Serratus anterior muscle is an important stabiliser of the scapula. Weakness or paralysis of the muscle is the commonest cause of scapular winging. The weakness can be caused by a nerve (neurogenic) or muscle (myopathic) condition. While it is more likely to be a problem affecting the nerve leading to muscle weakness, occasionally it could be due to a disease affecting the muscle directly. The distinctions are not necessarily obvious and would require specialist assessment. The treatments will also be very different, depending on the underlying cause.
Long Thoracic Nerve Palsy
The electrical signals to control the serratus anterior muscle are transmitted via the long thoracic nerve (LTN). When the muscle paralysis is thought to be due to a electrical wiring (neurogenic) problem, the condition is called long thoracic nerve palsy. This could arise as a result of:
- Trauma to the nerve itself (including surgery to the chest wall)
- Inflammation affecting the nerve (neuritis or neuralgic amyotrophy)
- Entrapment syndrome
Technical Information
Anatomy of Serratus Anterior Muscle
Serratus anterior is a fan-shaped, thin piece of muscle lying under the shoulder blade and spreading onto the side of the chest wall. It has complex arrangement and functions.
| Portions |
Roots |
Origin (ribs) |
Insertion on scapula |
Muscle fibres |
Scapular action |
| Upper |
C5 |
1st, 2nd |
Superior medial border |
Ascending |
Protraction, rotation |
| Intermediate |
C5,6 |
2nd - 4th |
Medial border |
Divergent |
Protraction |
| Lower |
C7 |
3rd - 8th |
Lower scapular angle |
Convergent |
Stabilisation |
Anatomy of Long Thoracic Nerve (LTN)
The LTN provides the link between our brain and the muscle. It is akin to the wiring of the muscle. It also has a complex course going from the spinal cord to the neck, passing through the armpit (axilla) and eventually lying on the side of the chest wall. The long and slender nature of the nerve has been thought to be the reason why it is vulnerable to injury and disease.
- Also known as Bell's nerve or the external respiratory nerve of Charles Bell
- Arises from C5, C6 and C7 nerve roots
- The C5, C6 branches join beneath the scalenus medius muscle to form the upper division of the nerve, which emerges from the scalenus medius about 1cm posteriorly and superiorly to the upper trunk region
- The C7 branch forms the lower division of the nerve
- The upper and lower divisions unite to form the LTN in the axilla
- Multiple branches are given off to innervate the serratus anterior muscle
Investigations
- EMG is central to the diagnosis of LTN palsy.
- MRI scan is used to exclude any abnormal lesion involving the LTN as well as to study the muscle bulk.
- Specialised blood tests (genetic analysis and enzymology) are sometimes requested to exclude myopathic conditions.
Treatment of LTN Palsy
- Physiotherapy is the mainstay of treatment.
- Input from a neurologist is required in certain cases.
- In chronic cases, surgery may be considered:
- Nerve surgery (neurolysis +/- nerve transfer) is recommended first
- Muscle transfer (pectoralis major transfer) is reserved as the salvage option
What questions should I ask before considering surgery?
- Do you suffer from scapular winging that is limiting your activities of daily living?
- Do you have EMG evidence of isolated long thoracic nerve palsy?
- Have you had a period of physiotherapy (and your progress is now static)?
- Do you accept the small risks of surgery?
If you answer Yes to all of the above questions, then nerve surgery may be considered.
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Rehabilitation after neurolysis of long thoracic nerve (91.2 KB)