Long thoracic nerve injury: the shortest route to recovery!

2019 European Games - Anna Balsukova in action during her bronze medal match REUTERS/Valentyn Ogirenko

The long thoracic nerve (LTN) is a motor nerve that innervates the serratus anterior muscle. Injury to this nerve may lead to insufficiency in serratus anterior function, which manifests as scapular ‘winging’. Scapular winging is a genuine problem for the overhead athlete because the full function of the serratus anterior is required to fully upwardly rotate and posteriorly tilt the scapula for full overhead movement.

Anatomy and biomechanics

The serratus anterior (SA) is a complex muscle, comprised of three distinct functional components (see figure 1). It is a broad and flat muscle that originates on the ribs and attaches to the medial scapular border. The superior component of SA functions to anchor the scapula to the ribs, which allows a pivot point for upward rotation. This region has thin fibers, which originate on the first and second ribs and insert onto the superior medial angle of the scapula.

The middle component is also flat and thin, but its primary role is to protract the scapula. It originates on the third to fifth ribs and attaches to the medial scapular border. The inferior component is the thickest and most distinct, made of thick fibers that originate on the sixth to ninth ribs and attach onto the inferior scapula angle. The main role for this portion of the muscle is to enable upward rotation of the scapula and posterior tilt motion that is crucial for ‘hand above head’ movements required in most overhead sports^(1,2).

Figure 1: Anatomy of the serratus anterior and the LTN

The LTN is solely a motor nerve with no sensory fibers and originates at the C5-C7 level of the cervical spine. The upper part (C5-6) passes either between the middle and posterior scalene - through the middle scalene muscle - or anterior to the middle scalene⁽³⁾. It then converges with the C7 branch (this runs anterior to the medial scalene)^(1,2,4). There are some anatomical variations in this respect^(5-7):

• In 13% of individuals, a contribution from C4 exists.
• In 8% of cases, the C7 component is absent.
• In another 8%, a branch is received from C8.
• In 20% of cadaver specimens studied, the lower part of the serratus anterior was innervated by the intercostal nerves.

The conjoined branches of C5/6/7 which form the LTN then pass over the second rib, and enter a fascial sheath(8)to continue down the lateral part of the thorax to the serratus anterior muscle^(1,4). It then divides into two or three branches to each digitation of the serratus anterior (see figure 1)⁽⁵⁾. The LTN is 22-24cm long on average, and its length it may make it more susceptible to injury^(9,10).

Injury to the LTN

Scapular winging is a pathomechanical abnormality of the scapula characterized by a failure to keep the scapula anchored to the rib cage. The first described case of scapular winging caused by LTN injury and serratus anterior palsy was described by Velpeau in 1837⁽¹¹⁾. The most common sources of injury to the LTN are traction, compression, and neuralgic amyotrophy^(7,12).

Other factors that may contribute to LTN injuries are as follows:

• Traction of the nerve as it exits the fascial sheath has often been cited as the most obvious source of damage and subsequent paralysis of the serratus anterior⁽¹³⁾. This can result from overuse in athletes with chronic overhead arm movements. This repetitive action lengthens and potentially shears the nerve⁽¹⁴⁾.
• Lateral flexion of the head away from the LTN also lengthens the nerve. When coupled with overhead arm movements that further stretch the nerve, the LTN may suffer damaging repetitive traction and compression⁽¹⁾. This action may occur in overhead athletes and account for up to 35% of LTN palsy cases⁽¹⁵⁾.
• Spasm of the middle scalene muscle may compress the LTN⁽¹⁴⁾.
• Compression can occur over the second rib as the LTN changes angles^(1,4,5). The approximation of the second rib and coracoid process is closer when the scapula is fully retracted medially.
• The inferior angle of the scapula represents another potential site of compression and traction on the nerve near the level of the eighth and ninth rib⁽⁹⁾. In asynchronous scapula motion, the inferior angle of the scapula may be the source of damage to the nerve.
• Four bursas, the subcoracoid, subscapular, accessory, and supracoracoid, could cause compression if inflamed⁽⁵⁾.
• Acute trauma to the nerve may occur from a clavicle or scapula fracture, a surgical procedure, or an infection, such as tonsillitis or bronchitis^(14,15). It may also occur following immunizations, carrying a heavy backpack, or sleeping in an awkward position.

Athletes most likely to suffer from LTN palsy are tennis players; however, it has been observed in wrestling, archery, golf, gymnastics and weightlifting athletes⁽¹⁶⁾. The common mechanism in these sports appears to be the repetitive overhead movements, which lends evidence to the theory that traction due to shoulder flexion is the greatest threat to the LTN^(1,17).

Signs and symptoms of LTN palsy

Typical signs and symptoms are as follows:

• Observable scapular winging at rest (usually inferior border only), with overhead movements, or resisted wall push-ups/push-ups whereby the entire scapula demonstrates winging (see figure 2).
• Pain around the base of the neck, deltoid, and scapula. This would be accounted for by spasm in the levator scapula and rhomboid muscles as they work unopposed against the serratus anterior⁽²⁾.
• Limitation of forward flexion and abduction of the shoulder (usually to less than 100 degrees)⁽¹⁾. Athletes may notice a lack of power in throwing movements. However, it may be possible to still achieve full upward rotation due to the action of the trapezius muscles⁽¹⁶⁾.

Figure 2: Left scapula winging during a push-up

To definitively identify the LTN as being the source of the scapula winging, electromyographic (EMG) testing is needed. EMG testing may show features such as resting denervation potentials, depressed motor unit recruitment, and polyphasic potentials during arm movements⁽¹⁴⁾. There’s some speculation that the type of winging observed is influenced by the part of the LTN that has been damaged. Due to the three distinct functional components of the LTN, an injury below the second rib level may still allow for an innervated superior portion of the serratus anterior. In this case, the scapula winging may appear to only involve the inferior angle and medial border. If the LTN is damaged in and around the scalenes, then the entire serratus anterior will be affected and total medial border liftoff will be seen⁽¹⁸⁾.

Treatment

With conservative treatment, most LTN injuries resolve on average within nine months, however, recovery can take anywhere from six to 24 months^(1,7). Some patients may be left with some residual scapula weakness due to incomplete healing of the nerve^(1,5). Axonal regeneration occurs at a rate of approximately 1mm per day; if the LTN palsy is not resolved by 24 months therefore, the paralysis is likely permanent and surgery may be required^(1,7). Surgical options in the event of failure of the nerve to recover, include muscle transfer (usually part of pectoralis major), neurolysis of the LTN (usually at the middle scalene level), fusion of the scapula to the thorax, and nerve transfers using the thoracodorsal nerve^(11,19).

Conservative rehabilitation consist of the following stages (adapted from Watson and Schenkman 1995)⁽²⁰⁾:

1. Protection from further nerve trauma (during the complete denervation period):
   1. Avoid overhead lifting and loaded movements.
   2. Avoid stretching the serratus anterior (avoid excessive retraction and downward rotation movements).
   3. Maintain range of motion. This is best achieved by actively moving the arm whilst supine so that the scapula is fixed and will not wing and cause further nerve traction.
   4. Scapular taping and/or bracing to sit the scapula against the chest wall.
2. Isolated activation (when the LTN shows some recovery and the serratus anterior shows activity with voluntary contraction):
   1. Gentle supine protraction drills focusing on serratus anterior and not the pectoralis minor (see exercise 1).
   2. Stretch pectoralis minor, rhomboids and levator scapulae to prevent adaptive shortening in these muscles (due to unopposed serratus anterior activity).
3. Functional retraining:
   1. As the LTN regains innervation to the serratus anterior, introduce direct functional serratus anterior exercises (see exercises 2 and 3 below).

Exercise 1: Reach for the sky

• Lie supine on the floor so that the floor stabilizes the scapula.
• Hold a Kettlebell or dumbbell in the hand of the affected shoulder. The hand should be positioned directly over the shoulder.
• Attempt to reach the hand towards the ceiling (scapula protraction) and simultaneously rotate the arm into external rotation and reach the hand to the level of the forehead. This external rotation and upward reach promotes upward rotation and eliminates the use of the latissimus dorsi and pectoralis minor.
• The free hand can be used to palpate serratus anterior contraction over the ribcage.
• Perform three sets of 15 reps.

Reach for the sky: Start position

Reach for the sky: Finish position

Exercise 2: Plank plus

• Place the elbows on a bench so that the shoulder is in approximately 100 degrees of flexion. This allows the scapula to be positioned in a posterior tilt.
• Holding a Theraband in the hands, gently pull against the bands (this will activate the external rotators).
• Push the chest away from the bench using a scapula protraction/thoracic flexion movement. This is the ‘plus’ movement.
• Perform three sets of 15 reps.

Plank plus

Exercise 3: Swiss ball twist

• Sit on a bench holding a Swiss ball between the forearms.
• Hold a Theraband in each hand and wrap around the back.
• Gently rotate away from the affected side, whilst simultaneously protracting the scapula.
• Perform three sets of 15 reps.

Swiss ball twist: Start position

Swiss ball twist: Finish position

Summary

LTN injury is an uncommon injury in athletes and the causes may be multifactorial. Its prevalence in overhead sports suggests that repetitive overhead arm movements may, in some cases, be the offending mechanism of injury. If the LTN is injured, the usual outcome is a winging scapula caused by serratus anterior dysfunction, which limits overhead movements and significantly reduces performance. Recovery is usually spontaneous and occurs on average nine months after diagnosis. For effective rehabilitation, it’s important for clinicians to instigate direct serratus anterior training exercises.

References

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