A Rich focus is a tuberculous granuloma occurring within the cortex or meninges of the brain that ruptures into the subarachnoid space, causing tuberculous meningitis.^[1] The Rich focus is named for Arnold Rice Rich, a pathologist at Johns Hopkins Hospital, who along with his colleague Howard McCordock first described the post-mortem finding of caseous foci within the cerebral cortex or meninges which appeared to predate the development of meningitis.^[2] Prior to their research the prevailing view had been that meningitis occurred as a result of the dissemination of tuberculous bacilli associated with miliary tuberculosis and that these processes occurred at the same time.

Developments

More recently a more comprehensive classification of the pathogenesis of tuberculous meningitis has been proposed by Donald et al.:^[1]

1. A Rich focus develops as a result of haematogenous dissemination from the primary complex, including miliary tuberculousis, subsequently rupturing into the sub-arachnoid space.
2. A Rich focus develops within the choroid plexus or ventricular walls as a result of haematogenous dissemination.
3. The mechanism described by Rich and McCordock in which haematogenous dissemination at the time of infection, or later, results in development of a Rich focus which is initially controlled but later ruptures into the subarachnoid space.
4. Direct extension from an adjacent structure such as the vertebrae results in development of meningitis.