[2] Primary inflammation of the subacromial bursa is relatively rare and may arise from autoimmune inflammatory conditions such as rheumatoid arthritis, crystal deposition disorders such as gout or pseudogout, calcific loose bodies, and infection.

[1] More commonly, subacromial bursitis arises as a result of complex factors, thought to cause shoulder impingement symptoms.

[3] If the pain resolves and weakness persists other causes should be evaluated such as a tear of the rotator cuff or a neurological problem arising from the neck or entrapment of the suprascapular nerve.

The inflammatory process causes synovial cells to multiply, increasing collagen formation and fluid production within the bursa and reduction in the outside layer of lubrication.

[6] Less frequently observed causes of subacromial bursitis include hemorrhagic conditions, crystal deposition and infection.

The bursa facilitates the motion of the rotator cuff beneath the arch, any disturbance of the relationship of the subacromial structures can lead to impingement.

[8] If the therapist performs a treatment direction test and gently applies joint traction or a caudal glide during abduction (MWM), the painful arc may reduce if the problem is bursitis or adhesive capsulitis (as this potentially increases the subacromial space).

The diagnosis of impingement syndrome should be viewed with caution in people who are less than forty years old, because such individuals may have subtle glenohumeral instability.

[citation needed] MRI imagining can reveal fluid accumulation in the bursa and assess adjacent structures.

Many non-operative treatments have been advocated, including rest; oral administration of non-steroidal anti-inflammatory drugs; physical therapy; chiropractic; and local modalities such as cryotherapy, ultrasound, electromagnetic radiation, and subacromial injection of corticosteroids.

[citation needed] In 1997 Morrison et al.[13] published a study that reviewed the cases of 616 patients (636 shoulders) with impingement syndrome (painful arc of motion) to assess the outcome of non-surgical care.

[citation needed] The authors were unable to posit an explanation for the observation of the bimodal distribution of satisfactory results with regard to age.

The poorer outcome for patients over 60 years old was thought to be potentially related to "undiagnosed full-thickness tears of the rotator cuff".