Hierarchical Attention for Sparse Volumetric Anomaly Detection in Subclinical Keratoconus

The detection of weak, spatially distributed anomalies in volumetric medical imaging remains a major challenge. The subtle, non-adjacent nature of early disease signals is often lost due to suboptimal architectural inductive biases: 2D/3D CNNs impose strong locality, while ViTs diffuse unconstrained global attention. This conflict leaves the optimal inductive structure for robust, sparse volumetric pattern recognition unresolved. This study presents a controlled comparison of sixteen modern deep learning architectures spanning 2D/3D convolutional, hybrid, and volumetric transformer families for subclinical keratoconus (SKC) detection from 3D anterior segment OCT volumes. We demonstrate that hierarchical attention models offer a superior and more parameter-efficient inductive bias, surpassing the performance of both 2D and 3D CNNs and ViTs. Our results show 21-23% higher sensitivity and specificity in the sparse anomaly (subclinical) regime. Mechanistic analyses reveal that this advantage stems from precise spatial scale alignment: hierarchical windowing produces effective receptive fields matched to the intermediate, multi-slice extent of subclinical abnormalities. This avoids excessive CNN locality and diffuse global attention. Attention-distance measurements confirm a key insight into architectural adaptation: the required spatial integration length shifts significantly based on the signal strength, with subclinical cases necessitating longer integration compared to both healthy and manifest disease states. Representational similarity and auxiliary age/sex prediction tasks further support the generalizability of these inductive principles. The findings provide design guidance for future volumetric anomaly detection systems, establishing hierarchical attention as a principled and effective approach for early pathological change analysis in 3D medical imaging.