How is hypotony (intraocular pressure (IOP) abnormality) defined and graded?

Definition and Grading of Hypotony

Hypotony is most commonly defined as intraocular pressure (IOP) ≤5 mm Hg, though published definitions vary widely from 4-8 mm Hg, and clinically significant hypotony should be defined by the presence of associated pathologic signs rather than IOP threshold alone. 1

Numerical Definitions

The literature reveals substantial heterogeneity in how hypotony is defined numerically:

• Most common threshold: IOP ≤5 mm Hg is the most frequently used definition in clinical practice and research 2, 3
• Range of published definitions: A systematic review identified 14 different numerical definitions ranging from 4-8 mm Hg 1
• Lack of consensus: Among 128 eligible papers reviewed, 41.4% did not define hypotony at all, and among those that did, definitions varied substantially 1

Clinically Significant Hypotony

The most clinically relevant approach focuses on hypotony with associated pathologic signs rather than IOP number alone 1:

Key Clinical Signs

• Hypotony maculopathy: Chorioretinal folds, optic disc edema, or vision loss 1
• Choroidal detachment: Serous or hemorrhagic 1, 2
• Shallow anterior chamber: May indicate wound leak or overfiltration 4
• Corneal changes: Striae, edema, or Descemet's folds 5

Temporal Considerations

Only 32% of published definitions include onset and duration as part of the hypotony definition 1:

• Transient hypotony: Early postoperative hypotony (first 24-48 hours) often resolves spontaneously without sequelae 2, 6
• Persistent hypotony: Defined as IOP ≤5 mm Hg at or after 6 months postoperatively in most studies 3
• Clinical significance: Duration matters—brief hypotony without signs may not require intervention, while persistent hypotony with maculopathy demands aggressive management 1

Grading Systems

While no universally accepted grading system exists, hypotony can be stratified by severity:

Mild

• IOP 4-5 mm Hg without clinical signs 1
• No visual symptoms 2
• Normal anterior chamber depth 4

Moderate

• IOP 2-3 mm Hg with shallow anterior chamber 4
• Choroidal effusions without macular involvement 2
• Transient visual symptoms 1

Severe

• IOP <2 mm Hg with hypotony maculopathy 1
• Extensive choroidal detachment 3
• Significant vision loss 4
• Risk of phthisis bulbi 5

Clinical Context and Measurement

The definition must account for measurement limitations and clinical context 5, 7:

• Goldmann applanation tonometry is less reliable in abnormal corneas 5
• Alternative methods (pneumotonometer, rebound tonometry, digital palpation) may be needed in corneal edema or post-surgical states 5, 7
• Chronic serous choroidal detachment may lead to hypotony and secondary corneal edema 5

Impact on Reported Outcomes

Definition-dependent hypotony rates in the same patient cohort varied from 1% to 59.3%, demonstrating that inconsistent definitions severely compromise the ability to compare outcomes across studies 1. This variation has major implications:

• Different definitions yield vastly different success/failure rates for surgical procedures 1
• Sensitivity and specificity for identifying clinically significant hypotony vary dramatically among published definitions 1
• A more robust universal definition focusing on clinically important signs is needed 1

Common Clinical Pitfalls

• Ignoring clinical signs: Focusing solely on IOP number without assessing for maculopathy or choroidal detachment misses clinically significant hypotony 1
• Premature intervention: Transient early postoperative hypotony often resolves spontaneously within 7 days and may not require treatment 2
• Measurement errors: Using Goldmann applanation in eyes with corneal edema, irregular surfaces, or post-surgical changes yields unreliable readings 5, 7
• Inconsistent follow-up: Not specifying when hypotony is measured (immediate postoperative vs. 6 months) makes comparison impossible 1