<P> In adults, the most common symptom of meningitis is a severe headache, occurring in almost 90% of cases of bacterial meningitis, followed by nuchal rigidity (the inability to flex the neck forward passively due to increased neck muscle tone and stiffness). The classic triad of diagnostic signs consists of nuchal rigidity, sudden high fever, and altered mental status; however, all three features are present in only 44--46% of bacterial meningitis cases . If none of the three signs are present, acute meningitis is extremely unlikely . Other signs commonly associated with meningitis include photophobia (intolerance to bright light) and phonophobia (intolerance to loud noises). Small children often do not exhibit the aforementioned symptoms, and may only be irritable and look unwell . The fontanelle (the soft spot on the top of a baby's head) can bulge in infants aged up to 6 months . Other features that distinguish meningitis from less severe illnesses in young children are leg pain, cold extremities, and an abnormal skin color . </P> <P> Nuchal rigidity occurs in 70% of bacterial meningitis in adults . Other signs include the presence of positive Kernig's sign or Brudziński sign . Kernig's sign is assessed with the person lying supine, with the hip and knee flexed to 90 degrees . In a person with a positive Kernig's sign, pain limits passive extension of the knee . A positive Brudzinski's sign occurs when flexion of the neck causes involuntary flexion of the knee and hip . Although Kernig's sign and Brudzinski's sign are both commonly used to screen for meningitis, the sensitivity of these tests is limited . They do, however, have very good specificity for meningitis: the signs rarely occur in other diseases . Another test, known as the "jolt accentuation maneuver" helps determine whether meningitis is present in those reporting fever and headache . A person is asked to rapidly rotate the head horizontally; if this does not make the headache worse, meningitis is unlikely . </P> <P> Other problems can produce symptoms similar to those above, but from non-meningitic causes . This is called meningism or pseudomeningitis . </P> <P> Meningitis caused by the bacterium Neisseria meningitidis (known as "meningococcal meningitis") can be differentiated from meningitis with other causes by a rapidly spreading petechial rash, which may precede other symptoms . The rash consists of numerous small, irregular purple or red spots ("petechiae") on the trunk, lower extremities, mucous membranes, conjuctiva, and (occasionally) the palms of the hands or soles of the feet . The rash is typically non-blanching; the redness does not disappear when pressed with a finger or a glass tumbler . Although this rash is not necessarily present in meningococcal meningitis, it is relatively specific for the disease; it does, however, occasionally occur in meningitis due to other bacteria . Other clues on the cause of meningitis may be the skin signs of hand, foot and mouth disease and genital herpes, both of which are associated with various forms of viral meningitis . </P>

Which layer of meninges is affected in meningitis