  Clinical Natural Language Processing (NLP) has become an emerging technology
in healthcare that leverages a large amount of free-text data in electronic
health records (EHRs) to improve patient care, support clinical decisions, and
facilitate clinical and translational science research. Recently, deep learning
has achieved state-of-the-art performance in many clinical NLP tasks. However,
training deep learning models usually requires large annotated datasets, which
are normally not publicly available and can be time-consuming to build in
clinical domains. Working with smaller annotated datasets is typical in
clinical NLP and therefore, ensuring that deep learning models perform well is
crucial for the models to be used in real-world applications. A widely adopted
approach is fine-tuning existing Pre-trained Language Models (PLMs), but these
attempts fall short when the training dataset contains only a few annotated
samples. Few-Shot Learning (FSL) has recently been investigated to tackle this
problem. Siamese Neural Network (SNN) has been widely utilized as an FSL
approach in computer vision, but has not been studied well in NLP. Furthermore,
the literature on its applications in clinical domains is scarce. In this
paper, we propose two SNN-based FSL approaches for clinical NLP, including
Pre-Trained SNN (PT-SNN) and SNN with Second-Order Embeddings (SOE-SNN). We
evaluated the proposed approaches on two clinical tasks, namely clinical text
classification and clinical named entity recognition. We tested three few-shot
settings including 4-shot, 8-shot, and 16-shot learning. Both clinical NLP
tasks were benchmarked using three PLMs, including BERT,BioBERT, and
BioClinicalBERT. The experimental results verified the effectiveness of the
proposed SNN-based FSL approaches in both NLP tasks.
