Why can untreated middle-ear infections lead to mastoiditis or meningitis?

Untreated middle-ear infections can lead to serious complications due to the anatomical proximity of the middle ear to surrounding structures, separated only by thin plates of bone.12

Mastoiditis

The middle-ear cavity is connected to the air cells within the mastoid process, a projection of the temporal bone located directly behind the ear. Because the mucous membrane lining the middle ear extends into these mastoid air cells, an infection can easily spread from the middle ear into the mastoid bone. If the infection is severe and untreated, it may erode the bone, leading to mastoiditis—an inflammation characterized by pain, swelling, and the potential development of an abscess.123

Meningitis

The brain cavity is situated just above and behind the middle ear and mastoid air spaces, separated by a thin bony plate. If an infection in the middle ear or mastoid bone is neglected, it can erode this bone and extend into the cranium. This proximity allows the infection to reach the protective coverings of the brain, resulting in meningitis or the formation of a brain abscess.1243