Meningitis and Encephalitis: What’s the Difference?

«Meningitis» and «encephalitis» are two words that pop onto most people’s radar screens from time to time, and usually in some scary context, like hearing of a cluster of cases in their child’s school, or reading media reports of epidemics occurring nationally or internationally. While most people understand that these words mean there is some sort of infection of the nervous system, other distinctions and implications are often left unstated and, as a result, can be vague or confusing. The basic concepts are built into the words themselves. Starting at the ends of the words and working forwards, «-itis» is the medical suffix meaning inflammation. Although it’s possible for inflammation to occur without an infection being present, as a practical matter, in most cases of meningitis or encephalitis the inflammation is indeed due to an infection. The next step in understanding these concepts is to analyze the first parts of the words. «Mening-» refers to the meninges which are the membranous coverings of the brain and spinal cord. So «meningitis» means inflammation or infection of these membranous coverings. By contrast, «encephal-» refers to the encephalon or brain (originating from the Greek word «enkephalos»), so «encephalitis» means inflammation or infection of the brain itself. Although no case of meningitis or encephalitis is trivial, depending on the particulars, some cases end up as temporary illnesses from which there is full recovery, while others can be severely damaging or even lethal. In a nutshell, cases of meningitis caused by viruses are usually associated with good outcomes (even without treatment), while cases of meningitis involving bacteria are very serious and require emergency treatment with powerful antibiotics. All cases of encephalitis–usually caused by viruses and not by bacteria–are serious, and antiviral treatment is available for some of the viruses involved, but not all. Most cases of either meningitis or encephalitis start fairly abruptly, sometimes following an obvious infection elsewhere in the body and sometimes not. As with most infections, a fever is usually present in meningitis or encephalitis, but is not necessarily striking. In both cases the patient feels miserable in general and often complains of pain in the head, neck, or both. Because encephalitis involves infection of the brain itself, symptoms of altered brain function–like confusion or decreased alertness–are usually present, while in cases of meningitis the patient is initially alert and, though understandably distracted by pain and misery, still in command of their mental processes. In either case, prompt medical assessment is important. In both meningitis and encephalitis a lumbar puncture (also known as a spinal tap) is usually crucial in detecting the presence of an infection, identifying the infecting organism, and guiding successful treatment. While an imaging test like a CT scan or an MRI scan is often included as part of the evaluation, they do not replace the lumbar puncture in identifying the essential features of the infection. A lumbar puncture is usually performed with the patient lying on one side, curled into a fetal position. The doctor preps and drapes the patient’s lower back to create a sterile field in which to work. After numbing the skin of the lower back the doctor inserts a needle in the middle of the spine, puncturing the meninges. In the lower back there is no spinal cord, so there is no risk of puncturing it, too. Watery fluid is collected into tubes as it drips out of the back of the needle. Then the needle is withdrawn. This watery fluid is called CSF–short for cerebrospinal fluid–and because it resides within the meninges (and outside of the brain and spinal cord) it holds some of the keys to diagnosing the infection. Laboratory personnel can perform several tests on the fluid right away, like measuring the concentrations of red and white blood cells, as well as the concentrations of protein and sugar. An increase in concentration of white blood («pus») cells and an increase in protein concentration are expected findings when the meninges are infected by either bacteria or viruses, with the changes more pronounced in bacterial infections than in viral infections. Reductions in sugar concentration are common in bacterial but not viral infections. Other tests on the CSF involve inherent delays, like trying to grow bacteria from the CSF in Petri dishes or other culture media. In truth, cases of encephalitis also usually involve inflammation of the meninges, so a stickler for linguistic accuracy could rightly maintain that they should be called «meningo-encephalitis» to reflect the involvements of both meninges and brain. But in common usage, the «meningo-» prefix is often dropped. So because CSF changes occur in cases of both meningitis and encephalitis, the main clinical feature that separates the two is the patient’s mental state, with confusion or decreased level of consciousness making a strong case for encephalitis. Once the CSF has been collected, the doctor can begin treatment without risk of obscuring the fluid’s diagnostic features. So long as there is any likelihood of bacterial infection, the doctor administers one or more antibiotic drugs, usually via an intravenous catheter. If the clinical findings could also be interpreted as due to a treatable virus, the doctor concurrently administers an antiviral drug. With the seriousness of these illnesses, the benefits of over-treatment exceed its risks, and once the dust settles and the diagnosis is clarified, needless treatment can be discontinued without harm. (C) 2005 by Gary Cordingley

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