There is provided a context detection apparatus for use with a medical monitoring device arranged to measure a physiological characteristic of a subject. The context detection apparatus comprises at least one context sensor comprising a camera; and a processing unit. The processing unit is arranged to receive image data acquired by the camera from the at least one context sensor; analyze the received image data to generate a plurality of different types of context information; wherein each type of context information comprises information which relates to a factor capable of influencing measured values of the physiological characteristic and which is not measurable by the medical monitoring device; and output a signal to the medical monitoring device based on the generated context information. The processing unit is arranged to analyze the received image data by detecting features in the received image data; and classifying each detected feature as relating to a particular one of the plurality of types of context information.