning) used to detect the early presence of brain abscesses have reduced mortality rates from about 40% to about 15%. This drop in mortality is also due to improved treatment methods, including intensive care. The probability that a patient will respond to such treatment is directly dependent on his or her alertness. Patients who are awake and responsive usually do well, whereas those patients in prolonged coma have a poorer prognosis.
How is a brain abscess treated in a neuro-ICU?
As mentioned, brain imaging is central to the diagnosis of brain abscess and thus essential to the early detection of such an infection.
Once diagnosed brain abscess is treated with antibiotic therapy. As is the case with any bacterial infection, the type of antibiotic given should be appropriate to the type of infectious bacteria. Typically, the origin of the abscess is indicative of the bacterial identity. Abscesses arising from the extension of a paranasal sinus infection often contain the bacterium Streptococcus, and abscesses resulting from trauma contain bacteria of the Staphylococcus variety. In the case of multiple infections spread via the blood from a distant source a variety of different bacteria are usually involved. Thus a corresponding wide spectrum of antibiotics is given for these abscesses.
Surgery is another mainstay treatment of brain abscess. There are two ways in which an abscess can be surgically drained. The abscess can be surgically excised if it is readily accessible, e.g. if it exists nearer to the brain surface. Alternatively, an abscess can be drained by aspiration (suctioning) through a hole in the skull. Aspiration is made possible by the use of a CT-guided sterotactic method. This technique is usually the preferred method of surgical drainage of an abscess. If the abscess is small or in an early stage of its development, or if the causative bacteria is known, medical treatment with an antibiotic regimen may be sufficient. In most other instances surgical aspiration is used. Aspiration has two advantages, it gives a sample of the abscess for bacterial analysis, and, it serves to alleviate the "mass effect" of the abscess.
The cerebral edema and increased intracranial pressure associated with brain abscess are also specifically treated in a neuro-ICU. Techniques are used to measure intracranial pressure by ICP monitoring. Finally, seizures associated with brain abscess are treated in the neuro-ICU by the use of rapid acting anticonvulsants such as diazepam and lorazepam. Diazepam infusion is commonly followed-up with a longer lasting anticonvulsant known as phenytoin.
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August 27, 2014