Timothy C. Hain, MD, Chicago IL. • Page last modified: November 30, 2019
Allodynia means that there is enhanced perception of somatosensory stimuli that are generally not painful, and greater pain from stimuli that are normally painful. For example, shaving, showering, wearing earrings and glasses, and brushing the hair can cause pain (Harriott and Schwedt, 2014).
This can be measured with thermal pain thresholds and mechanical pain thresholds. While we use the word "threshold", this is not the same thing as being able to detect heat/cold or pressure. The sensors are not better in migraine, but the scaling between their output and pain is increased. Castien et al (2018) reviewed 17 studies and reported that pain thresholds were reduced in migraine and chronic tension headache patients compared to controls.
According to Harriott (2014), "Cephalic allodynia likely results from sensitization of trigeminal nucleus caudalis neurons that receive convergent signals from the dura and cutaneous regions of the face, whereas extracephalic allodynia probably results from more widespread sensitization of thalamic and cortical neurons". This doesn't really say much -- obviously the parts of the brain that process sensory input from the head must be more sensitivit (i.e. nucleus caudalis), or a more central circuit such as the thalamus or cortex must be responsible.
Some suggest that fibromyalgia (where everything hurts more), is analogous to migraine (where just the head hurts more). (Nicolodi and Sicuteri, 1996).