What is the differential diagnosis for a patient presenting with an ankle sprain?

Differential Diagnosis for Ankle Sprain

When evaluating a patient with suspected ankle sprain, you must systematically rule out fractures, syndesmotic injuries, osteochondral defects, Achilles tendon rupture, peroneal tendon injuries, and high ankle sprains before confirming the diagnosis of lateral ligament injury.

Immediate Diagnostic Priorities

Fracture Exclusion

• Apply the Ottawa Ankle Rules (OAR) to determine need for radiography with 99.7% accuracy in ruling out fractures 1
• Order ankle radiographs if any of the following are present 1:
  • Bone tenderness along the posterior edge or tip of either malleolus (distal 6 cm)
  • Bone tenderness at the base of the fifth metatarsal
  • Inability to bear weight immediately after injury and for four steps in the emergency department
• The OAR correctly avoid unnecessary radiography in 299 out of 300 patients 1

Key Differential Diagnoses to Consider

Syndesmotic (High Ankle) Sprain:

• Perform crossed-leg test: pressure applied to medial knee producing pain in the syndesmosis area indicates high ankle sprain 1
• Requires MRI if suspected, as this injury has different treatment implications and longer recovery 1

Complete Lateral Ligament Rupture (Grade III):

• Look for all four cardinal signs: swelling, hematoma, pain on palpation around distal fibula, and positive anterior drawer test 1
• When all four findings present, 96% likelihood of lateral ligament rupture versus only 14% when absent 1
• Delay physical examination 4-5 days post-injury for optimal sensitivity (84%) and specificity (96%) of anterior drawer test 1

Osteochondral Defects:

• Suspect when persistent symptoms occur despite appropriate treatment 1
• MRI has excellent sensitivity (93-96%) and specificity (100%) for visualization 1
• Cannot be adequately assessed by physical examination alone 1

Achilles Tendon Rupture:

• Palpate for swelling or pain in Achilles tendon region 1
• Perform Thompson test (calf squeeze test)
• This is a critical diagnosis not to miss as it requires different management

Peroneal Tendon Injuries:

• May occur with lateral ankle trauma, particularly with traction injury to peroneal nerve 2
• Delayed response time of peroneal muscle detected after lateral ankle injuries 1

Occult Fractures:

• MRI indicated when clinical suspicion remains high despite negative radiographs 1
• Particularly important in patients with persistent inability to bear weight

Clinical Examination Algorithm

Initial Assessment (Day 0-2)

• Mechanism of injury: forced plantar flexion and inversion suggests lateral ligament injury 3
• Ability to bear weight immediately after injury 1
• Presence of hematoma or rapid swelling 1
• Note: Excessive swelling and pain can limit examination accuracy up to 48 hours after injury 1

Delayed Assessment (Day 4-5)

• This is the optimal timing for definitive ligament assessment 1
• Anterior drawer test at this timepoint has 84% sensitivity and 96% specificity 1
• Reexamination at this interval is crucial to distinguish partial tears from complete ruptures 1

Advanced Imaging Indications

Ultrasonography:

• Similar sensitivity (92%) to delayed examination but lower specificity (64%) 1
• Depends on availability of experienced technician 1
• Not routinely recommended over delayed clinical examination

MRI:

• Reserved for suspected high-grade ligament injuries, osteochondral defects, syndesmotic injuries, or occult fractures 1
• Not needed for uncomplicated anterior talofibular ligament rupture as delayed physical examination is sufficient 1
• Use when persistent symptoms occur despite appropriate treatment 1

Obsolete Modalities:

• Stress radiographs should not be used due to limited diagnostic value and pain during acute testing 1
• Arthrography is invasive with no advantage over delayed physical examination 1

Common Diagnostic Pitfalls

• Failing to apply OAR systematically, leading to unnecessary radiographs or missed fractures 1
• Performing definitive ligament testing too early (before day 4-5) when swelling obscures findings 1
• Missing syndesmotic injuries by not performing crossed-leg test 1
• Assuming all ankle injuries are simple sprains without considering Achilles rupture or peroneal tendon injury 1, 2
• Ordering MRI for straightforward lateral ligament injuries when delayed examination would suffice 1