Serotonin regulates the constriction and dilation of blood vessels. Prodrome aura of visual disturbances that can include zigzag lines and flashes of light. The constant stimulation of both decreased blood flow and increased nerve involvement causes optical effects. Bending over or lying down often adds just enough pressure to gravity to cause reflux. Also, lax connective tissue can cause hiatal hernia problems.
Trigger Points can also result in itch.
For example, the coracobrachialis referred pattern on the forearms often becomes itch, rather than pain. The masseter TrP can cause a maddening itch inside the ear. Cold numbs the receptors. Dryness creates an enhanced pressure reception. Check for possible causative TrPs.