Shoulder Impingement: Mimickers, Differential Diagnosis, and When to Seek Help

What can be mistaken for shoulder impingement?

Shoulder pain presenting similarly to impingement can stem from various conditions, including rotator cuff tears, biceps tendinopathy, AC joint pathology, cervical radiculopathy, and less common issues, necessitating careful differential diagnosis.

Understanding Shoulder Impingement Syndrome

Shoulder impingement syndrome, often termed subacromial impingement, occurs when the rotator cuff tendons and the subacromial bursa become compressed and irritated as they pass through the narrow space (subacromial space) beneath the acromion. This compression typically happens during overhead arm movements. Common symptoms include:

• Pain with overhead activities: Reaching up, throwing, lifting.
• Pain when reaching behind the back: Such as fastening a bra or tucking in a shirt.
• Night pain: Especially when lying on the affected shoulder.
• Weakness or limited range of motion: Due to pain.

While these symptoms are classic for impingement, many other shoulder and even neck conditions can present with similar complaints, making accurate diagnosis crucial for effective treatment.

Common Mimickers of Shoulder Impingement

Distinguishing between shoulder impingement and its mimickers requires a detailed understanding of anatomy, biomechanics, and careful clinical assessment.

• Rotator Cuff Tears (Partial or Full Thickness)

  • Overlap: Tears of the rotator cuff tendons (supraspinatus, infraspinatus, teres minor, subscapularis) often cause pain with overhead movements, weakness, and night pain, similar to impingement. In fact, chronic impingement can lead to rotator cuff tears.
  • Key Differentiators: Tears may present with more acute onset following a specific injury, significant weakness that is disproportionate to pain, a "catching" or "grinding" sensation, and specific positive findings on special orthopedic tests (e.g., drop arm test, empty can test) and imaging (MRI).
  • Anatomy: Disruption of the tendon fibers of one or more of the four rotator cuff muscles.

• Biceps Tendinopathy (Long Head)

  • Overlap: Inflammation of the long head of the biceps tendon can cause anterior shoulder pain, often radiating down the arm, and may be aggravated by overhead activities or lifting.
  • Key Differentiators: Pain is typically localized to the bicipital groove (front of the shoulder), and specific special tests like Speed's test or Yergason's test will elicit pain. The pain may be more pronounced with resisted elbow flexion and supination.
  • Anatomy: Inflammation or degeneration of the tendon of the long head of the biceps muscle as it passes through the bicipital groove and into the shoulder joint.

• Acromioclavicular (AC) Joint Pathology (Osteoarthritis, Sprain)

  • Overlap: Conditions affecting the AC joint (where the collarbone meets the shoulder blade) can cause pain in the top of the shoulder, which might be mistaken for impingement, especially during movements that load the joint.
  • Key Differentiators: Pain is sharply localized to the AC joint itself and is often exacerbated by horizontal adduction (reaching across the body) or direct palpation of the joint. In contrast, pure impingement pain is usually deeper and more generalized around the deltoid region.
  • Anatomy: Degeneration of the articular cartilage or sprain of the ligaments supporting the joint between the acromion and the clavicle.

• Cervical Radiculopathy

  • Overlap: Pain originating from nerve root compression in the neck can radiate into the shoulder, upper arm, and even down to the hand, mimicking shoulder pathology.
  • Key Differentiators: This condition typically involves neck pain, stiffness, and neurological symptoms such as numbness, tingling, or weakness in specific dermatomal (sensory) or myotomal (motor) patterns in the arm. Special tests like Spurling's maneuver, which compresses nerve roots, will often reproduce symptoms. Shoulder range of motion may be full and pain-free until specific neck movements are performed.
  • Anatomy: Compression or irritation of a nerve root as it exits the cervical spine, often due to disc herniation, degenerative changes, or osteophytes.

• Adhesive Capsulitis (Frozen Shoulder) - Early Stages

  • Overlap: In its early, painful "freezing" stage, adhesive capsulitis can cause generalized shoulder pain with movement, which might initially be confused with impingement.
  • Key Differentiators: As the condition progresses, adhesive capsulitis leads to a global and progressive restriction of both active and passive range of motion in multiple planes, a hallmark not typically seen with isolated impingement. The stiffness is often more profound and impacts all movements, especially external rotation.
  • Anatomy: Inflammation and fibrosis of the glenohumeral joint capsule, leading to thickening and contraction.

• Glenohumeral Osteoarthritis

  • Overlap: Degenerative changes in the main shoulder joint can cause deep, aching pain and stiffness, which may worsen with activity.
  • Key Differentiators: This condition often presents with crepitus (grinding sensation) during movement, a progressive loss of range of motion (especially rotation), and characteristic findings on X-rays. While impingement can be a component, the primary pathology is joint degeneration.
  • Anatomy: Wear and tear of the articular cartilage lining the head of the humerus and the glenoid fossa of the scapula.

• Thoracic Outlet Syndrome (TOS)

  • Overlap: Though less common, TOS can present with shoulder and arm pain, numbness, and tingling, particularly with arm elevation.
  • Key Differentiators: TOS involves neurovascular symptoms (pain, numbness, tingling, weakness, swelling, discoloration) that are often reproduced by specific postures or maneuvers that compress nerves and/or blood vessels in the thoracic outlet region (between the neck and armpit).
  • Anatomy: Compression of the brachial plexus nerves, subclavian artery, or subclavian vein as they pass through the thoracic outlet.

The Importance of Differential Diagnosis

Given the wide array of conditions that can mimic shoulder impingement, accurate differential diagnosis is paramount. Misdiagnosis can lead to ineffective treatment, prolonged pain, and potentially worsening of the underlying condition. A thorough assessment typically involves:

• Detailed History: Understanding the onset, nature, aggravating, and alleviating factors of the pain.
• Physical Examination: Including observation, palpation, active and passive range of motion assessment, manual muscle testing, and a battery of specific orthopedic special tests designed to isolate different structures.
• Neurological Screening: To rule out cervical spine involvement.
• Imaging Studies: X-rays can rule out fractures, arthritis, or bony abnormalities. MRI or ultrasound may be used to visualize soft tissue structures like tendons, ligaments, and bursae, helping to identify tears, inflammation, or other pathologies.

When to Seek Professional Help

If you are experiencing persistent shoulder pain, especially if it is worsening, accompanied by significant weakness, loss of motion, numbness, tingling, or radiating pain, it is crucial to consult a qualified healthcare professional. This could include a physician, physical therapist, or sports medicine specialist. They can accurately diagnose your condition and guide you toward the most appropriate and effective treatment plan.