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b. Direct trauma or strain of the deltoid muscle.
c. Cervical spondylosis of the C4-C5 and C5-C6 spinal segments (i.e. C5-C6 spinal nerves).
d. Axillary nerve neuralgia (ANN).

As we mentioned above, the deltoid muscle is innervated by the axillary nerve. The axillary nerve arises from the posterior part of the brachial plexus (C4-C5), runs down, and then passes posteriorly to the proximal end of humerus. Fig. 328 shows anatomical pathway of the axillary nerve.

Fig. 328. Anatomical pathway of the axillar nerve.

 Axillary nerve supports the sensory innervation of the skin over the deltoid muscle. Motor branches provide innervation of the deltoid muscle and teres minor muscle. ANN is caused by a direct trauma of nerve, especially after anterior dislocation of shoulder or after fracture of the proximal end of the humerus, especially in area of the surgical neck. ANN is also caused by cervical spondylosis.

2. Clinical symptoms.
The main complaint of patients with DS is pain. It appears when the patient tries to abduct the arm, or put the arm behind the back and head. The pain is local and usually does not radiate lower than the level of the elbow joint. During palpation, hypertonuses and trigger points in the deltoid muscle can be found (see Fig. 332).
Patients who suffer from ANN complain about weakness and fatigue of the deltoid muscle. They have problems or are sometimes unable to adduct the arm at the shoulder joint. This sensory deficit shows up as pain, paraesthesia, or numbness in the area of the deltoid muscle (see Fig. 329). 

Fig. 329. Area of sensory deficit in the case of ANN.