Is the hummingbird sign in Magnetic Resonance Imaging (MRI) exclusive to Progressive Supranuclear Palsy (PSP) or seen in other diseases?

The Hummingbird Sign in MRI: Not Exclusive to Progressive Supranuclear Palsy

The hummingbird sign on MRI is highly specific but not exclusive to Progressive Supranuclear Palsy (PSP), as it can also be observed in idiopathic Normal Pressure Hydrocephalus (iNPH) and Guam Parkinsonism-Dementia Complex (PDC). 1, 2

Diagnostic Value of the Hummingbird Sign

Specificity and Sensitivity

• The hummingbird sign demonstrates excellent specificity (99.5%) and positive predictive value (96.1%) for PSP diagnosis 3
• However, its sensitivity is suboptimal at only 51.6%, meaning it's absent in nearly half of confirmed PSP cases 3
• In early, clinically unclassifiable parkinsonism, sensitivity drops further to just 35.3% 3

Other Conditions Showing the Hummingbird Sign

1. Idiopathic Normal Pressure Hydrocephalus (iNPH)

   • Recent research shows 65% of iNPH patients may demonstrate the hummingbird sign 1
   • This creates significant diagnostic overlap with PSP

2. Guam Parkinsonism-Dementia Complex (PDC)

   • Case reports document the hummingbird sign in patients with this rare condition 2
   • This condition shares clinical and pathological features with PSP

Anatomical Basis of the Hummingbird Sign

The hummingbird sign represents:

• Atrophy of the midbrain tegmentum (both rostral and caudal portions) 4
• Relative increase in the length of the interpeduncular fossa compared to the anteroposterior diameter of the midbrain tegmentum 4
• Involvement of the rostral interstitial nucleus of the medial longitudinal fasciculus 4

Diagnostic Accuracy Considerations

Quantitative vs. Qualitative Assessment

• Measuring sagittal midbrain area (quantitative approach) is more accurate and reliable than visual assessment of the hummingbird sign 1
• A midbrain area cutoff of 90 mm² yields better diagnostic performance (AUC = 0.86) than visual assessment scores (AUC = 0.83) 1

Morning Glory Sign Comparison

• The "morning glory flower sign" is another radiological finding in PSP
• It has even higher specificity (97.7%) but lower sensitivity (36.8%) than the hummingbird sign 3

Clinical Implications

When the hummingbird sign is observed on MRI:

• Consider PSP as the primary diagnosis, especially when accompanied by:

  • Vertical supranuclear gaze palsy (particularly downward gaze initially)
  • Early falls and postural instability
  • Axial rigidity more prominent than appendicular rigidity
  • Poor response to levodopa therapy 5

• Rule out alternative diagnoses, particularly:

  • Idiopathic Normal Pressure Hydrocephalus (iNPH) - look for the triad of gait disturbance, cognitive impairment, and urinary incontinence 1
  • Guam PDC - consider geographical origin and family history 2

Diagnostic Algorithm When Hummingbird Sign is Present

1. Assess for clinical features of PSP:

   • Vertical supranuclear gaze palsy
   • Early falls within first year
   • Poor levodopa response
   • Age of onset ≥40 years 5

2. Consider alternative diagnoses:

   • Evaluate for iNPH triad and ventriculomegaly
   • Check geographical/ethnic background for Guam PDC
   • Rule out other parkinsonian syndromes (PD, MSA, CBD) 6

3. Use additional imaging markers:

   • Measure midbrain area (cutoff <90 mm²)
   • Look for other PSP-specific findings 1

4. Consider functional imaging:

   • FDG-PET/CT may show hypometabolism in medial frontal and anterior cingulate cortices, striatum, and midbrain 6
   • Ioflupane SPECT/CT shows decreased striatal uptake but cannot differentiate between parkinsonian syndromes 5

The presence of the hummingbird sign should raise suspicion of PSP, but clinicians must carefully consider alternative diagnoses, particularly iNPH, before finalizing the diagnosis.