An occipital nerve block is an injection of numbing medicine next to the occipital nerves. There are occipital nerves on either side of the head, located just underneath the scalp on the back of the head. These are the greater and lesser occipital nerves. The greater occipital nerve provides sensation to the back and top part of the head. They can be a source of headache pain and what is called occipital neuralgia.
Therefore, an occipital nerve block is designed to stop the pain sensation traveling through the occipital nerves. There are two occipital nerves, the greater and lesser. The injection is most often used to diagnose and treat cervicogenic headaches along with occipital neuralgia, and has also been shown to provide pain relief for migraine sufferers. So it has two purposes, including both a diagnostic one and a therapeutic one.