Background and aims Fatty infiltration is known as a physiologic hallmark of thymic involution, starting at puberty (age-related involution). However, the knowledge of fatty infiltration in thymuses whose paediatric patients suffering from acute illness (stress-related involution) has never been studied. The purpose of this work was to evaluate the frequency and degree of fatty infiltration and to correlate the degree of fatty infiltration with the severity of involution in thymic tissues belonging paediatric patients who died from various causes of illness.

Methods Thymic tissues from paediatric autopsy series were collected and processed for histopathologic examination. The degree of fatty infiltration was divided semi-quantitatively as absence, minimal (<5%), occasional (5–50%) and diffuse (>50%). The severity of thymic involution was graded as 1 (resting state), 2 (more thymic lobule separation) and 3 (advanced stromal fibrosis).

Results Fatty infiltration (Figure 1) was found 36/130 cases (28%) and tended to accumulate in grade 3 thymic involution (p = 0.01). In most cases, the degree was minimal (11 cases; 9%) or occasional (18 cases; 14%) and no statistical correlation with any clinical information. There were 7 cases (5%) showing diffuse fatty infiltration and all of them died from infection (bronchopneumonia = 3, meningitis = 2, acute myocarditis with abscesses = 1, acute pyelonephritis = 1), regardless of specific organism, patient age or duration of illness.

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Abstract PS-372 Figure 1

Thymic fatty infiltration.

Conclusion In stress-related thymic involution, the frequency of diffuse fatty infiltration was rare, but such change was an independent finding associated with paediatric infection.