Mastoid infection in a 10 year old boy eroding through the outer coex of bone results in:

Acute Mastoiditis is defined as the inflammation of mucosal lining of mastoid air cell system, usually as a result of acute infection. Common causative organisms include beta hemolytic streptococci, pneumococcus and staphylococcus aureus. As the mucoperiosteal lining of the air cells is involved it leads to swollen and hyperemic mucosa (hyperemic decalcification). The accumulation of pus under tension fuher leads to resorption of septa (coalescence of mastoid air cells) and as the pus continues to increase in amount it escapes the confines of mastoid bone and presents as subperiosteal abscess. Periosteitis gives an ironed out appearance on palpation in post aural area. If the patient is not treated at this stage, periosteum give way and pus escapes in subcutaneous plane which can fuher escape on skin presenting as post aural sinus.