The thoracic inlet, also known as the superior thoracic aperture, connects the neck to the chest cavity, or thorax. This narrow anatomical passage represents the top opening of the body’s protective cage, allowing for the transfer of necessary structures between the head, neck, and the trunk. Defined by a rigid bony ring, its function is to permit the passage of neurovascular bundles and visceral tubes. The crowding of essential structures in this area makes it a region of significant clinical interest.
Anatomical Boundaries of the Thoracic Inlet
The location of the thoracic inlet is defined by a bony ring that forms a restricted space at the top of the chest. This ring is oriented obliquely, sloping downward from the back toward the front of the body. The posterior limit of this aperture is formed by the body of the first thoracic vertebra (T1).
Moving laterally, the boundary is established by the inner margins of the first pair of ribs and their costal cartilages. These ribs create C-shaped curves that complete the lateral aspect of the ring. The anterior boundary is created by the superior border of the manubrium, the uppermost segment of the sternum.
Because the posterior boundary (T1) is positioned higher than the anterior boundary (manubrium), the plane of the inlet is tilted, angling forward and downward. This oblique orientation means that the apex of the lungs, along with the protective pleura, naturally extends slightly upward into the root of the neck, above the level of the clavicle. The overall shape of the inlet is often described as kidney-shaped due to the slight inward projection of the T1 vertebral body.
Critical Structures Traversing the Inlet
A large collection of vital tubes, vessels, and nerves must pass through the confined space of the thoracic inlet to connect the head and upper limbs with the torso. These contents can be broadly grouped into visceral structures and the neurovascular bundles. The primary visceral components traversing the inlet are the trachea (windpipe) and the esophagus (food passage).
The neurovascular structures include the major arteries and veins that supply the neck and upper extremities. The subclavian arteries and veins, the main vessels for the arms, cross through this area. Specifically, the subclavian artery passes between the anterior and middle scalene muscles, while the subclavian vein travels anterior to the anterior scalene muscle.
Key nerves also make their way through the inlet, including the phrenic and vagus nerves, which regulate the diaphragm and internal organs. Components of the brachial plexus, a complex network of nerves that controls movement and sensation in the shoulder, arm, and hand, pass through the inlet. The collective passage of these densely packed structures makes the thoracic inlet an area where any reduction in space can have immediate and significant consequences.
Understanding Thoracic Outlet Syndrome
Thoracic Outlet Syndrome (TOS) is the primary clinical condition related to the restricted anatomy of this upper chest region. This disorder occurs when the nerves or blood vessels passing through the thoracic outlet—a space closely associated with the inlet—become compressed. Compression typically happens in the space between the clavicle, the first rib, and the scalene muscles.
TOS is classified into three main types based on the structure being compressed. Neurogenic TOS is the most common, accounting for about 95% of cases, and involves compression of the brachial plexus. Symptoms include pain in the neck and shoulder, numbness, and tingling in the arm and hand. The other, less common forms are vascular TOS, which involves compression of the subclavian artery or vein.
Venous TOS can cause swelling and bluish discoloration in the arm. Arterial TOS, the rarest form, may result in coldness, pallor, or a diminished pulse in the affected limb. Common causes of the syndrome include traumatic injuries, such as whiplash, or repetitive stress from certain sports or occupations. Anatomical variations, such as a person being born with an extra rib, known as a cervical rib, can also narrow the passageway and predispose individuals to TOS.