Axillary Sheath

vagina axillaris

The axillary sheath is a tube of fascia that surrounds the axillary artery, axillary vein and the cords of the brachial plexus as they pass from the neck into the axilla. It is an extension of the prevertebral layer of deep cervical fascia, dragged down around the neurovascular bundle as it leaves the posterior triangle of the neck.

Anatomical Data

Boundaries & Contents

Location	From the root of the neck through the cervicoaxillary canal into the axilla, ending around the proximal arm.
Boundaries	Continuous above with the prevertebral fascia Surrounds the neurovascular bundle within the axilla Fades distally into the brachial fascia
Contents	Axillary artery Axillary vein Cords and branches of the brachial plexus

Biomechanics

Function & Actions

Binds the main nerves and vessels of the upper limb together and creates a closed space used for regional anaesthesia.

Clinical Relevance

Clinical Notes

The sheath is the target space for brachial plexus blocks: local anaesthetic injected into it spreads along the plexus, which is the basis of the axillary, infraclavicular and supraclavicular approaches. Septa inside the sheath can limit spread and explain patchy blocks. A haematoma inside the sheath after axillary artery puncture can compress the plexus.

Pathology

Common Injuries & Conditions

Axillary Sheath Haematoma

Bleeding inside the closed sheath after arterial puncture or catheterisation can compress the brachial plexus cords and cause arm weakness and numbness, sometimes needing surgical decompression.

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