How do you interpret Octopus (perimeter) test results in an adult patient with a history of eye conditions or neurological disorders?

Interpreting Octopus Perimeter Visual Field Results

Octopus perimetry provides automated threshold-based visual field assessment that is particularly valuable for detecting and monitoring glaucomatous defects, with superior sensitivity compared to manual perimetry, though it has limitations in neurological field defects where Goldmann perimetry remains superior. 1

Understanding Key Octopus Output Parameters

The Octopus perimeter generates several critical field indices that guide interpretation 2:

• Mean Differential Light Sensitivity (mDLS): Represents average threshold sensitivity across tested points, declining approximately 0.5-1 dB per decade of age 3
• Mean Defect (MD): Quantifies overall depression of the visual field compared to age-matched normals 2
• Loss Variance (LV): Measures irregularity of the field, with increased values indicating localized defects 2
• Corrected Loss Variance (CLV): Adjusts LV for short-term fluctuations, providing more reliable assessment of true scotoma depth 2

Systematic Interpretation Approach

Step 1: Assess Test Quality

• False positive/negative rates: Should be <15% for reliable interpretation 4
• Test duration: Modern G-TOP algorithm typically completes in approximately 2 minutes 12 seconds 4
• Fixation stability: Poor fixation can create artifactual defects, particularly in patients with central scotomas 5

Step 2: Identify Pattern of Defects

For Glaucoma Suspects 4, 6:

• Diffuse defects: Generalized depression with MD typically >20 dB, seen in 17% of glaucomatous eyes 4
• Localized defects (83% of glaucomatous eyes) 4:
  • Paracentral scotomas (most common early finding)
  • Nasal step defects
  • Arcuate defects
  • Half ring-shaped defects
  • Concentric defects with central island (advanced disease)
• Inferior hemifield involvement: Most frequently affected in glaucoma 4

Critical Pitfall: Increased fluctuations around normal threshold represent the earliest detectable glaucomatous changes, often before visible scotomas develop 6. These fluctuations are reversible in early relative scotomata 6.

Step 3: Correlate with Clinical Findings

The Octopus results must be interpreted alongside 5, 4:

• Optic nerve appearance: Cup-to-disc ratio, rim thinning, disc hemorrhages
• Intraocular pressure: Current and historical measurements
• Central corneal thickness: Affects IOP accuracy
• Best-corrected visual acuity: MD and sLV correlate with BCVA (P<0.001) 4
• Number of prior glaucoma surgeries: Correlates with field defect severity 4

Step 4: Recognize Age-Related Changes

Normal aging causes predictable field changes 3:

• Linear decline in threshold sensitivity: Approximately 0.5 dB per decade at fixation
• Accelerated peripheral decline: Sensitivity at 30° eccentricity declines twice as rapidly as at fixation 3
• Volume and surface area reduction: Progressive with age 3

Avoid This Pitfall: Do not over-interpret mild peripheral depression in elderly patients as pathologic without considering age-matched normative data 3.

Special Considerations for Neurological Defects

Octopus has significant limitations for neurological field defects 1:

• Goldmann perimetry remains superior for detecting and characterizing hemianopsias, quadrantanopsias, and other neurological patterns 1
• Octopus may miss subtle homonymous defects that Goldmann kinetic testing readily identifies 1
• For suspected chiasmal lesions, stroke, or other neurological causes, supplement Octopus with Goldmann or confrontation testing 5

Monitoring Progression

Threshold perimetry is essential for detecting progression 6:

• Suprathreshold stimuli are too crude to identify subtle worsening 6
• Serial Octopus testing allows mathematical/statistical trend analysis using Delta program 6
• Compare MD, LV, and CLV values across visits 2
• Rapid fluctuations in scotoma depth are normal and do not necessarily indicate progression 6

Clinical Decision Points

When Octopus shows defects 5:

• Mild glaucoma: Definite optic nerve/RNFL abnormalities with normal visual field
• Moderate glaucoma: Field abnormalities in one hemifield not within 5° of fixation
• Severe glaucoma: Defects in both hemifields and/or loss within 5° of fixation in at least one hemifield 5

Screening efficiency: Octopus identifies >90% of patients with glaucomatous field loss, making it excellent for screening suspected glaucoma 1.