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Differential Diagnosis

It is important to carefully evaluate for other sources of shoulder pain. These may include acromioclavicular arthrosis rotator cuff tear (partial or complete), instability, adhesive capsulitis, glenohumeral arthritis, biceps tendonosis, labral pathology, and cervical radiculopathy. It is also important to remember that there can be more than one source of pain. The two most common coexisting conditions are AC arthrosis and rotator cuff tears.

Patients with AC arthrosis often point directly at the AC joint as the source of pain. They are point tender over this area and have pain with cross arm adduction. An injection into this joint may result in a decrease in symptoms and x-rays will often show joint degeneration.

Full thickness tears of the rotator cuff result in weakness of the particular muscle group involved. Isolated muscle strength testing with comparison to the asymptomatic extremity can pick up even subtle differences. Active range of motion in forward flexion and abduction may be less than passive range of motion. In chronic conditions, muscle atrophy is often present.

Differential injections can be helpful in making the diagnosis of impingement syndrome. Injections may be given in the subacromial space, AC joint, or glenohumeral joint. The original impingement injection test was described as a valuable method for separating impingement lesions from other causes of shoulder pain.

This test involves injection of 10 ml of 1% lidocaine into the subacromial bursae. If after injection, “the painful arc is considerably reduced or abolished, it establishes the anatomic site of the lesion but does not give an indication of the precise pathology or extent of the lesion”.

The most common indication for selective injection involves differentiating subacromial and AC joint pain. These injections may also be used in the biceps tendon sheath. They typically contain a local anesthetic and often a corticosteroid.

The effect of the local anesthetic should begin almost immediately. It is important to have the patient move their arm and document what percentage of pain relief was obtained. The effect of the steroid can be determined at a follow up visit quantifying amount and duration of pain relief.

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