Neurophysiology of Accommodation

• Accommodation involves changing the dioptric power of the lens via ciliary muscle contraction.
• All stimuli to accommodate occurs at the retina, specifically the macula (fovea).
• Accommodation is parvo-mediated.

Neurophysiology of Accommodation

• The afferent pathway begins with stimulation of cones due to a defocused retinal image.
• The signal travels from the DLGN to cortical area 17, then parieto-temporal areas.
• This neural signal transforms to a motor command in the midbrain oculomotor nucleus complex/Edinger-Westphal nucleus.
• The efferent pathway involves the III cranial nerve -> ciliary ganglion -> short ciliary nerve -> ciliary muscle.

Components of Accommodation

• Accommodation includes four basic components: tonic, proximal, blur/reflex, and vergence.
• Tonic accommodation is the amount exerted in a totally dark room.
• It's present in total darkness and provides a constant stimulus, whether eyes are open or closed.
• Proximal accommodation is related to the apparent nearness of a target and is psychophysical.
• Blur or reflex accommodation is driven by optical divergence from the target.
• Vergence-accommodation is vergence accommodation dependent on the CA/C.
• Pure convergence drives accommodative response.
• This is a minimal component, difficult to measure in a clinic.
• Blur reflex accommodation is most focused on when measuring.

Stimuli

• Stimuli for accommodative response includes proximity, retinal blur, vergence/accommodation relationship, and chromatic aberrations.
• Accommodative dysfunctions are common functional treatable problems in school-aged children.
• Most are functional, indicating a problem between processing visual stimulus to exerting a motor response.
• Most activities are done at near.

Accommodative Skills Evaluation

• Evaluating accommodative skills requires assessing the state of the accommodative system.

Amplitude of Accommodation

• Amplitude of accommodation refers to how much accommodation a patient has available, assessing its facility and accuracy.
• Methods to assess:
• Push-up/Donder's: Involves moving a target closer, increasing magnification, and noting when the image blurs.
• Starting at 20/20 at 40 cm, moving to 20 cm results in 20/40.
• The closer the target, the larger the magnification.
• Pull-away method: A procedure involving moving a target further away
• Sheard's/minus to blur (standard test): Conducted under monocular conditions.
• As minus lenses increase, the target is minified.
• Add the working distance to the blur value to get the Amplitude of Accommodation (AoA).
• For example, adding 6 D until blur makes AoA 8.50 due to the patient exerting 2.50 D initially.
• Bell retinoscopy: Near point retinoscopy performed under binocular conditions, but less accurate for measuring AoA.
• Compare amplitude results with expected values based on:
• Donder's table
• Hofstetter's formula (for 8 years or older):
• Maximum: Max = 25 - 0.4(age)
• Average: Avg = 18.5 - 0.3(age)
• Minimum (most used): Min = 15 - 0.25(age)

Accommodative Facility

• Accommodative facility is how efficiently a patient can stimulate and relax accommodation.
• Assessment method uses +2.00 flippers.
• Procedure: monocular and binocular (indirect).
• Under monocular conditions, measure facility by placing lenses and asking the patient to indicate when clear while reading aloud for a full minute.
• Observations include the cycles per minute, the lens causing difficulty, and whether cycles remain steady or increase in latency.
• Expected findings are more important than the three observations.
• Monocular norms include for 6 y/o: 5.5 +/-2.5 cpm, 7 y/o: 6.5 +/- 2.0 cpm, 8-12 y/o: 7 +/- 2.5 cpm, 13-30 y/o: 11 +/-5 cpm
• Binocular norms include for 6 y/o: 3 +/- 2.5 cpm, 7 y/o: 3.5 +/-2.5 cpm, 8-12 y/o: 5 +/- 2.5 cpm, 13-30 y/o 8 +/- 5 cpm, 30-40 y/o 9 +/- 5 cpm
• Amplitude scaled facility does not provide accurate facility information, as it measures AoA under binocular conditions (NPC).
• It considers the patient's amplitude of accommodation and the test distance (= 45% of amplitude = 1/(amp x 0.45), rounded to the nearest 0.5 cm.
• The lens power range = 30% of amplitude, rounded to the nearest 0.25 cm.
• This is used in non-presbyopic patients.

Accommodative Response

• Accommodative response determines the accuracy of exerted accommodation compared to the stimulus to accommodate.
• Problems with accommodative response testing: All indirect tests performed under binocular conditions for accommodative/vergence response may affect results.
• Factors affecting accommodative response include depth of focus due to pupil size, AC/A ratio, pupil size, and target color. Measurement looks for a lag of accommodation.
• Pupillary miosis necessitates increasing depth of focus, leading to the accommodation system exerting the minimum effort for a clear target.
• Methods of Assessment:
• MEM retinoscopy (near point ret) is performed binocularly in full room illumination outside of the phoropter, with the patient wearing a trial frame distance Rx.
• The average expected lag of accommodation is +0.50 (±0.50), reaching up to +1.00.
• Negative values indicate a problem; OVER +1.00 indicates a significant issue.
• The doctor places the near card and retinoscope at Harmon's distance (knuckle to forearm) for testing.
• Correct the eyes fully, estimate the lens needed to move WITH motion, and use loose lenses briefly to neutralize the movement (less than 0.5 seconds).
• The patient reads aloud.
• No significant difference is expected; a >0.25 difference suggests a refraction error.
• Record MEM results and compare.
• Binocular crossed cylinder (BCC, FCC, X-cyl): BCC is indirect, +0.50 +/- 0.50 is the expected finding. Negative means a problem and anything over +1.00.
• Negative and positive relative accommodation (NRA/PRA) are performed with plus lenses, then minus lenses. NRA cannot exceed 2.50; exceeding this indicates misunderstanding or incorrect refraction.
• PRA can be over 2.50 in young patients, stop usually at -3.00.
• NRA +2.00 +/-0.50, PRA -2.37 +/-1.00 are the method and expected results.
• Adding lenses changes the accommodative stimulus more than the convergence stimulus, but changes the accommodative and vergence responses.
• Other near-point retinoscopies: Bell retinoscopy, stress-point retinoscopy, and book retinoscopy can also be used.

Accommodative Diagnosis

• Accommodative diagnoses have specific patterns.
• 6-20% of the general population have accommodative dysfunctions
• Symptomatic patients sees more, with 60-80%.
• Most cases are functional. Diagnoses may be combined. General symptoms:
• Variable distance/near acuity, tendency to fall asleep/lose concentration when reading, eyestrain/fatigue, headaches above the eyes (usually absent on weekends), light sensitivity, difficulty seeing small print, holding reading material close, tendency to miss-call words.
• General signs include variable near acuity during assessment, fluctuating pupillary response while reading, refractive error inconsistencies, fluctuating retinoscopy reflex, inability to obtain clear stable acuity.
• An accommodative case analysis should rule out refractive conditions and pathology to find accommodations-related findings.
• Determine findings within/outside expected ranges, and determine general behavior. Each diagnosis has a pattern of difficulty.
• Potential issues include difficulty with minus/plus lenses, stamina, equal latency, or increased latency with lens changes.

Accommodative Insufficiency

• Accommodative Insufficiency means not having the minimum expected A of A and is the most prevalent type of issue, at 84%.
• General symptoms: common for all
• Classic: blurred vision when looking from distance to near and errors copying from the blackboard
• Signs:
• A of A is 2D less than the minimum expected.
• Patients reject minus/accept plus.
• Under monocular conditions at near, these patients reject minus as they struggle to accommodate, and accept positive as they reduce the stimulus.
• Flippers will show difficulty with minus and cpm is under expected.
• Diagnoses also entail monocular and binocular difficulty with minus lenses.

Organic Causes

• Causes may include orthophoria, esophoria, or exophoria.
• Organic conditions can include toxicity, cranio-cervical/head trauma, infections, alcoholism, malnutrition, dental issues, psychological problems.
• functional if treatable, or can only manage if non-organic.
• The organic nature must be addressed if present.

Management Of Toxicity

• Treatment and management involves either VT or bifocal lenses.
• Vision therapy emphasizes addressing difficulties with stimulating the accommodative system via minus lenses.
• Patients struggle with minus, so therapy makes it more challenging, similar to weight training.
• Tests include binocular crossed cylinder: It measures the lag of accommodation so we expect patient to have a high score
• Sheard's: Indicates a reduced minus to blur, so it will be low.
• Deviation: Can be eso or exo, but varies at near.
• Easy for patients: NRA
• Difficult for patients: PRA
• Bifocal lenses: Romero does not prefer near point add for kids becuase it reduces the need to accommodate.
• Patient may have some accommodation to exert to clear te target.
• Ways to determine NRA + PRA divided by BCC.
• By MEM finding of values which are over 1.00 indicates the amount of near point add to be prescribed BUT, this is a managment approach, it does not cure
• Goal is to Reduce symptoms by reducing the stimulus with the acc, but does not stimulate an increase in the amp of acc.

Accommodative Infacility

• Accommodative infacility is the inability to relax or stimulate accommodation efficiently, aka inertia.
• 12% pts with acc problems
• General and classic symptoms include trouble focusing when switching from distance to near vision and vice versa.
• Signs:
• Normal A of A, low BCC, low MEM
• It's common to find accommodative insufficiency and infacility together
• Low NRA & PRA.
• Difficulty maintaining latency throughout the minute of testing on flippers, key finding difficulty and low cpm even when equal latency.
• Usually functional cases often stem from organic causes.
• IE; Graves' disease, cerebral palsy, uncontrolled diabetes, Adie's syndrome, or drugs affecting the CNS.
• Managing with VT or lenses, but these patients does not justify bifocals: these patients accept plus lens
• emphasis on response speed as a timed factor w/ minus and plus while monitoring that it is still done the time and meets the metronome.

III-Sustained Accommodation

• III-sustained accommodation is where the stamina is a problem, and is 1% of patients with acc. problems.
• General: common for all
• Classic: intermittent clear/blurred vision when reading, losing that stamina and it becomes blurred Signs:
• A of A - in range.
• Tend to be normal.
• Main sign: patients are aware of intermittent clear/blurred vision.

Accommodation Excess

• Accommodation Excess means exerting too much accommodation, an accomendation response is greater than stimulus response.
• It is at 3% of pts, with accommodative anomalies
• The problem is exertion*
• Blurred vision is reported at DV after NV work.
• It is generally normal, except PRA is high.