The petromastoid canal in the young child: appearance on computed tomography.

Objective: The adult petromastoid canal (PMC) is a thin anatomical structure, but in young children, it can be wide on high resolution CT (HRCT) scans. We performed this study to evaluate the year-by-year change in width and shape of the PMC in the young child.

Materials And Methods: We retrospectively reviewed temporal bone HRCT scans, performed between 2007 and 2012. Eighty children were included (age range: 0.3-6.9 years; median age: 3.1 years; 56% male). All scans had a slice thickness of ≤ 1 mm.

Results: The average width of the PMC was 1.95 mm in children < 2 years, compared to 0.83 mm in children ≥ 2 (p < 0.001). We categorized the PMC into 4 subtypes. A bulky (type III) PMC was predominant < 2 years. A thin curvilinear (type I) PMC was more prevalent in the older children. Type II and IV PMC were found infrequently. The PMC width correlated inversely with the degree of pericapsular ossification (p<0.001), and mastoid pneumatisation (p < 0.001).

Conclusion: A wide PMC (≥ 1 mm) is a common finding in children < 2 years. As children grow older, the PMC progressively narrows. This correlates to increased temporal bone pneumatisation and ossification of the otic capsule.