The most common meningeal pathogens are normal inhabitants of the nasopharynx in a significant part of the population and depend on antiphagocytic capsular or surface antigens for survival in the tissues of the infected host. To a large extent they express their pathogenicity by extracellular proliferation. It is evident from the frequency with which the carrier state is detected that nasal colonization is not a sufficient explanation of infection of the meninges. Factors that predispose the colonized patient to invasion of the bloodstream, which is the usual route by which these bacteria reach the meninges, are obscure but include antecedent viral infections of the upper respiratory passages or, in the case of S. pneumoniae, infections of the lung. Once blood-borne, pneumococci, H. influenzae, and meningococci possess a predilection for the meninges, although the precise factors that determine this tropism are not known. Whether the organisms enter the CSF via the choroid plexus or meningeal vessels is also unknown. It has been variously postulated that the entry of bacteria into the subarachnoid space is facilitated by disruption of the blood–CSF barrier by trauma, circulating endotoxins, or an initial viral infection of the meninges. These organisms, being commensal in most persons, create immunity, but bacteria may nonetheless penetrate the mucosa. Certain features of the organisms enhance their ability to cause infection; this is particularly true of the meningococcus (Rosenstein et al).
Avenues other than the bloodstream by which bacteria can gain access to the meninges include congenital neuroectodermal defects; craniotomy and spinal operative sites; diseases of the middle ear and paranasal sinuses, particularly perilymphatic fistulas; skull fractures; and, in cases of recurrent infection, dural tears from remote minor or major trauma. Occasionally, a brain abscess may rupture into the subarachnoid space or ventricles, thus infecting the meninges. The isolation of anaerobic streptococci, Bacteroides, Actinomyces, or a mixture of microorganisms from the CSF should suggest the possibility of a brain abscess with associated meningitis.
The early clinical effects of acute bacterial meningitis are fever, headache, usually severe, and stiffness of the neck (resistance to passive movement on forward bending), and less often initially, generalized convulsions and a disorder of consciousness (i.e., confusion, drowsiness, stupor, and coma). Flexion at the hip and knee in response to forward flexion of the neck (Brudzinski sign) and resistance to completely extending the legs with the hips flexed (Kernig sign) have the same significance as stiff neck but are less consistently elicitable. Basically, all of these signs are part of a flexor protective reflex (one of the “nocifensive” responses in Fulton’s terms). Stiffness of the neck that is part of paratonic or extrapyramidal rigidity should not be mistaken for that of meningeal irritation. The former is more or less equal in all directions of movement, in distinction to that of meningitis, which is present only or predominantly on forward flexion. Whether it is stiffness in the initial few degrees of flexion of the neck or in the subsequent part of the movement that is more specific for meningitis has been debated; our experience has been that the latter is more sensitive but also occurs in other disorders; thus the first may be more specific for meningitis.
Diagnosis of meningitis may be difficult when the initial manifestations consist only of fever and headache, when stiffness of the neck has not yet developed, or when there is only pain in the neck or abdomen or a febrile confusional state or delirium. Also, stiffness of the neck may not be apparent in the stuporous or comatose patient or in the infant or the elderly, as indicated further on.
These signs and symptoms comprising the meningitic syndrome are common to the three main types of bacterial meningitis, but certain clinical features and the setting in which each of them occurs correlate more closely with one type than another.