What is the cause of sudden shoulder pain in a postpartum female with no history of trauma, normal X-ray and MRI results?

Postpartum Shoulder Pain with Normal Imaging: Referred Pain from Diaphragmatic Irritation

The most likely cause is referred pain from diaphragmatic irritation due to intra-abdominal blood or fluid accumulation, not a primary shoulder pathology. When imaging of the shoulder is completely normal (both X-ray and MRI), and there is no trauma history, you must consider non-musculoskeletal etiologies, particularly in the postpartum period.

Critical Clinical Context

The postpartum period creates unique risk factors that are not addressed in standard shoulder pain guidelines:

• Diaphragmatic irritation from postpartum hemorrhage, retained products, or peritoneal fluid causes referred pain to the shoulder via the phrenic nerve (C3-C5 distribution)
• This presents as sudden shoulder pain, typically worse with lying flat or deep breathing
• The shoulder itself is structurally normal, explaining why advanced imaging shows no abnormalities

Algorithmic Approach to Diagnosis

Step 1: Assess for Red Flags

• Vital signs: Check for tachycardia, hypotension, or fever suggesting hemorrhage or infection
• Abdominal examination: Assess for tenderness, distension, or peritoneal signs
• Timing: Pain onset within days to weeks postpartum strongly suggests obstetric complication
• Position dependence: Pain worse when supine suggests free fluid irritating diaphragm

Step 2: Rule Out Obstetric Complications

Order abdominal/pelvic imaging:

• Ultrasound as first-line to assess for free fluid, hematoma, or retained products
• CT abdomen/pelvis if ultrasound is non-diagnostic and clinical suspicion remains high

Step 3: If Abdominal Imaging is Normal, Consider Primary Shoulder Pathology

While the provided guidelines focus on traumatic shoulder pain 1, they note that 40% of patients with nonspecific shoulder pain have no significant pathology on ultrasound 1. However, your patient already has normal MRI, which is superior to ultrasound for detecting soft tissue pathology 1.

Adhesive capsulitis is a consideration in postpartum patients:

• Can occur without trauma and presents with painful, gradual loss of active and passive motion 2, 3, 4
• However, this typically develops over weeks to months, not suddenly 4
• Plain radiographs are usually normal because pathology involves capsular fibrosis, not bony changes 5
• MRI would show capsular thickening and inflammation if present

Common Pitfalls to Avoid

• Do not assume normal shoulder imaging means benign etiology - referred pain from serious intra-abdominal pathology can present with isolated shoulder pain
• Do not miss postpartum hemorrhage - this can be life-threatening and requires urgent intervention
• Do not order additional shoulder imaging when X-ray and MRI are already normal - the ACR guidelines indicate MRI is the gold standard for soft tissue pathology 1
• Do not delay abdominal evaluation in a postpartum patient with sudden shoulder pain and normal shoulder imaging

If All Imaging Remains Normal

Consider:

• Nerve entrapment (suprascapular or long thoracic nerve) - though this typically causes weakness, not just pain
• Cervical radiculopathy - obtain cervical spine imaging if shoulder and abdominal workup negative
• Functional pain syndrome - diagnosis of exclusion only after thorough evaluation

The key distinguishing feature is the postpartum timing combined with sudden onset and completely normal shoulder imaging - this clinical picture demands evaluation for referred pain from intra-abdominal pathology before attributing symptoms to primary shoulder disease.