The term ‘nerve block’ refers specifically to an injection of local anesthetic that is aimed at inactivating a nerve.  These blocks typically work by chemically and temporarily disabling a nerve’s capability to conduct an impulse.  The result can be numbness in the case of a sensory nerve (i.e. a nerve that provides sensation to a part of the body), paralysis in the case of a motor nerve (i.e. a nerve that moves a muscle) or both in the case of a mixed motor & sensory nerve. 

The duration of action of a nerve block will depend on which local anesthetic or combination of anesthetics are used, but is typically between 1-8 hours. There are two important things to note about nerve blocks with regard to chronic headaches. First, blocks are good diagnostic tools, but are not good treatment tools.  In other words, the effects of the block will only last so long as the medicine is in effect.  Once the body absorbs the medicine, the effects of the block wear off.  Anyone who has ever had their mouth numbed up at the dentist for a filling knows that the numb feeling goes away after a few hours. 

However, in the case of chronic headaches, sequential nerve blocks can aid in determining which nerve or nerves may be causing your problem by eliminating one nerve at a time and seeing if doing so has an effect on your pain.  The second thing to understand about nerve blocks is that they are distinct from trigger point injections.  The latter are injections performed at specific points where a person may be experiencing pain with the goal of reducing inflammation or irritation of muscle or fascia. Trigger point injections are not intended to inactivate a specific nerve.  So while they may numb an area that is painful, they are less helpful from a diagnostic standpoint.