1.Anatomy of the parapharyngeal space The parapharyngeal space is an inverted cone-shaped space of loose connective tissue from the base of the skull down to the plane of the hyoid bone, enclosed by thick deep cervical fascia, located on both sides of the pharynx. The stem and its fascial extension to the palatofantoid muscle divide the gap into anterior and posterior parts, which are also called the parapharyngeal anterior gap and the parapharyngeal posterior gap. The anterior interstitium is small and contains a small portion of the deep lobe of the parotid gland and fat and is crossed by a small branch from the Vth cerebral nerve to the palatine palpebralis muscle, which can lead to salivary gland-derived tumors and lipomas, but rarely to neurogenic tumors. The posterior interstitial space is larger and contains internal jugular arteries, veins, palatal ascending artery, pharyngeal ascending artery, linguopharyngeal nerve, vagus nerve, parasympathetic nerve, hypoglossal nerve, cervical sympathetic nerve and lymphatic tissues. 2.Parapharyngeal space tumor Because of its complex anatomical structure and deep location, the tumor in this space is hidden and not easy to be found in the early stage, and it is often found by the patient or during physical examination only when the tumor increases to a certain size. Tumors in the parapharyngeal space are not common, accounting for about 0.5% of head and neck tumors, of which 80% are benign and 20% are malignant. The pathological types of parapharyngeal space tumors vary greatly, with neurogenic tumors predominating, followed by salivary gland-derived tumors. Salivary gland-derived tumors occurring in the anterior parapharyngeal space rarely have symptoms of neurological involvement and can mostly be removed completely. Neurogenic tumors mostly occur in the posterior parapharyngeal space. If the tumor is a nerve sheath tumor, it can be resected completely and the function of the nerve of origin is usually not affected after surgery. Rarely, chemoreceptor tumors can occur in the vagus body or carotid body. Vagus body tumors are often associated with paralysis of the recurrent laryngeal nerve (hoarseness), and those that develop intracranially and grow around the internal carotid artery are very difficult to resect. The pathological types of parapharyngeal space malignant tumors are more diverse. Because of their infiltrative growth and unclear boundary with surrounding tissues, CT and M or MRI examination should be performed before surgery in order to select the surgical pathway before surgery. Treatment of parapharyngeal space tumor The treatment of parapharyngeal space tumor is based on surgery. Due to the deep location and complex anatomical relationship of the parapharyngeal space, it is difficult to accurately estimate the true location and scope of the tumor by general physical examination. Therefore, imaging examination should be performed before surgery. CT and M or MRI can clearly show the location, shape and scope of the lesion, which can evaluate the surgery in detail. Since 80% of parapharyngeal space tumors are benign, the results are good if complete debridement can be done during surgery. However, this area is covered by important blood vessels and nerves leading into the skull, and the lateral ascending branches of the mandible and parotid gland, which are not easily exposed. Therefore, it is important to choose the correct surgical route. Currently, there are four common surgical routes, each with its own advantages and disadvantages and indications. (1) Transoral route: In recent years, due to the application of plasma knife, there are more and more reports on the use of this route for resection of parapharyngeal space tumors. The advantages of the plasma knife are less bleeding, clear field, and both cutting and suction effects. Therefore, this pathway does not require a cervical-facial incision and has the advantage of being cosmetic and minimally invasive. The plasma knife has been used in our department to remove parapharyngeal space tumors via the oropharyngeal pathway, and the procedure is safe and reliable. However, this approach has limited field, difficult exposure, blind separation of the tumor, risk of injury to important blood vessels and nerves, and is not easy to stop bleeding when there is more intraoperative bleeding. Therefore, this approach is only suitable for cases with small tumors that protrude significantly from the pharyngeal cavity, and the surgeon should have experience in open surgery. (2) Trans-cervicomaxillary approach: Most scholars now advocate this approach to remove tumors. It has the following advantages: large field, clear exposure, easy to isolate the internal jugular artery and protect it from accidental injury; it is aseptic surgery, reducing the chance of trauma infection. The cervicomandibular approach with median fracture of the mandible and external rotation has also been reported in foreign literature, which can clearly expose the important vascular and neural structures on the lateral side of the neck and is suitable for malignant tumors, huge tumors and tumors that are fragile. It also helps to expose and resect most of the malignant tumors in the retrosternal space and the hemangioma surrounding the internal carotid artery at the carotid artery foramen. The cervicomandibular approach has been proven to be the preferred, safe and effective approach for parapharyngeal space tumor surgery. Regardless of the size of the tumor, this route can be used for resection. (3) Transcervical parotid pathway: This pathway can be used for deep lobe parotid tumors that invade the parapharyngeal space. This approach can identify the facial nerve and the vascular and nerve structures in the posterior stromal space, and provide excellent exposure and resection of the tumor and the deep lobe of the parotid gland as a whole. The superficial lobe of the parotid gland is usually removed first, followed by the deep lobe of the parotid tumor in the parapharyngeal space. If the tumor is large, the submandibular gland should be pulled back or removed to enlarge the operative field. Except for deep lobe parotid tumors invading the parapharyngeal space, the transcervical parotid pathway should be avoided to reduce the risk of facial deformity and facial palsy. (4) Trans-lateral skull base approach: For parapharyngeal gap tumors invading the lateral skull base, a trans-lateral skull base approach should be used for resection. A curved incision behind the ear on the cervical side can fully expose all parts of the tumor in the lateral skull base, temporal bone and parapharyngeal space. Jugular vein bullae tumor and one case of giant nerve sheath tumor. Intraoperative attention should be paid to the anatomical landmarks of the skull base. The stem process should be identified first, because all the above important vascular nerves are located in its deep side, and also the stem process is one of the signs of facial nerve exit from the skull, and it is safer to operate in its lower lateral side. It is safer to operate on the lower lateral side, which can not only remove the tumor more completely, but also reduce the complications. Because of the mucosal edema and nerve injury after surgery, it is easy to cause respiratory difficulty, so tracheotomy is needed if necessary, but it should depend on the condition. For small tumors, tracheotomy can be avoided in cases with little surgical trauma; however, for larger tumors, it is better to perform tracheotomy to ensure the postoperative airway patency. Tracheotomy is required for all patients who undergo median mandibular dissection. 4. Complications of parapharyngeal space tumor surgery are mainly nerve injury and bleeding in the operative cavity. The former includes hoarseness, choking, tongue extension deviation, facial palsy, Horner syndrome, etc., which are more common especially in malignant tumors.