W8 - Accommodation Vergence relationship

Analytical approaches (overview)

• There are multiple approaches to analyse accommodation and vergence clinical data.

• Goals across approaches:

  • Diagnose binocular vision dysfunctions.

  • Confirm normal binocular function after clinical assessment.

• Approaches include:

  • Graphical analysis

  • Analytical analysis

  • Morgan’s normative analysis (normative analysis)

  • Fixation disparity analysis

  • Integrative analysis approach (framework of this course)

• Each approach uses different data representations and decision rules, often complementing one another in clinical decision-making.

Graphical analysis

• Purpose: Plot clinical accommodation and vergence findings to determine whether the patient has clear, single, comfortable binocular vision.

• Clinical measures commonly plotted:

  • Dissociated phoria

  • Negative fusional vergence (BI): blur/break/recovery

  • Positive fusional vergence (BO): blur/break/recovery

  • Negative relative accommodation (NRA)

  • Positive relative accommodation (PRA)

  • Amplitude of accommodation

  • Amplitude of vergence (calculated using near point of convergence (NPC) and PD)

• Distance scale at bottom, near scale at top.

  • 40cm working distance at 0 vergence demand.

  • Cross is accommodative demand (2.50D)

• Visual representation: x-axis = vergence (Δ); y-axis = accommodative stimulus (D).

• Coordinate mapping:

  • Vergence at 6 m is on the bottom axis; vergence at 40 cm is on the top axis.

  • Accommodative stimulus (D) on the y-axis.

• Reference lines and lines:

  • Horizontal dotted line at 2.50 D accommodative stimulus (corresponding to 40 cm = 1/0.40 m).

  • Vertical dotted line at 15 Δ BO (convergence demand at 40 cm with PD = 60 mm, i.e., 6.0/0.40 = 15 Δ).

  • Demand line (bold curved line) showing combined accommodative and convergence demand across levels of accommodative stimulus.

  • Orthophoria at all working distances along the demand line when aligned with the visual axes.

• Key plotted data points (examples from slides):

  • Dissociated phoria

  • BO (base-out) blur/break/recovery

  • BI (base-in) blur/break/recovery

  • NRA/PRA

  • Amplitude of accommodation

  • Amplitude of vergence (via NPC and PD)

Graphical analysis – procedure (stepwise)

• Step 1: Plot amplitude of accommodation and amplitude of convergence.

  • Amplitude of accommodation = AA=\frac{1}{\text{NPA}}

  • Example: if NPA = 11 cm = 0.11 m, then \text{AoA} = \frac{1}{0.11} = 9\,\text{D}

  • Amplitude of convergence = A_{conv} = \left(\frac{1}{NPC}\right) \times PD

  • Example given: NPC = 7 cm (0.07 m), PD = 6 cm; A_{conv} = \left(\frac{1}{0.07}\right) \times 6\text{ cm} \approx 86\Delta

    • 70-90 is the common range.

• Step 2: Plot distance and near phorias.

  • Connect with a straight line and extend the line to the amplitude of accommodation line.

  • ORTHO is bottom scale

  • base in = exo

  • base out = eso

  • near working distance - plot point at 40cm working distance line (horizontal dotted line).

• Step 3: Plot BO to blur / break findings.

  • Connect with a straight line and extend to the amplitude of accommodation.

  • Plot BO to break findings similarly.

  • repeat for distance and near

• Step 4: Plot BI to blur / break findings.

  • Connect with a straight line and extend to the amplitude of accommodation.

  • If there is no blur point, draw the line through the break finding.

  • Plot BI to break findings analogously.

  • repeat for distance and near

• Step 5: Plus lens to blur finding (NRA).

  • Represents the limit of positive fusional vergence as plus is added.

  • Should line up with the BO to blur finding.

• Step 6: Minus lens to blur finding (PRA).

  • Represents the limit of negative fusional vergence.

  • Generally, does not line up with the BI to blur finding due to minification effects of minus lenses.

Graphical analysis – interpretation

• Zone of clear, single binocular vision (BSV):

  • Bounded by BI and BO to blur lines.

    • clear zone - purple lines on graph

      • NRA/ PRA line to be in the centre

    • zone of binocular vision - blue lines on graph

  

  • Zero accommodative stimulus line marks alignment with the baseline.

  • The line representing amplitude of accommodation sets the top boundary.

  • Zone height depends on AoA (amplitude of accommodation).

  • Zone width depends on the amplitude of fusional vergence (positive and negative) at corresponding accommodative stimulus levels.

• Accommodative convergence (AC/A) relationship:

  • Indicated by the slope of the phoria line.

  • Steeper slope => lower AC/A ratio; flatter slope => higher AC/A ratio.

• AC/A ratio considerations:

  • Low AC/A: smaller change in vergence for a given accommodation change; potential for convergence insufficiency or accommodation-driven issues.

  • High AC/A: larger vergence change per diopter of accommodation; potential for convergence excess or accommodative problems influencing vergence.

• Percival’s criterion (PC):

  • Demand line should lie in the middle third of the zone of clear, single BV.

  • If not, there is an increased risk of asthenopia.

• Sheard’s criterion:

  • Compensating vergence reserve should be about twice the initial phoric demand.

  • If not, there is an increased risk of asthenopia.

  • green point to vertical dotted by dotted to purple.

Advantages of Graphical Analysis:

• Helpful for understanding relationships between individual parameters

  • Width of zone of clear BSV and zone of BSV

  • Relationship between phoria and fusional reserves

  • Relationship between NRA/PRA and fusional reserves

  • Limits imposed by NPA and NPC

  • AC/A ratio

Disadvantages of graphical analysis:

• Doesn’t cover key parameters

  • Accommodative posture

  • Accommodative facility

  • Vergence facility

  • Fixation disparity

• These are covered in integrative analysis approach

• Cumbersome and time consuming

  • Usefulness limited to consolidate your understanding of binocular vision, through visually representing data

Analytical analysis

• Developed by OEP (optometric extension program)

• 21-point examination with precise instructional sets

• clinical findings are then compared to a table of expected findings

• data is grouped and conditions identified

Limitations

• rigidity of 21 point examination

• must be familiar with specific OEP protocols; requires further study / training

• OEP has different definitions for accommodation, convergence, phoria etc.

Morgan’s normative analysis (Normative analysis)

• Developed approach by Morgan; also called normative analysis.

• Core idea: treat results as group data, not single-point deviations.

• Procedure:

  • Compare individual findings to Morgan’s table of expected findings.

  • Look for a trend across Group A and Group B findings.

  • If a group, as a whole, varies in a given direction, it is clinically significant.

• Group data descriptions:

  • Group A data: distance NFV (break), near NFV (blur/break), near NFV (break), PRA, AoA.

  • Group B data: distance PFV (blur/break), near PFV (blur/break), binocular + monocular fused cross-cylinder, NRA.

  • Group C data: Phoria, AC/A ratio

• Interpretive rules:

  • If Group A findings are high and Group B findings are low: suggests a convergence problem.

  • If Group A findings are low and Group B findings are high: suggests accommodative fatigue.

  • Group C findings help decide whether lenses, prism, or vision therapy should be recommended.

• Limitations:

  • Does not account for accommodative facility, vergence facility, fixation disparity.

Fixation disparity - revision

• Occurs in binocular single vision

• Image is seen singly despite slight under/over convergence of visual axes, when disparate retinal points are WITHIN Panum’s area

  • Manifest deviation of the eyes

  • Generally <10 mins of arc (always less than 1 PD / 30 mins of arc)

  • Measured with Sheedy disparometer, Wesson card

• Aligning prism/associated phoria = amount of prism required to reduce fixation disparity to zero (correct misalignment)

  • Occurs within Panum’s fusional area (no diplopia)

  • Measured with Mallett unit, Bernell lantern

Fixation disparity analysis

• Fixation disparity curves are generated

• As vergence demand is altered, fixation disparity may change

• Plot fixation disparity in 3\Delta increments (both BI then BO, until diplopia)

• Typically, an increase in BO increases exo fixation disparity and an increase in BI increases eso fixation disparity

• Four diagnostic characteristics of curve are analysed:

  • Type

  • Slope

  • x-intercept

  • y-intercept

Curve type (Fixation disparity curves)

• Curve type reflects how the system adapts to prism demands

• Generally adapt more in one direction (BI or BO) than the other; variations determine curve shape (type)

• Type I = equal adaptation to BI and BO; asymptomatic

• Type II = more adaptation to BO prism, less adaptation to BI prism

• Type III = more adaptation to BI prism, less adaptation to BO prism

• Type IV = unstable binocularity

Type I curve

• Type I = 60% of population

• x- and y-intercept = 0

• Vision therapy has best prognosis

Type II curve

• Type II = 25% of population

• ‘Eso’-curve (but not always)

• Prism has best prognosis

Type III curve

• Type III = 10% of population

• ‘Exo’-curve (but not always)

• Prism or vision therapy (fusional reserves) may be useful

Type IV curve

• Type IV = 5% of population

• Unstable findings

• Strong tendency to prism adapt

• Poor prognosis

SHELLY SAYS DON’T SPEND A LOT OF TIME UNPACKING THIS - she skipped over it in the lecture.

Curve slope

• Estimated by determining change in fixation disparity between prism demands of 3\Delta BO and 3\Delta BI

• Flat fixation disparity curves:

  • Prism is effective at shifting centre of symmetry (middle of flattest portion) towards y-axis

  • Reduces symptoms, improving binocularity and allows lower prism than dissociated/associated phoria measures

• Steep fixation disparity curves:

  • Treat with vision therapy

  • If curves do not flatten with vision therapy, prism may be prescribed based on associated phoria

x-intercept (associated phoria) and y-intercept (fixation disparity)

• x-intercept = associated phoria; prism amount to reduce fixation disparity to 0

• Use of only lateral associated phoria for prism prescribing is not adequate

• Need to consider slope, curve type and fixation disparity

• Otherwise, can overestimate prism corrections for esodeviations

• Guideline: for horizontal prism in esodeviations, start with \frac{1}{3} of distance phoria, then assess vergence ranges, associated phoria, and stereoacuity

• y-intercept (fixation disparity) = small misalignments of visual axis under binocular conditions

y-intercept (fixation disparity)

• Small misalignments of visual axis under binocular conditions

• Fixation disparity measurements are generally not measured clinically

• Common clinical tools: Wesson card, disparometer

Advantages of fixation disparity analysis

• Clinical measures are taken under binocular conditions (more natural)

• Useful in determining which patients are likely to have symptoms

• Guides to optimal management strategy (prism or vision therapy)

• An effective method for determining the amount of prism to prescribe

Disadvantages of fixation disparity analysis

• Does not provide direct measures of accommodation or ocular motility

Integrative analysis

• Three distinct steps:

  • Compare individual tests to a table of expected findings

  • Group findings that deviate from expected findings

  • Identify syndrome/condition

• Similar concept to analytical analysis but without the rigidity of the 21-point clinical tests

Integrative analysis - expected findings (overview)

• Tables of expected values cover binocular vision testing and accommodative testing

• Tests include: cover test, lateral phoria, AC/A ratio, smooth vergence, base-out/in; break/recovery/blur values; vergence facility; NPC; MEM; FCC; accommodative tests

• Important to consider both mean values and standard deviations (SD) when interpreting resultsIntegrative analysis - expected findings: NSUCO SACCADE TEST minimal acceptable score by age and sex

• Provides age- and sex-specific normative scores for accuracy, head movement, and body movement during saccades

• Used to evaluate ocular motor function as part of integrative analysis

• Scores decline with age; differences exist between sexes in some categories

Integrative analysis – grouping data

• Data are grouped into 6 test groups:

  • Positive fusional vergence (PFV)

    • NPC

    • NRA

    • positive fusional vergence

  • Negative fusional vergence (NFV)

    • PRA

    • minus lenses for binocular accommodative facility

  • Accommodative system

  • Vertical fusional vergence

  • Ocular motor system

  • Motor alignment and interaction test

Integrative analysis – PFV group data

• PFV components included:

  • PFV from smooth vergence testing - Risley Prism

  • PFV from step vergence testing - prism bar

  • PFV from vergence facility testing - BO

  • Negative relative accommodation (NRA)

  • Binocular accommodative facility (BAF, through plus)

  • Near point of convergence (NPC)

  • MEM retinoscopy / FCC

• PFV behavior:

  • As BO is added, patient must converge to maintain bifoveal fixation and maintain accommodation at a given level (D or N)

  • As patient converges, accommodative response increases due to increased vergence accommodation

  • Patient must relax accommodation to counterbalance increased vergence accommodation (NRA)

  • When relaxation limit is reached, blur occurs

  • Accommodative convergence cannot be used when maintaining a fixed accommodation level; when accommodative convergence assists, blur is reported

Integrative analysis – PFV group data (continued)

• Vergence facility testing:

  • Needs to maintain accommodative level at 2.50 D while using 12\Delta of PFV to restore bifoveal fixation

  • If lag of accommodation is normal, accommodative response for a 2.50 D stimulus will be 1.75–2.00 D

  • If sufficient fusional vergence, single and clear; if need both fusional and accommodative vergence, may be single but blurred

  • If cannot restore binocularity – diplopia

• Negative relative accommodation (NRA):

  • Indirect measure of PFV; plus lenses added

  • Patient must relax accommodation to keep target single and clear; relaxation reduces accommodative convergence

  • Amount depends on AC/A ratio; NRA endpoint may reflect decreased PFV or inability to relax accommodation

    • Differentiate via monocular testing

• Binocular accommodative facility (BAF):

  • Similar logic to NRA; must relax by ~2.00 D to keep clear

  • If AC/A = 5:1, divergence will be 10\Delta; thus 10\Delta PFV required to compensate for divergence

• NPC:

  • Engages accommodative convergence, PFV and proximal vergence; reduced PFV may recede NPC; indirect measure of PFV

• MEM/FCC:

  • Tested binocularity; normal MEM/FCC is +0.25 to +0.50 D; low PFV or exophoria often show less plus (overaccommodation); substituting accommodative convergence for PFV

Integrative analysis – NFV group data

• NFV components include:

  • NFV from smooth vergence testing

  • NFV from step vergence testing

  • NFV from vergence facility testing

  • Positive relative accommodation (minus lenses used)

  • Binocular accommodative facility (BAF, through minus)

  • MEM retinoscopy / FCC

• NFV behaviour:

  • As BI is added, patient must diverge to maintain bifoveal fixation and maintain accommodation at a given level (D or N)

  • As patient diverges, accommodative response decreases due to decreased vergence accommodation

  • Patient must stimulate accommodation to counterbalance decreased vergence accommodation

Integrative analysis – NFV group data (continued)

• Vergence facility testing:

  • Needs to maintain accommodative level at 2.50 D while using 3\Delta of NFV to restore bifoveal fixation

  • If lag of accommodation is normal, accommodative response for a 2.50 D stimulus will be 1.75–2.00 D

  • If sufficient fusional vergence, will be single, clear; if using a decrease in accommodative convergence to aid fusional vergence, will be single but blurred; if cannot restore binocularity – diplopia

• PRA (positive relative accommodation):

  • Indirect measure of NFV; patient must maintain convergence at a given level while changing accommodative response

  • Minus lenses added to stimulate accommodation; this increases accommodative convergence

  • Amount depends on AC/A ratio

  • NFV is required to counteract convergence (increasing accommodative convergence) to avoid diplopia

  • PRA endpoint may reflect decreased NFV or inability to stimulate accommodation

    • Differentiate through monocular testing

• Binocular accommodative facility (BAF):

  • Similar to PRA; must stimulate 2 D of accommodation to keep clear; reflex increase in accommodative convergence

  • If AC/A = 5:1, convergence will be 10\Delta; thus 10\Delta NFV required to compensate for convergence

• MEM/FCC:

  • Tested binocularly; normal MEM/FCC is +0.25 to +0.50 D

  • When NFV is low, there may be more plus (underaccommodation) to substitute for PFV

  • Reducing accommodation can reduce demand on NFV

Integrative analysis – accommodative system group data

• Group includes:

  • Monocular amplitude of accommodation (AA)

  • Monocular accommodative facility (MAF) with plus and minus

  • MEM retinoscopy

  • Fused cross-cylinder (FCC)

  • Negative relative accommodation (NRA) and Positive relative accommodation (PRA)

  • Binocular accommodative facility (BAF)

  • Binocular amplitude of accommodation

• Monocular amplitude of accommodation:

  • Total amount of accommodation available

  • Repeated testing (3–4x or at end of exam) provides information on sustaining ability

  • Low MAA in pre-presbyopic patients suggests accommodative insufficiency; supporting findings include poor MAF through minus, poor BAF through minus, reduced PRA

  • Adequate MAA does not guarantee absence of accommodative anomaly (e.g., facility issues or reduced NRA)

• Monocular accommodative facility (MAF):

  • Ability to make rapid and large changes in accommodative level and sustain over 60 seconds

  • Outcomes:

    • Adequate performance with both plus and minus

    • Inadequate performance with both plus and minus

      • Indicative of accommodative infacility

      • Supporting findings: reduced BAF (plus and minus), low PRA and NRA

    • Adequate performance with minus, inadequate with plu

      • Overaccommodation, accommodative spasm or accommodative excess

      • Can be secondary to binocular vision condition; e.g. convergence insufficiency will overaccommodate to use accommodative convergence, constant overaccommodation can lead to accommodative spasm

    • Adequate performance with plus, inadequate with minus

      • Difficulty stimulating accommodation, will have reduced amplitude of accommodation

      • Accommodative insufficiency, ill-sustained accommodation

        Supporting findings: low PRA, high MEM and FCC

• MEM retinoscopy / FCC:

  • Measure of actual accommodative response

  • Normal: +0.25 to +0.50 D for a 2.50 D stimulus

  • More plus suggests accommodative insufficiency; supportive findings: low PRA, high FCC, reduced MAF/BAF through minus

  • Less plus suggests accommodative spasm or excess; supportive findings: low NRA, reduced MAF through plus

  • lead = over accommodation

    • might be sign of an accommodative spasm

  • lag = under accommodation.

    • accommodative insufficiency

• Binocular accommodative facility (BAF):

  • Useful for identifying an accommodative anomaly when binocular findings are normal

  • If abnormal binocular findings, BAF alone cannot distinguish between isolated accommodative anomaly or binocular vision anomaly

Integrative analysis – vertical fusional vergence data

• Supravergence and infravergence

  • Right infravergence is a compensatory mechanism for right hyperdeviation

    • BU in front of RE

  • Right supravergence is a compensatory mechanism for right hypodeviation

    • BD in front of RE

  • Also applicable to left eye

• Vertical fixation disparity testing

  • Associated phoria = prism amount that reduces fixation disparity to zero

  • Accepted method of prism correction for vertical deviations

• vertical fusional vergence is common in patients with a 4th nerve palsy.

Ocular motor data

• Fixation status

• Saccadic ability

• Pursuit ability

Diagnosing conditions (Integrative analysis - diagnosing condition)

• Heterophoria with:

  • Low AC/A

    • Convergence insufficiency or Divergence insufficiency (depending on pattern)

  • Normal AC/A with:

    • Fusional vergence dysfunction (orthophoria D and N)

    • Basic esophoria or basic exophoria

  • High AC/A

    • Convergence excess or Divergence excess

  • Vertical heterophoria

• Accommodative anomalies:

  • Accommodative insufficiency

  • Ill-sustained accommodation

  • Accommodative excess

  • Accommodative infacility

• Ocular motor dysfunction

If signs & symptoms are greatest at near → convergence insufficiency or excess

• insufficiency: exo at near

• excess: eso at near

  • give minus at near due to high AC/A

if signs & symptoms are greater at distance → divergence insufficiency or excess

• insufficiency: eso at distance

• excess: exo at distance

  • give minus at distance due to high AC/A

low AC/A (insufficiency) don’t respond well to plus or minus → generally require prism (BO)

Integrative analysis - advantages

• Includes analysis of ocular motor, accommodative facility, vergence facility, MEM retinoscopy and fixation disparity data

• No other approach uses all of this data