Topographic anatomy of the interscalene triangle
Normally, nerve trunks and blood vessels glide freely within the so-called interscalene triangle. This anatomical space has clear boundaries:
- Anteriorly and posteriorly it is formed by the anterior and middle scalene muscles.
- Inferiorly it is bounded by the superior surface of the 1st thoracic rib.
The trunks of the brachial plexus emerge through this gap toward the arm, accompanied by the subclavian artery (a. subclavia). In a healthy body, the width of the gap is sufficient for the unobstructed functioning of these structures. However, in the presence of anatomical predispositions, the situation changes drastically.
Pathogenesis of compression
The primary cause of the syndrome is a significant narrowing of the interscalene triangle. Most frequently, this occurs due to the presence of a cervical rib in the patient.
The mechanism of injury is associated with constant friction. The brachial plexus (plexus brachialis) and the artery are compressed in the space between the anterior scalene muscle, the 1st rib, and the anomalous cervical rib itself. Compression is often exacerbated by a fibrous band that attaches the end of the accessory rib to the first thoracic rib. As a result, local ischemia of the nerve fibers and blood flow impairment occur, making these structures extremely vulnerable.
Clinical presentation
The symptoms of scalenus syndrome comprise neurological and vascular manifestations that depend on the degree of compression:
- Pain syndrome. This is the leading complaint. Pain radiates into the arm and directly depends on the position of the body or limb.
- Sensory disturbances. Manifested as paresthesias and hypoesthesia. The typical localization of these disorders is the medial (ulnar) side of the hand.
- Motor disorders. Muscle weakness develops, presenting as Klumpke's paralysis (lower brachial plexus palsy).
- Vasomotor disorders. Occur due to damage to sympathetic nerve fibers. These autonomic fibers run through the interscalene triangle alongside the subclavian artery.
Note: with a prolonged course of the disease and a lack of treatment, further severe complications develop.
Anatomy of the affected plexus
To understand the essence of neurological deficits, it is important to remember the structure of the brachial plexus. According to anatomical nomenclature, it is formed by the spinal nerves C5–Th1 (with a contribution from the C4 branch) and consists of three main trunks:
- Superior trunk (segments C5–C6).
- Middle trunk (segment C7).
- Inferior trunk (segments C8–Th1).
Long terminal branches arise from the trunks: the musculocutaneous, median (n. medianus), radial (n. radialis), axillary, and ulnar (n. ulnaris) nerves. In scalene muscle syndrome, the lower part of this complex is affected.
Position among other entrapment syndromes
Anterior scalene syndrome belongs to the group of isolated peripheral nerve lesions caused by their compression in anatomically narrow zones. A similar mechanism is characteristic of other entrapment neuropathies:
- Carpal tunnel syndrome (compression of the median nerve beneath the transverse carpal ligament).
- Cubital tunnel syndrome (lesion of the ulnar nerve on the medial side of the elbow joint).
- Tarsal tunnel syndrome and peroneal nerve injury.
Unlike trauma or ischemia associated with infections, entrapment syndromes require precise knowledge of topography to identify the level of compression.
To memorize the boundaries of the interscalene triangle: «ladders» (anterior and middle scalene muscles) stand in the front and back, and the floor beneath them is the 1st rib. The plexus and artery hide inside this gap.
Class and exam questions
Why do vascular disorders occur in scalene muscle syndrome?
Sympathetic nerve fibers pass through the interscalene triangle along with the subclavian artery. Their compression and friction against the ligaments lead to pronounced vasomotor disorders.
Which area of the hand loses sensation in scalenus syndrome?
Sensory disturbances (paresthesias and hypoesthesia) are most frequently localized on the medial, i.e., ulnar, side of the hand.
What is Klumpke's paralysis in the context of this syndrome?
This is a specific variant of muscle weakness characteristic of lower brachial plexus lesions. It develops when nerve trunks are compressed in the thoracic outlet.
How do the causes of brachial plexus lesions differ from lumbar plexus lesions?
The lumbar plexus (L1–L3) is better protected anatomically. Inflammatory lesions are almost uncharacteristic of it, but it suffers significantly more often from metabolic diseases such as diabetes mellitus.
What else to study on this topic
- Formation and branches of the cervical plexus (including the ansa cervicalis and phrenic nerve)
- Short and long branches of the brachial plexus: innervation zones
- Carpal tunnel syndrome: nocturnal pain (brachialgia parestetica nocturna) and thenar atrophy
- Complex regional pain syndrome in median nerve lesions
- Specifics of sacral plexus lesions (L4–S3)
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