What is the diagnostic approach for missing muscle syndrome due to a blowout fracture?

Diagnostic Approach for Missing Muscle Syndrome in Blowout Fractures

Computed tomography (CT) with coronal views is the essential diagnostic imaging modality for identifying missing muscle syndrome in blowout fractures, showing absence of the inferior rectus muscle in multiple contiguous coronal CT cuts, which requires urgent surgical intervention. 1, 2

Clinical Presentation and Initial Assessment

• Key clinical findings suggestive of muscle entrapment:

  • Severe vertical diplopia, especially in upgaze
  • Restricted ocular motility
  • Positive forced duction test (indicating mechanical restriction)
  • Oculocardiac reflex symptoms (bradycardia, nausea, vomiting, dizziness)
  • Minimal external signs of trauma (particularly in "white-eyed" blowout fractures)

• Vital signs monitoring:

  • Check for bradycardia or heart block
  • Monitor for symptoms of oculocardiac reflex (dizziness, nausea, vomiting, loss of consciousness)

Imaging Evaluation

• CT scan is the primary imaging modality:

  • Coronal views are critical for identifying the "missing muscle syndrome"
  • Look for absence of the inferior rectus muscle in multiple contiguous CT cuts
  • Assess for small linear-type fractures which are more likely to cause severe muscle entrapment
  • Evaluate for muscle herniation or entrapment at fracture site

• MRI considerations:

  • Only if no concern for metallic foreign bodies
  • Provides better soft tissue detail of extraocular muscles
  • Can be performed dynamically in some cases

Diagnostic Pearls

• Missing muscle syndrome characteristics:

  • Often associated with small "trapdoor" fractures rather than large defects
  • More common in pediatric patients and elderly patients
  • Can be easily misdiagnosed as increased intracranial pressure due to similar symptoms 3
  • May involve muscle sheath entrapment even when the muscle itself appears intact 4

• Specialized clinical tests:

  • Forced duction testing (critical to distinguish restriction from paresis)
  • Forced generation testing
  • Double Maddox rod, Lancaster red-green, or Hess screen tests

Management Algorithm Based on CT Findings

1. If CT shows "missing muscle" with entrapment:

   • Urgent surgical intervention is required
   • Delay increases risk of Volkmann's ischemic contracture of the extraocular muscles 5
   • Recovery time is typically longer (>1 month) even with proper intervention 6

2. If CT shows muscle entrapment with oculocardiac reflex:

   • Immediate surgical repair is indicated 1, 7
   • Monitor vital signs closely pre-operatively

3. If CT shows linear-type fracture with muscle involvement:

   • Early surgical intervention (within 2 weeks) is recommended
   • Patients with "missing rectus" on CT have higher risk of residual diplopia 6

4. If CT shows minimal or no muscle involvement:

   • Observation may be appropriate if:
     • Minimal diplopia (not in primary or downgaze)
     • Good ocular motility
     • No significant enophthalmos or hypoglobus

Common Pitfalls to Avoid

• Misdiagnosis: White-eyed blowout fractures with muscle entrapment can be misdiagnosed as increased intracranial pressure, leading to delayed treatment and permanent sequelae 3

• Delayed intervention: Elderly patients, hypotensive patients, and those with small fractures are at higher risk for developing Volkmann's contracture of extraocular muscles if surgical intervention is delayed 5

• Inadequate imaging review: Clinician review of radiologic studies is crucial, as radiologists may not specifically comment on the "missing muscle" finding 2

• Underestimating small fractures: Small linear-type fractures can cause more severe muscle entrapment than larger fractures due to the "trapdoor" mechanism 6