The axilla is the name given to an area that lies underneath the glenohumeral joint, at the junction of the upper limb and the thorax. It is a passageway by which neurovascular and muscular structures can enter and leave the upper limb.
- Apex: Also known as the axillary inlet, this is formed by lateral border of the first rib, superior border of scapula, and the posterior border of the clavicle.
- Lateral wall: Formed by intertubercular groove of the humerus.
- Medial wall: Consists of the serratus anterior and the thoracic wall (ribs and intercostal muscles).
- Anterior wall: Contains the pectoralis major and the underlying pectoralis minor and the subclavius muscles.
- Posterior wall: Formed by the subscapularis, teres major and latissimus dorsi.
The size and shape of the axilla region varies with arm abduction. It decreases in size most markedly when the arm is fully abducted – at this point, the contents of the axilla are at most risk of injury.
contents of the axilla
The main, and clinically important contents of the axilla region include muscles, nerves, vasculature and lymphatics:
- Axillary artery – It is the main artery supplying the upper limb. It is commonly referred as having three parts, one medial to the pectoralis minor, one posterior to pectoralis minor, and one lateral to pectoralis minor. The medial and posterior parts travel in the axilla.
- Axillary vein – The main vein draining the upper limb, its two largest tributaries are the cephalic and basilic veins.
- Brachial plexus – A collection of spinal nerves that form the peripheral nerves of the upper limb.
- Biceps brachii and coracobrachialis – These muscle tendons move through the axilla, where they attach to the coracoid process of the scapula.
- Axillary Lymph nodes – The axillary lymph nodes filter lymph that has drained from the upper limb and pectoral region. In women, axillary lymph node enlargement is a non-specific indicator of breast cancer.
Passageways Exiting the Axilla
There are three main routes by which structures leave the axilla.
The main route of exit is immediately inferiorly and laterally, into the upper limb. The majority of contents of the axilla region leave by this method.
Another pathway is via the quadrangular space. This is a gap in the posterior wall of the axilla, allowing access to the posterior arm and shoulder area. Structures passing through include the axillary nerve and posterior circumflex humeral artery (a branch of the axillary artery.
The last passageway is the clavipectoral triangle, which is an opening in the anterior wall of the axilla. It is bounded by the pectoralis major, deltoid, and clavicle. The cephalic vein enters the axilla via this triangle, while the medial and lateral pectoral nerves leave.
Thoracic Outlet Syndrome
The apex of the axilla region is an opening between the clavicle, first rib and the scapula. In this apex, the vessels and nerves may become compressed between the bones – this is called thoracic outlet syndrome.
Common causes of thoracic outlet syndrome include:
- Trauma – e.g. fractured clavicle.
- Repetitive movements – seen commonly in occupations that require lifting of the arms.
- Cervical rib – an extra rib which arises from the seventh cervical vertebra.
It often presents with pain in the affected limb (the distribution of pain is dependant on which nerve is compressed), tingling, muscle weakness and discolouration.
Lymph Node Biopsy
Approximately 75% of lymph from the breast drains into the axilla lymph nodes, so can be biopsied if breast cancer is suspected.
If breast cancer is confirmed, the axillary nodes may need to be removed to prevent the cancer spreading. This is known as axillary clearance. During this procedure, the long thoracic nerve may become damaged, resulting in winged scapula.
Radial nerve palsy
Axillary region compression or stretching leading to damage can result in palsy (paralysis with tremors) of the radial nerve. This can also result due to midshaft humeral fracture. The radial nerve supplies the triceps brachii, as well as all the extensor muscles of the forearm.
It also supplies cutaneous sensation to some of the dorsal portions of the hand. The radial nerve is particularly vulnerable as it resides deep in the axilla. Compression from prolonged pressure on the axilla can therefore result in wrist drop. Grip strength is dependent on the wrist being in slight extension, which is not possible in cases of wrist drop.
The long thoracic nerve (arises from the C5-7 nerve roots) supplies the serratus anterior. This nerve descends in the axillary region and compression or damage of the nerve may give rise to winged scapula deformity.
The serratus anterior is no longer able to protract or stabilize the scapula against the thoracic wall. Compression of the nerve may be a result from a tumour, an enlarged lymph node, or an aneurysm.
Breast cancer is the most common known cancer to affect women worldwide. The vast majority of lymphatic drainage of the breast flows through the axillary lymph nodes. Fine needle aspiration of lymph nodes can aid in diagnosis of breast cancer, as well as determining if the cancer has spread.
Chest tube (drain) insertion
If a patient has a pneumothorax, pleural effusion, or haemothorax (air, fluid, or blood in the intrapleural space, respectively), a chest tube is likely to be inserted. Local anaesthetic and skillful technique are required for the procedure to be safe.
The safe region to insert this is in the triangle of safety. This is marked by the lateral border of pectoralis major anteriorly, the apex is just below the axilla, the horizontal line at the level of the nipple marks the base, and posteriorly by the anterior border of latissimus dorsi.
The tube should also be placed above the 6th rib, in the 5th intercostal space. This avoids the intercostal neurovascular bundle under the 5th rib. The drain should be kept below the level of the heart to ensure drainage. Risks of the procedure include damage to the lung, inducement of a pneumothorax, or infection (pleuritis) of the intrapleural space.