What does the Neurological Pupillary Index (NPI) on a pupillometer indicate?

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Understanding the Neurological Pupillary Index (NPi) on Pupillometry

The Neurological Pupillary Index (NPi) is a quantitative measure of pupillary reactivity obtained through automated pupillometry that ranges from 0 (non-reactive) to 5 (normal), with values below 3 indicating abnormal pupillary function. 1

Clinical Significance of NPi Values

  • NPi combines multiple parameters of the pupillary light reflex including pupil size, constriction percentage, constriction velocity, and latency to provide an objective assessment of pupillary reactivity 1
  • An NPi value between 0 and 2.40 within the first 24 hours after return of spontaneous circulation (ROSC) in post-cardiac arrest patients predicts unfavorable neurological outcomes with 100% specificity 1
  • NPi values are categorized as:
    • 0: Non-reactive pupil
    • <3: Abnormal pupillary reactivity
    • ≥3: Normal pupillary reactivity 1, 2

Prognostic Value in Critical Care

  • The American Heart Association recommends using quantitative pupillometry at 72 hours or more after ROSC for predicting neurological outcome in comatose adults after cardiac arrest 3
  • An NPi ≤2 at 48 hours has 100% specificity and 18.8% sensitivity for predicting poor neurological prognosis 1
  • An NPi ≤2 at 72 hours maintains 100% specificity with 16.9% sensitivity for predicting poor neurological outcomes 1
  • In patients with ECMO support, an NPi <3 at any time between 24-72 hours after cannulation has 100% specificity for 90-day mortality 1

Relationship with Other Neurological Indicators

  • NPi shows a strong correlation with Glasgow Coma Scale (GCS) scores (r=0.609, p<0.001) and can significantly predict modified Rankin Scale (mRS) scores, indicating its value as a prognostic tool 4
  • NPi has an inverse relationship with intracranial pressure (ICP) - decreasing pupil reactivity correlates with increasing ICP 2
  • Patients with abnormal pupillary light reactivity (NPi <3) had an average peak ICP of 30.5 mmHg versus 19.6 mmHg for those with normal pupil reactivity 2
  • Abnormal NPi can precede ICP elevation by approximately 15.9 hours, making it a valuable early warning sign 2

Clinical Application Considerations

  • NPi should be interpreted as part of a multimodal approach to neurological evaluation, not as an isolated test 1
  • Sedatives and muscle relaxants can affect results, which is why evaluation after 72 hours is recommended to minimize these effects 1, 3
  • NPi and constriction velocity (CV) are distinct variables - a brisk CV does not rule out an abnormal PLR, and a slow CV does not necessarily indicate an abnormal PLR 5
  • NPi measurements may show circadian patterns that can be disrupted in patients with poor outcomes 6
  • In pediatric patients with neurological injuries, those with abnormal NPi (<3) have higher mortality and worse short-term and long-term neurological and functional outcomes compared to those with normal NPi (≥3) 7

Practical Implementation

  • For post-cardiac arrest patients: Use NPi after 72 hours for greater specificity in predicting neurological outcomes 3, 1
  • For patients with suspected increased ICP: Monitor NPi trends as early indicators of worsening neurological status 2
  • For patients with subarachnoid hemorrhage: Track NPi values over time as they tend to reflect the course of neurological illness 6
  • Consider variance in NPi measurements, as higher variance has been associated with more severe clinical conditions 6

References

1
Guideline

Clinical Significance of the Neurological Pupillary Index (NPi)

Praxis Medical Insights: Practical Summaries of Clinical Guidelines, 2025

3
Guideline

Guideline Directed Topic Overview

Dr.Oracle Medical Advisory Board & Editors, 2025

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Professional Medical Disclaimer

This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.

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