The pain in the back of the left side of the head is caused by the following reasons: 1. Consider the occipital neuralgia, because the back of the head belongs to the region of the occipital nerve distribution, occipital neuralgia manifests as episodic, lightning-like, knife-like pain, which is usually aggravated when the patient walks or sneezes. 2. Consider the pain in the back of the left side of the head is also caused by migraine. Migraine headache is a throbbing pain, which is caused by vasoconstriction dysfunction. 3. If the pain on the left side of the back of the head is sore and swollen, it should be considered to be caused by cervical spine. For example, improper posture of the patient may lead to continuous tension and contraction of the posterior occipital and cervical muscles, which may easily strain the left back of the head and subscalp fascia, resulting in soreness and swelling of the left back of the head.