What is frozen shoulder, its triggers, pathogenesis, and symptoms?

What is Frozen Shoulder?

Frozen shoulder, also known as adhesive capsulitis, is a painful musculoskeletal disorder characterized by progressive inflammation of the glenohumeral joint capsule followed by capsular fibrosis, resulting in severe restriction of both active and passive shoulder motion, particularly external rotation. 1, 2

Definition and Clinical Characteristics

Frozen shoulder is a common shoulder disorder that involves:

• Gradual onset of spontaneous shoulder pain combined with progressive stiffness of the glenohumeral joint 1
• Equal restriction in both active and passive range of motion, distinguishing it from rotator cuff pathology where passive motion may be preserved 3
• No focal weakness on resistance testing, unlike rotator cuff tears 3
• Absence of swelling or muscle atrophy on physical examination 3

The condition primarily affects middle-aged women and is usually unilateral 2.

Triggers and Risk Factors

Primary (Idiopathic) Form

The cause of primary frozen shoulder remains unknown, though it occurs spontaneously without clear precipitating factors 1, 2.

Secondary Form Triggers

In post-stroke patients, frozen shoulder has a dramatically elevated incidence:

• Up to 72% of stroke patients experience at least one episode of shoulder pain during the first year 4, 3
• Up to 67% incidence of shoulder-hand-pain syndrome occurs in stroke patients with combined motor, sensory, and visuoperceptual deficits 5

Specific triggers in post-stroke patients include:

• Improper handling during rehabilitation, particularly overhead pulley exercises that encourage uncontrolled abduction 4, 5, 3
• Shoulder subluxation and motor weakness, which have strong covariance as predictive factors 4
• Inadequate protection of the hemiplegic limb leading to trauma 5
• Shoulder tissue injury including effusion, tendinopathy, or rotator cuff tears found in approximately one-third of acute stroke patients 5
• Spasticity, though definitive causation remains unconfirmed 5

Other predictors include:

• Older age 4
• Left hemiplegia 4
• Tactile extinction and reduced proprioception in the affected limb 4
• Reduced passive shoulder abduction and external rotation 4

Pathogenesis

The pathophysiology involves a two-stage process:

Stage 1: Synovial Inflammation

• Initial phase characterized by synovial inflammation of the glenohumeral joint 1
• Development of pain as the predominant feature 1

Stage 2: Capsular Fibrosis

• Progressive capsular fibrosis and contracture following the inflammatory phase 1
• The rotator interval and axillary recess are the primary anatomical structures involved in capsular thickening and contracture 5
• Results in severe motion restriction 1

In post-stroke patients, additional pathophysiological factors include:

• Shoulder tissue injury with abnormal ultrasound findings (effusion in biceps tendon or subacromial bursa, tendinopathy of biceps/supraspinatus/subscapularis, rotator cuff tears) 4, 5
• Abnormal joint mechanics related to altered movement patterns and shoulder subluxation 4
• Central nociceptive hypersensitivity contributing to pain 4
• Capsular stiffness with altered resting position of the scapula in lateral rotation in chronic cases 4

Clinical Stages and Symptoms

Frozen shoulder progresses through three distinct stages: 2, 6

Freezing Phase (Painful Stage)

• Gradual increase in pain of spontaneous onset 1
• Progressive loss of range of motion 6
• Pain often worse at night 6
• Duration varies but typically lasts months 2

Frozen Phase (Adhesive Stage)

• Severe restriction of motion with less pain than freezing phase 6
• External (lateral) rotation is the most significantly affected motion and relates most strongly to shoulder pain onset 4, 3
• Abduction is severely restricted 5, 3
• Internal rotation is least affected, creating the classic capsular pattern: external rotation > abduction > internal rotation 3
• Both active and passive motion equally restricted 3

Thawing Phase

• Gradual improvement in range of motion 6
• Progressive reduction in pain 6
• The condition is usually self-limiting and typically persists for 2-3 years, though some patients may continue to suffer from pain and limited range of motion beyond this time 2

Additional Symptoms in Post-Stroke Patients

• Shoulder-hand syndrome may develop with pain, tenderness, and edema in metacarpophalangeal and proximal interphalangeal joints 5
• Delayed rehabilitation and functional recovery, potentially masking motor function improvement 5
• Limited use of assistive devices (canes, wheelchairs) for ambulation due to pain 5
• Depression and sleeplessness contributing to reduced quality of life 5

Common Pitfalls

Do not confuse frozen shoulder with rotator cuff pathology:

• Rotator cuff tears show focal weakness with specific resistance testing 3
• Rotator cuff pathology may have preserved passive motion despite limited active motion 3
• Frozen shoulder demonstrates equal restriction in both active and passive motion 3

Avoid overhead pulley exercises in at-risk patients (especially post-stroke) as they encourage uncontrolled abduction and can worsen shoulder complications 4, 3.