Introduction

The scalp is a descriptive anatomical term that refers to the subcutaneous tissue that covers the cranium and comprises 5 layers: skin, dense connective tissue, epicranial aponeurosis, loose areolar connective tissue, and the periosteum. The first 3 layers move as 1 unit as they are tightly bound together. The veins of the scalp are present within the dense connective tissue, which also contains nerves and arteries, and the loose areolar connective tissue, which also contains emissary veins that connect to the diploic veins of the skull and the intracranial venous sinuses (see Image. Anterior Divisions, The Nerves fo the Scalp, Face, and Side of the Neck).

The venous drainage of the scalp divides superficially and deeply. The superficial veins of the scalp, starting anteriorly and moving posteriorly, are the supratrochlear and supraorbital veins, respectively, the superficial temporal veins and their branches, the posterior auricular vein, and the occipital vein and its branches (see Image. Veins in the Skull). The deep venous drainage is via the pterygoid venous plexus.

Structure and Function

Venous anatomy starts with a thick outer layer of connective tissue called the tunica externa or adventitia, a middle layer of smooth muscle called the tunica media, and an interior layer of endothelial cells called the tunica intima.[1] As veins do not function like arteries in a primarily contractile manner and are not subject to the same high pressures to which arteries are exposed, veins' smooth muscle layer is less muscular than arteries. Veins also contain valves to prevent backflow.[2] The scalp's veins drain the deoxygenated blood from the scalp muscles and back to the right heart via the internal and external jugular veins and the superior vena cava. As previously mentioned, the valveless emissary veins connect the superficial veins to intracranial venous sinuses and diploic veins of the skull. Here, the blood supply and lymphatics of the scalp veins, starting with the superficial components anteriorly and moving posteriorly, are described (see Image. Lymphatic Supply for the Scalp).

Supratrochlear and Supraorbital Veins

The supratrochlear vein originates on the forehead, draining the superficial muscles and skin of the forehead and the front of the scalp supplied by the supratrochlear artery into the angular vein; there is no associated lymph tissue. The supraorbital vein originates on the forehead, and its anastomosis with the frontal branch of the superficial temporal vein serves to drain the area of the forehead, eyebrow, and upper eyelid supplied by the supraorbital artery that anastomoses with the frontal branch of the superficial temporal vein.[3] The vein travels caudally over the frontal bone and superficial to the frontalis muscle before splitting to join the supratrochlear vein inferiorly and the superior ophthalmic vein medially; it also has no associated lymph tissue.

Superficial Temporal Vein

The superficial temporal vein has frontal and parietal branches, which drain the superficial muscles and skin of the temporal region. It originates from a venous plexus on the side of the head, travels laterally from the temporal region over the zygomatic arch, and enters the parotid gland to become the retromandibular vein, where it joins the transverse facial vein.[4][5] The retromandibular vein drains into the internal jugular via its anterior branch and the external jugular via its posterior branch. The frontal branch also connects to a parietal emissary vein that runs from the side of the head to the top of the head, where it pierces the cranium and communicates with the superior sagittal sinus.[6] The parietal branch anastomoses with the occipital vein. The parotid and auricular lymph vessels are located both anteriorly and posteriorly of the superficial temporal vein and artery, and the superficial parotid and preauricular lymph nodes located posteriorly drain the area supplied by the branches of the superficial temporal artery.

Posterior Auricular and Occipital Veins

The posterior auricular vein assists with draining the muscles and skin of the scalp (see Image. Veins of the Scalp). It originates from small vessels behind the ear superficial to the temporal fascia and anastomoses with the occipital vein superiorly.[7] It runs just behind the ear, confluences with the occipital and deep cervical veins, and then receives blood from the posterior branch of the retromandibular vein before turning into the external jugular vein. The posterior auricular vein runs alongside the posterior auricular artery, originating from the external carotid artery. It is surrounded anteriorly by the mastoid lymph nodes that drain into the parotid and auricular lymph vessels and posteriorly by sternocleidomastoid lymph nodes that drain into the occipital lymph vessels. These lymph vessels then drain into the jugular lymph vessels.

The occipital vein originates from small vessels on the scalp's posterior aspect and drains the occipital region's superficial muscles and skin. It is superficial to the occipital fascia and the occipital artery and anastomoses laterally with the parietal branch of the superficial temporal vein and inferolateral to the posterior auricular vein. It then runs into the inferior portion of the posterior auricular vein, where it becomes the external jugular vein. The occipital vein also has an anastomosis with the intracranial confluence of the sinuses via the occipital emissary vein.[8] The occipital lymph nodes and vessels drain lymph from this area and into the jugulodigastric lymph node, draining into the jugular lymph vessels.

Deep Venous Drainage

The venous drainage of the deep scalp layers is via the pterygoid venous plexus, an extensive plexus of veins located between the lateral pterygoid and temporalis muscles, draining into the maxillary vein. It receives branches corresponding with the branches of the maxillary artery.[9] These veins include:

• Alveolar
• Buccinator
• Deep temporal (anterior and posterior)
• Infraorbital
• Masseteric
• Middle meningeal
• Pterygoid
• Sphenopalatine

The pterygoid venous plexus also receives blood from some branches of the palatine veins and contains a communicating vein that travels through the inferior orbital fissure to connect the ophthalmic vein to the cavernous sinus.

Embryology

The scalp veins are derived from the mesoderm of the first and second branchial arches, embryologic structures that form between the fourth and seventh week of gestation. The branchial arches go on to form the blood vessels, bone, cartilage, and muscles of the face, among other structures.[10] The muscles these veins drain also derive from the first and second branchial arches. Second branchial arch anomalies account for most (95%) of branchial arch anomalies, with first branchial arch anomalies accounting for only 1 to 4% of cases.[11]

Nerves

Several nerves are relevant and proximal to the scalp veins, with varying relationships anatomically to these structures.

• The supratrochlear nerve is located medially in the supratrochlear vein, whereas the medial branch of the supraorbital nerve is just deep in the supratrochlear vein.
• The lateral branch of the supraorbital nerve is just medial and deep to the supraorbital vein.
• The temporal branches of the facial nerve are located between the zygomatic orbital vein (a non-scalp vein) inferiorly and the frontal branch of the superficial temporal vein superiorly and posteriorly.
• The auriculotemporal nerve runs anteriorly and deep to the superficial temporal artery and vein.
• The posterior auricular artery and vein are sandwiched between the posterior auricular nerve anteriorly and the great auricular nerve just deep and posteroinferiorly.
• The occipital artery and vein are the lesser and greater occipital nerve branches, superficially anteriorly and posteriorly.[12]

Muscles

The scalp veins primarily drain the deoxygenated blood from the epicranial aponeurosis and the frontal and occipital belly of the occipitofrontalis muscle, the temporalis muscle, the auricular superior muscle, the temporoparietalis muscle, the auricular anterior muscle, and the auricular posterior muscle.

Physiologic Variants

The parietal emissary vein most commonly communicates with the superficial temporal vein, specifically the frontal branch, but may communicate with other scalp veins. The occipital emissary vein may not be present in all individuals. The multiple anastomoses of the scalp may also have variability in which branches they interconnect with.[13][14]

Clinical Significance

Deep lacerations to the scalp are inclined to bleed profusely for several reasons.[15] These reasons are as follows:

• Vasoconstriction is prevented due to the close-fitting adherence of the dense connective tissue to the blood vessels.
• The pulling effect of the occipitofrontalis muscle precludes the closing of the actively bleeding vessel and the surrounding skin.
• Numerous anastomoses of the scalp blood vessels exist, such as the anastomosis of the occipital and posterior auricular veins.

Scalp arteriovenous malformations (AVM), or cirsoid aneurysms, are uncommon lesions whose etiologies are most commonly congenital, post-infectious, or traumatic.[16] These arteriovenous malformations often comprise arterial blood from the superficial temporal or occipital arteries and venous outflow into extracranial venous structures. Treatment of arteriovenous malformations typically consists of endovascular embolization, followed by surgical resection; therefore, variant arterial supply and venous drainage should be considered in these cases.[17]

Subgaleal hematoma is an abnormal collection of blood beneath the galea aponeurosis of the scalp, commonly caused by traumatic shearing of emissary veins in the loose areolar tissue. This condition is more commonly seen in neonates, infants, and small children, although there have been case reports of subgaleal hematoma occurring in adults.[5] Mild trauma typically occurs via contusion, hair pulling, or vacuum-assisted vaginal delivery. Treatment is typically conservative, principally with bandage compression.[18] The ‘danger area of the scalp’ refers to the layer of loose areolar connective tissue as it contains the valveless emissary veins that connect the superficial veins in the subaponeurotic space with the intracranial venous sinuses. This connection allows an infection to spread from the scalp to the meninges. See Image. Relationship of the Meninges to the Skill and Brain.

Cavernous sinus thrombosis is the development of a blood clot within the cavernous sinus. The cavernous sinus is 1 of the dural venous sinuses, a cavity at the base of the brain that helps drain deoxygenated blood from the brain via the superior and inferior ophthalmic veins, the superficial middle cerebral veins, the sphenoparietal sinus, and the inferior cerebral veins. The cavernous sinus connects to the pterygoid venous plexus via the inferior ophthalmic, deep facial, and emissary veins. Due to this connection, superficial facial infections from the nose, sinuses, ears, or teeth may spread to the cavernous sinus to cause this disorder.[19] Symptoms may include proptosis, unilateral periorbital edema, decrease or loss of vision, headaches, and possible cranial nerve paralysis, particularly of the delicate abducens nerve that runs through the cavernous sinus. There are 2 types of cavernous sinus thromboses: septic and aseptic. Treatment of septic disease includes initial broad-spectrum IV antibiotics covering staph and strep, the most commonly associated bacteria, then a prolonged course (3 to 4) weeks of targeted IV antibiotics. Surgical drainage may be necessary in cases of sphenoidal sinus infection.[20]

Scalp veins may be used for venous catheterization as they offer easy access with little risk.[21] Most commonly, scalp catheterization is utilized in infants and neonates after unsuccessful attempts at cannulation of extremity veins. The scalp veins in this population have less overlying subcutaneous fat, are more prominent compared to other peripheral sites, and are less obscured by hair. These factors allow for easier visualization and, therefore, cannulation. The most commonly used veins are the superficial temporal, supratrochlear, and occipital.