CV - Clinical Assessment 2

Three tests discussed

  • Nagel Anomaloscope

  • City University Colour Vision Test

  • Medmont C-100

Nagel Anomaloscope

  • Gold standard instrument for red green (RG) colour vision deficiency (CVD) classification.

  • only test able to reliably classify dichromats from anomalous trichromats (AT) and protan types (exact wording in transcript).

  • Test setup:

    • 2-degree circular field, split horizontally.

    • Upper hemifields: mixture of spectral red (670 nm) and green (546 nm) at constant luminance.

    • Lower hemifields: spectral yellow (589 nm) with variable luminance.

  • Based on the “Rayleigh Equation”: R+G=YR+G=Y

  • Scales:

    • Red/Green scale (x-axis): from 0 (pure green) to 73 (pure red).

    • Yellow luminance scale (y-axis): from 0 to 87 (bright yellow maximum).

  • Subject’s matching mid-point, range, and profile define their congenital RG CV status.

  • Key concepts:

    • Mid-point (SFM): the subject’s exact colour-match point on RG vs Y fields.

    • Range: how far the RG field can be shifted while still achieving an exact match via adjustments of the Y field.

    • Profile: the overall shape of the RG vs Y matching across the x-y plane.

Explain to subject how to respond

  • An exact match is required in both colour and brightness; otherwise the subject must report “no match.”

Testing procedure (two-step)

  • Step 1: Subject makes several exact colour matches by simultaneous adjustment of the RG and Y fields.

    • First match is the Subject’s First Match (SFM).

    • If an exact match cannot be achieved, the subject reports “no match.”

    • Repeat this process twice; average the SFM coordinates (x, y) from 3 attempts.

    • Plot this averaged SFM point on the graph.

  • Step 2: Examiner sets the RG field (x-axis) at 5 or 10 units on either side of the SFM and the subject adjusts the Y field (brightness; y-axis) to attempt exact matches.

    • Stop when no match is possible on both sides of the SFM, or if the x-axis limit is reached (0 or 73).

    • Record the range achieved on the graph.

NA - Example 1

  • Step 1 matches: 42/12, 40/13, 41/13 → Average SFM = 41/13.

  • Step 2: Examiner adjusts x-axis to ±5 units around 41 (i.e., x = 36 and x = 46).

    • x = 36: No Match → record 36/ no match

    • x = 46: No Match → record 46/ no match

  • only graph the matched points

NA - Example 2

  • Step 1 matches: 33/15, 25/16, 35/14 → Average SFM = 31/15.

  • Step 2: Examiner sets x-axis at several points and subject’s matches recorded:

    • x=26, subject matches y=16: record 26/16

    • x=21, matches y=16: record 21/16

    • x=16, matches y=17: record 16/17

    • x=11, no match: record 11/No Match

    • x=36, matches y = 14: record 36/14

    • x=41, no match: record 41/No Match

    • Graph shows x-range from 16 to 36 with mid-point at 26.

Nagel Anomaloscope interpretation

  • CV normals: match a single point (typical around ~42/13, with ±2 in x and small y-range).

    • CV can have small x - axis range (1-5 units)

  • Dichromats: will match for all x-axis settings (0–73)

    • cannot differentiate hue settings on RG scale

    • Y-axis matches reflect brightness only.

  • Deuteranopes (deuteranopes)

    • Y-axis value relatively constant (flat) across x-range.

  • Protanopes (protanopes)

    • Sloping function

    • Lower red sensitivity → less bright and Y brightness reduced to match

Anomalous Trichromats (AT) characteristics

  • Mid-points lie on one or the other side of the normal match (42/13):

    • Deuteranomals: x-midpoints < 40 → need to add more green to match.

    • Protanomals: x-midpoints > 50 → need to add more red to match.

  • Range reflects AT severity: a larger range indicates more severe CVD.

  • Severe AT often matches to one end of the x-axis

  • AT usually reject the normal 42/13 match but may accept if the AT is very severe.

Graph interpretation notes (visual patterns from the data)

  • Normal: single, consistent match near 42/13 with small x-range.

  • Dichromats: broad ability to match across x-range; Y-brightness alignment dominates.

  • AT (deutanomal, protanom): midpoints shift left or right; broader ranges indicate greater severity.

  • Severe AT: may cluster near an end of the x-range (0 or 73) and can be difficult to obtain a normal-match.

Severity ranges for AT

  • PA range (severity measure):

    • mild < 10

    • mod 10–15

    • severe > 15

  • Double for DA (likely referring to a dichromat anomaly or a dual-type assessment)

  • Red-green discrimination index (RGI):

    • Formula: RGI = 1 - (patients age / 74)

    • CV normal: RGI ≈ 1 (graphically blue region)

    • Dichromats: RGI = 0 (will accept any RG mixture)

    • AT: RGI falls between 0 and 1 depending on range

City University Test (CUT)

  • Matching test using the D-15 Munsell colours.

  • Format: 4 alternate forced choice, with 10 plates (2nd edition).

  • Page layout: 5 colours per page; 1 central colour and 4 peripheral colours.

  • Task: Select the peripheral dot that most closely matches the central dot.

  • Confusion lines: One peripheral colour is adjacent to the D-15 sequence; the other 3 lie on the D, P, and T confusion lines.

  • Scoring: 2 errors or more on CUT predict a pass/fail on D-15 well.

  • Reasonable test with severity classification similar to D-15.

  • Less sensitive than Ishihara for RG-CVD detection.

  • P vs D sensitivity ~60% (similar to Ishihara).

  • D-15 is better at distinguishing protans from deutan

  • Protans make fewer errors, due to luminous contrast variations

Medmont C-100

  • Handheld electronic device with a small circular flickering disk.

  • Mechanism: Flickering red and green lights in counter-phase.

  • Operational setup: 40 cm working distance; normal room lighting; switch off fluorescent lights if flicker is detected.

  • Test: patient adjusts RG ratio to eliminate or minimize flicker perception (the “null point”).

  • Null point ranges:

    • CV normals: between −1 and +1.

    • Protans: between −2 and −5 (reduced red sensitivity).

    • Deutans: between +2 and +5 (reduced green sensitivity).

  • Experimental protocol: average of 3 settings recorded.

  • Classification role:

    • Colour classification test for RG defects only.

    • Excellent sensitivity/specificity for protan vs deutans.

    • Protans may be less occupationally safe (in both dichromats and AT).

    • Protans show reduced red signal detection by up to ~40%.

    • Cannot screen for CVD; classifies RG defect once RG defect is confirmed.

      • Useful in conjunction with Ishihara.

    • Cannot assess CVD severity.

      • Dichromats cannot be distinguished from AT by this test.

Colour Naming & Occupational Lantern Tests

  • Lantern tests are occupational tools that test colour naming of signals relevant to specific industries (aviation, maritime, armed forces, police, railway, etc.).

    • Determines if signal lights would be identified accurately in the field

  • Generally RG based using:

    • Small stimuli to simulate distance

    • Dimming filters to simulate unfavourable weather conditions

  • Key lanterns:

    • Farnsworth Lantern (FALANT)

    • Holmes Wright Lantern A and B (HW A/B)

    • Eldridge-Green and other lanterns


Farnsworth Lantern (FALANT)

  • Setup: paired red, green, and white stimuli arranged vertically

    • white may appear yellowish and lie on RG confusion lines for CVD.

  • Filters used to remove brightness cues.

  • Procedure:

    • Distance: 2.4 m; moderate room lighting.

    • 1 run= presents 9 colour pairs (all possible combinations) for 2 seconds each.

    • The patient names the top and bottom colour for each pair; responses must be red, green, or white only.

    • Any miscalls (one or both lights) count as 1 error.

  • Interpretation:

    • Errors accumulate across runs; a failure is declared after predefined error totals (as per example runs shown in the transcript).

Holmes Wright B Lantern (HW B Lantern)

  • Used in maritime occupations to simulate ship navigation lights

    • horizontally arranges pairs of red, green, white colours

  • Distance: 1.5 nautical miles; 9 pairs shown after 10 minutes of dark adaptation.

  • Two forms:

    • HW-B (full lantern, brighter/larger lights) and HW-A (vertical arrangement, lights are brighter and larger).

  • Test procedure specifics (illustrated in sample runs):

    • 3 runs are typically used; a single error can lead to failure.

  • Diagnostic tendency:

    • Approximately 98% of CV abnormals will probably fail.

    • About 8% of CV normals may also fail.

Who should be assessed? (General considerations)

  • All school children:

    • Colour often used as an aid to learning; abnormal CV can impact learning.

    • Males more likely to have CV defects, but implications similar for females.

    • Severe CV defects observed early (incorrect colour naming or choosing colours) may be noted from an early age.

    • If CV defect is mild, errors may be subtle or unnoticed; screening helps identify risk.

  • Identifying CV deficiencies in adulthood is often late for career planning.

  • If a CV defect is detected, discuss implications for career paths early.

Assessment of CV in children

  • Age considerations:

    • Testing can start around age 4.

    • By age 6+, tests become more reliable.

    • Screening is more likely until around age 8.

    • D-15 and Nagel tests become more reliable from around age 10+; Nagel may not be reliable under 10.

  • always confirm findings as child ages (tentative dx for young children)

  • treat with urgency if evidence of acquired CVD in a child → Possible neuro-ophthalmic disease

  • Screening tools for children:

    • Ishihara PIC plates: generally adequate for RG defects by age 6; Ishihara for children from age 4; shapes better than trails.

    • Matching approaches: Colour Vision Testing Made Easy (CVTME) – pictures/shapes but RG defects only.

    • Matsubara/Ishihara for Children (Matsubara Ishihara for Children).

    • Guy’s tests; CVTME; Matsubara

    • others exist but validation varies.

    • early role is identifying risk and managing parental anxiety by providing information and reassurance

Who should be assessed:

  • All px with the folloiwng?

    • reporting colour disturbances or difference between eyes

    • taking medications known to affect CV

    • with confirm or suspected ocular, neurological or systemic disease (e.g., reduced VA, VF defects, RAPD)

  • All px considering specific occupations:

    • With known CV occupational requirements

      • Aviation, maritime, armed forces, police, railways, fire brigade

    • Where normal CV would be an advantage, but not a requirement

      • Textiles, fashion, paint, electronics

Test battery in general practice

  • Recommended minimum:

    • PIC plates (Ishihara) or Richmond HRR;

    • D-15 panel test for severity indication (P, D, T dichromats and AT severity);

    • Medmont C-100 to differentiate Protan vs Deutan defects (not CVD severity).

  • Practical note: In practice, clinics may have fewer tests; the goal is to identify RG defects and provide basic vocational/lifestyle guidance.

Test battery in a CV clinic

  • Ishihara PIC (and Richmond HRR PIC) used for initial screening.

    • Pass on Ishihara: further testing not routinely indicated.

    • Fail on Ishihara: proceed to classification of type and severity.

  • Medmont C-100 differentiates Protan/Deutan defects but does not provide severity.

  • D-15: Identifies and classifies Protan, Deutan, and Tritan dichromats and moderate/severe AT

    • cannot separate mild AT from very mild AT.

    • L’anthony Desat panel provides separation of mild vs very mild AT.

  • Anomaloscope: definitive RG CVD classification (gold standard).

  • Farnsworth Lantern: vocational aptitude assessment for aviation and other industries.

Classifications: Protans vs Deutans

Classifications: Tritans

Determining CVD severity

Agreement between tests

  • Performance on CV tests does not always align across tests.

    • Ishihara is a poor predictor of CV severity and does not reliably predict outcomes on D-15, Nagel Anomaloscope, or Farnsworth Lantern.

  • Can clinical CV tests predict if people with CVD pass or fail the Farnsworth Lantern?

    • D-15: 67% of those who fail D-15 (2+ diam crossing) fail Farnsworth Lantern; 94% who pass D-15 pass FL.

    • Nagel: 87% who fail Nagel (range > 10 units) fail FL; 57% who pass Nagel pass FL.

    • Neither Nagel nor D-15 alone is a perfect predictor of FL performance.

  • Approximately 35% of CVD cases pass Farnsworth Lantern

    • mostly mild deuteranomals

    • some may still fail simulated flight tasks (e.g., PAPI).