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Scalp Anesthesia

Overview

The scalp is an envelope of 5 separate soft tissue layers that combine to cover the skull. It stretches from the supraorbital margins on the anterior aspect to the external occipital protuberance of the posterior aspect of the skull. It contains a thick layer of skin (within which is contained hair and sebaceous glands), connective tissue, epicranial aponeurosis, loose areolar tissue, and the pericranium.

The skin, connective tissue, and aponeurosis are bound together in one single unit that slides atop the areolar tissue. The aponeurosis comprises the underside of the skin-connective tissue unit and serves as the insertion site for the muscles of the scalp. The pericranium is a thin fascial layer fixed to the outer portion of the skull. It is continuous with the endosteum of the skull bones, so subperiosteal hematomas typically take the shape of the skull bones underlying the bleeding.

A mnemonic device for remember these layers is as follows:

S – Skin

C – Connective tissue & cutaneous vessels & nerves.

A – Aponeurosis (epicranial aponeurosis)

L – Loose areolar tissue

P – Pericranium (periosteum of skull bones)

An image depicting scalp anatomy can be seen below.

Scalp anatomy.

Scalp anatomy.

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Blood supply to the scalp comes from both the internal and external carotid arteries, and these arterioles anastomose randomly along the connective tissue plane of the scalp. Because these vessels are contained within the connective tissue, compromise of the vessels that would typically lead to vasospasm elsewhere in the body is prevented by the rigid tissue, leading to the classical large volume bleeding associated with scalp lacerations.

Nerve supply of the scalp comes from the fifth cranial nerve (trigeminal), as well as the cervical plexus. The forehead is innervated by branches of the V1 division, specifically the supraorbital and supratrochlear nerves. The vertex and lateral region of the scalp receives its nerve supply from the V2 and V3 divisions (zygomaticotemporal, and temporomandibular and auriculotemporal nerves, respectively). The posterior scalp sensation is supplied by the greater auricular and occipital nerves. All of these nerves become superficial (and accessible to anesthetic access) above an imaginary line drawn from the occipital protuberance to the eyebrows, passing along the upper border of the ear. In the scalp, the nerves travel between the connective tissue and aponeurosis.

See Scalp Anatomy for more information.

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