Meningism is the syndrome of the triad of symptoms of headache, neck stiffness, and photophobia caused by irritation of the meninges. While it is often associated with a diagnosis of meningitis, it is also present in other conditions causing meningeal irritation such as subarachnoid haemorrhage, trigeminal neuralgia, migraine, or febrile illness in children. Meningitis is process of inflammation of the meninges, which may or may not be due to an infectious agent. Strictly speaking, it is a pathological diagnosis, but in lieu of the impracticability of biopsying the meninges, surrogate markers are used to infer inflammation. These include raised cerebrospinal fluid (CSF) white cell count and protein; and meningeal enhancement using contrast enhanced MRI or CT of the brain. Encephalitis is process of inflammation of the brain parenchyma. Strictly speaking, it is again a pathological diagnosis, and again surrogate markers are used to infer this inflammation, although it is slightly more difficult due to the protected nature of the brain. CSF white cell count and protein are expected to be elevated and parenchymal inflammation may be seen on contrast enhanced MRI. Meningoencephalitis is a combination of the above with inflammation of both the meninges and the adjoining brain parenchyma. Aseptic meningitis is said to be present when there is meningism and signs of meningeal inflammation on CSF and imaging, but no bacterial cause is found on culture or molecular diagnostics. Viral meningitis is the commonest cause, although post-neurosurgical aseptic meningitis is often chemical in nature. Meningism plus fever are the classic symptoms of meningitis. The onset may be acute, subacute, or chronic, depending on the cause. Neck stiffness may range from mild discomfort to an almost rigid neck and is not a sensitive test in young children or elderly. While not used routinely and with low sensitivity particularly in young children and elderly, Kernig’s and Brudkzinski’s signs, both of which stretch the meninges worsening the irritation and increasing pain, have a good positive predictive value. Non-specific signs of intracranial pathology may be present, such as signs of raised intracranial pressure (ICP), vomiting, reduced level of consciousness, focal neurological signs, seizures, or irritability, especially in the immunocompromised, elderly, and young children who may not have classic signs and symptoms.
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