Introduction

The sphenopalatine artery (SPA) is a well-known vessel to otolaryngologists, deemed the artery of epistaxis. Epistaxis is among the most common ear, nose, and throat related emergency, and roughly 60% of the population will experience epistaxis sometime during their life.[1] Most epistaxis cases are anterior bleeds and occur at the Keisselbach's plexus.  In the rare case of posterior epistaxis, the SPA or branch of the SPA is likely responsible. It presents a challenge when treating as the vessel is hard to visualize and presents with profound bleeding.[2] The sphenopalatine artery predominantly branches into two major vessels, the septal artery, and posterior-lateral nasal artery, however numerous additional branches may be present along with a highly variable course within the nasal cavity. Knowledge of the anatomical variations of the sphenopalatine artery and the surrounding structures is crucial to controlling posterior epistaxis that is unresponsive to conservative measures.[3]

Structure and Function

The sphenopalatine artery is a terminal branch of the internal maxillary artery originating from the external carotid artery system.[4] The SPA is the major blood vessel to the nasal cavity mucosa: supplying the superior, middle, and inferior turbinate; lateral nasal wall; and nasal septum. The sphenopalatine artery travels within the pterygopalatine fossa and enters the nasal cavity through the sphenopalatine foramen within the posterior portion of the superior meatus.[5] The superior boundary of the sphenopalatine foramen is the body of the sphenoid, anteriorly by the orbital process of the palatine bone, posteriorly by the sphenoidal process of the palatine bone, and inferiorly by the perpendicular plate of the palatine bone. 

The SPA branches into two major vessels, the septal artery and posterior lateral nasal artery, before exiting the sphenopalatine foramen.[6]  The septal artery exits the sphenopalatine foramen, courses through the anteroinferior wall of the sphenoid sinus, and distributes on the nasal septum.  The posterior lateral nasal artery exits the sphenopalatine foramen, courses downward anteriorly to the posterior end of the middle turbinate along the lateral nasal wall, and runs inferiorly on the perpendicular plate of the palatine bone giving off branches to the inferior and middle turbinate.[7]

The sphenopalatine artery and its branches form an anastomotic region within the nasal cavity. Kiesselbach's plexus (Little's area) is the arterial anastomosis found in the anterior nasal septum formed by terminal branches of the internal carotid artery (ICA) and the external carotid artery (ECA). The Kiesselbach's plexus is formed by the anterior and posterior ethmoidal artery (branches of the ICA) and ECA branches such as the superior labial artery, greater palatine artery, and the posterior septal branch of the SPA.[8]

Embryology

During developmental weeks four and five, the aortic sac gives rise to the aortic (pharyngeal) arches. The maxillary artery develops from the mesoderm of the first aortic arch and subsequently gives rise to the sphenopalatine artery.[5]. The number of developing branches and the diameter of the sphenopalatine artery determine the number and shape of the sphenopalatine foramina in the palatine bone.[6] 

Nerves

The maxillary nerve (CN V2) is the second branch of the trigeminal nerve and runs close to the maxillary artery. It courses from the middle cranial fossa into the pterygopalatine fossa via the foramen rotundum.[9] While in the pterygopalatine fossa, the maxillary nerve gives off multiple branches, including the infraorbital, nasopalatine, zygomatic, superior alveolar, pharyngeal, and the greater and lesser palatine nerves. The maxillary nerve also communicates with the pterygopalatine ganglion, which is suspended by two small trunks known as the pterygopalatine nerves.[10]

The pterygopalatine ganglion is located deep within the pterygopalatine fossa near the sphenopalatine foramen.[11] It is the largest parasympathetic ganglion of the maxillary nerve branches and primarily receives nerve supply from the greater petrosal branch of the facial nerve via the Vidian nerve. The Vidian nerve forms from the junction of the greater petrosal nerve containing preganglionic parasympathetic fibers and the deep petrosal nerve from the carotid plexus containing postganglionic sympathetic fibers.  Only the preganglionic parasympathetic fibers synapse with the pterygopalatine ganglion. The postganglionic parasympathetic fibers then distribute via their respective nerves to provide a secretomotor function to the lacrimal gland, mucosal glands of the oral cavity, nose, and the pharynx.[12]  The nasopalatine branch travels alongside the sphenopalatine artery and also enters the nasal cavity through the sphenopalatine foramen. The nasopalatine nerve travels across the roof of the nasal cavity below the orifice of the sphenoid sinus to reach the septum. It then runs in an oblique direction between the periosteum and mucous membrane of the lower part of the septum to connect with the greater palatine nerve.[13]

Physiologic Variants

The sphenopalatine artery anatomy has been studied extensively by numerous researchers. Ninety-seven percent of these cadaveric studies have demonstrated at least two arterial branches at the plane of the sphenopalatine foramen.[7] These branches are the posterior lateral and septal branches, as discussed earlier. Although two to four branches off of the SPA is the most common arrangement, some studies have demonstrated as many as ten branches.[14] Once the sphenopalatine artery exits the sphenopalatine foramen, the more distal and posterolateral contributions have been shown to have greater variability.[15] Additionally, it has been observed that some portions of the artery may run anteriorly to the posterior maxillary wall.

The individual arrangement of the arterial branches is highly variable with distributions superior or inferior to the ethmoidal spine, crisscrossing upon exiting the foramen, and demonstrating asymmetry between nasal cavities within the same cadaver. However, the ethmoidal crest serves as a consistent landmark that can help navigate within the nasal cavity, regardless of any variation seen in the SPA or its branches.[7][14][15]

Surgical Considerations

Sphenopalatine Artery Ligation

Surgical management of epistaxis is typically only for those patients whose bleeding is refractory to more conservative therapies. Knowledge of the vascular anatomy and surrounding landmarks is necessary to avoid surgical failure and lower the risk of postoperative bleeding.[7] The sphenopalatine artery can be exposed endoscopically by raising a posterolateral mucosal flap over the orbital process of the palatine bone.[15] A vertical incision is then made inferior to the posterior portion of the middle turbinate, 1 cm anterior to its posterior tip. Raising the mucoperiosteal flap posteriorly and superiorly will expose the ethmoid crest.[7] The ethmoid crest represents a significant landmark for locating the position of the sphenopalatine artery and is consistently anteromedially to the sphenopalatine foramen.[6] Resection of the ethmoid crest enhances the exposure of the sphenopalatine artery and helps identify its terminal branches. As the flap is raised and the ethmoid crest resected, the fibro neurovascular bundle including the SPA and nasopalatine nerve, will be reachable at the sphenopalatine foramen. After isolating the artery and its branches, the next step is either cauterizing them with bipolar forceps, occluding with clips, or using a combination of both. If bleeding is not controlled distally, the sphenopalatine artery may be traced proximally through the sphenopalatine foramen into the pterygopalatine fossa to ligate the vessel before its terminal bifurcation. Upon attaining hemostasis, the mucoperiosteal flap is returned and covered with an oxidized cellulose polymer hemostatic agent.[2] However, the large variations in SPA branches and possible multiple foramina can produce a varied response SPA ligation.

Nasoseptal Flap

A nasoseptal flap is a surgical technique used in skull base reconstruction following anterior skull base tumor removal, transsphenoidal approaches for pituitary adenoma resections, and repairing sella and clival cerebral spinal fluid (CSF) leaks.[16] The use of a nasoseptal flap provides an effective barrier for the prevention of CSF leaks. The flap is pedicled based on the posterior septal artery, a terminal branch of the sphenopalatine artery. A mid septum mucoperiosteal flap is transected inferiorly along the maxillary crest and dorsally below the olfactory epithelium. The posterior septal flap pedicled on the posterior septal branch gets rotated around the pedicle and into the defect. Because of its specific septal branch blood supply, the nasoseptal flap provides reliable revascularization and robust coverage for skull base reconstruction.[17]

Clinical Significance

Posterior Epistaxis

The sphenopalatine artery carries the most significant volume of blood to the nose and is the source of most posteriorly based nose bleeds.[2]  Epistaxis caused by posterior bleeding accounts for 10% of cases, with 80% of bleeding coming from the sphenopalatine artery. Posterior epistaxis is most often associated with atherosclerotic disease, hypertension, and diabetes, but can occur iatrogenically or from idiopathic causes. Posterior bleeding may remain asymptomatic or may present insidiously as hematemesis, anemia, or melena. Additional clinical presentations of posterior epistaxis include the presence of blood in oropharynx; a history of recurrent epistaxis; failure to control epistaxis by nasal packing; the presence of clots in the area of sphenopalatine foramen; the presence of clots in maxillary sinus ostium; and absence of bleeding areas in the anterior nasal septum. Treatment of posterior epistaxis is challenging due to the inaccessible location of the bleeding vessel.[18]  The sphenopalatine artery is not readily visualized on anterior nasal speculum rhinoscopy and often only visualized with nasal endoscopy. Traditional methods used for control involve anteroposterior packing and the use of balloon catheters. Posterior epistaxis nasal packing involves placing a postnasal pack in the nasopharynx, blocking both the posterior choana and bilateral firm anterior nasal packing deep into the nasal cavity. The pack stops bleeding by exerting pressure on the bleeding vessel. Nasal balloon catheters are easy to insert and cause less pain while providing control at the point of maximum pressure, instead of compressive effect on the entire surface like the nasal packing, decreasing the risk of mucosal ischemia. When bleeding is severe or resistant to packing, direct cauterization, embolization, or ligation of the responsible vessel may be necessary.[2]

Juvenile Nasopharyngeal Angiofibroma

Juvenile nasopharyngeal angiofibroma (JNA) is an uncommon benign tumor that originates in the lateral wall close to the sphenopalatine foramen. This tumor predominantly affects adolescent males and is locally aggressive, often invading the surrounding structures and tissues. JNA commonly presents with painless, progressive unilateral nasal obstruction, epistaxis, and rhinorrhea. JNA has a rich vascular supply primarily from the sphenopalatine artery. Macroscopically the tumor appears as a rounded circumscribed non-encapsulated mucosa covered mass. Microscopically the tumor shows haphazardly arranged collagen with an irregular vascular pattern. Because of the rich vascular supply, the embolization of the sphenopalatine artery and surgical resection of the tumor provides a good prognosis if JNA is diagnosed early enough.[19][20]