Unit 3: Objective Retraction (Static and Dynamic) | Visual Optics-II | 4th Semester of Bachelor of Optometry

Objective Refraction (Static & Dynamic)

Objective refraction refers to the clinical methods of determining the refractive error of an eye without relying on the patient’s subjective responses. Unlike subjective refraction, where patients provide feedback on clarity, objective refraction uses optical principles, light reflexes, and retinoscopic techniques to estimate refractive status. It is especially important in young children, uncooperative patients, patients with communication difficulties, and in cases where subjective refraction is unreliable.

Streak Retinoscopy

Streak retinoscopy is the most widely practiced technique for objective refraction in optometry and ophthalmology. It allows the examiner to determine the refractive state of the eye by projecting a streak of light into the pupil and observing the motion of the retinal reflex. This technique is fundamental because it does not depend on the patient’s subjective responses and can be performed even in young children, non-verbal individuals, or uncooperative patients. A thorough understanding of the principle, procedure, common difficulties, and interpretation of findings is essential for accurate clinical use.

Principle of Streak Retinoscopy

Retinoscopy is based on the optical relationship between the far point of the patient’s eye and the examiner’s retinoscope. When the examiner shines light into the eye, it is reflected from the retina and emerges through the pupil. By placing lenses of different powers before the patient’s eye, the emergent rays can be focused to coincide with the examiner’s working distance. The neutral point is reached when the far point of the patient’s eye lies at the plane of the examiner’s retinoscope. At neutrality, the reflex fills the pupil uniformly and exhibits no motion.

Equipment

• Streak retinoscope: A handheld instrument with a light source that projects a linear streak of light. The streak can be rotated to align with any meridian of the eye.
• Trial lenses/trial frame: Used to introduce different lens powers in front of the patient’s eye to achieve neutrality.
• Target for fixation: A distant target is provided to relax accommodation during static retinoscopy.

Procedure

1. Seat the patient at a distance of 50–67 cm from the examiner (depending on working distance preference).
2. Ask the patient to fixate on a distant non-accommodative target to ensure relaxed accommodation.
3. Direct the streak of light into the pupil and sweep it along the horizontal and vertical meridians separately.
4. Observe the direction of the reflex relative to the movement of the streak:
• “With” motion: Reflex moves in the same direction as the streak → indicates hyperopia, emmetropia, or low myopia (< working distance).
• “Against” motion: Reflex moves opposite to the streak → indicates myopia greater than the reciprocal of the working distance.
• Neutrality: No movement of reflex → far point coincides with examiner’s plane.
5. Add or subtract spherical and cylindrical trial lenses until neutrality is achieved in each meridian.
6. Subtract the dioptric equivalent of the working distance (e.g., 1.50 D for 67 cm, 2.00 D for 50 cm) from the lens power at neutrality to obtain the final refraction.

Difficulties Encountered

• Small pupils: Make reflex dim and harder to interpret.
• Media opacities: Cataracts or corneal scars reduce reflex brightness.
• High astigmatism: Reflex is distorted and difficult to neutralize; requires careful meridional assessment.
• Unstable accommodation: Common in children; may cause fluctuating reflex. Cycloplegia is often needed.
• Examiner error: Incorrect working distance or poor alignment can lead to inaccurate results.

Interpretation of Findings

The lens power needed to reach neutrality in each meridian reveals the refractive error. Once both principal meridians are neutralized:

• If equal → spherical correction.
• If unequal → cylindrical correction is prescribed for the meridian difference.
• Final prescription is transposed into the desired form (plus or minus cylinder).

Clinical Importance

• Provides an objective baseline for refractive correction.
• Essential in pediatric practice to detect hyperopia, astigmatism, or anisometropia—important causes of amblyopia and strabismus.
• Critical in non-communicative patients (infants, disabled, or language barrier cases).
• Used in cycloplegic conditions to reveal latent hyperopia.
• Forms the foundation for subjective refinement of refraction in cooperative patients.

Principle, Procedure, Difficulties, and Interpretation of Retinoscopy Findings

Retinoscopy is a cornerstone of objective refraction, enabling clinicians to determine the refractive status of an eye by observing the movement of the retinal reflex. While the technique is conceptually straightforward, accurate results require careful understanding of its principle, systematic procedure, recognition of difficulties, and correct interpretation of findings. This subtopic explores these four aspects in detail to provide a comprehensive understanding.

Principle of Retinoscopy

The principle of retinoscopy is based on the concept of conjugate foci. When light is projected into the eye, it is reflected from the retina and emerges from the pupil. The direction of this emergent light depends on the refractive error of the eye. The examiner’s retinoscope serves as the observation point:

• If the far point of the patient’s eye coincides with the examiner’s retinoscope, the reflex appears neutral (no motion).
• If the far point lies behind the examiner (hyperopia or emmetropia at working distance), the reflex moves with the streak.
• If the far point lies in front of the examiner (myopia greater than working distance), the reflex moves against the streak.

Neutralization is achieved by introducing trial lenses until the far point is shifted to the examiner’s working distance, after which the working distance correction is subtracted to calculate the true refractive error.

Procedure of Retinoscopy

1. Preparation: Dim the room lighting. Ensure the patient fixates on a distant non-accommodative target to relax accommodation (for static retinoscopy).
2. Positioning: The examiner sits at a fixed working distance (commonly 67 cm = 1.50 D or 50 cm = 2.00 D).
3. Initial Observation: Direct the streak of light into the pupil and sweep it horizontally and vertically.
4. Identify Motion:
   • “With” motion → add plus lenses.
   • “Against” motion → add minus lenses.
5. Neutralization: Adjust trial lenses until no motion of the reflex is seen. At this point, the far point coincides with the examiner’s retinoscope.
6. Calculation: Subtract the dioptric value of working distance from the neutralizing lens power to obtain the refractive error.
7. Meridional Assessment: Repeat the procedure for each principal meridian if astigmatism is suspected. Record sphere, cylinder, and axis.

Difficulties in Retinoscopy

Retinoscopy is simple in principle but may be challenging in practice due to several factors:

• Patient factors: Poor fixation, unstable accommodation in children, or fatigue may produce variable reflexes.
• Small pupils: Reflex is dim and narrow, making neutrality harder to judge.
• Media opacities: Corneal scars, cataracts, or vitreous opacities reduce reflex clarity.
• High ametropia: Reflex is very fast in high hyperopia (hard to control) or very dim in high myopia.
• Examiner error: Incorrect working distance, misalignment, or over-reliance on subjective impression of reflex speed may cause inaccuracy.
• Astigmatism: Reflex appears distorted or changes motion in different meridians, requiring careful meridional testing.

Interpretation of Retinoscopy Findings

Once neutrality is achieved, the examiner must correctly interpret the results:

• Neutralization Lens Power: The trial lens that produces neutrality indicates the vergence needed to move the far point to the examiner’s working distance.
• Working Distance Correction: Always subtract the dioptric value of working distance (e.g., 1.50 D for 67 cm).
• Final Refractive Status:
  • If both meridians neutralize with the same lens → spherical error.
  • If different → astigmatism. Cylinder power and axis are determined by the difference between meridians.
• Transposition: Results may be written in plus or minus cylinder form depending on clinical convention.

Clinical Example

An examiner performs retinoscopy at 67 cm. Neutralization is achieved at +2.75 D sphere in both meridians. Subtracting 1.50 D (working distance), the true refractive error is +1.25 D hyperopia. Prescription: +1.25 DS.

Transposition and Spherical Equivalent

In clinical practice, the refractive error of the eye is often expressed in different formats depending on local conventions and the optical instruments used. For example, optometrists generally use minus cylinder form, while ophthalmologists often use plus cylinder form. The process of converting one form into another is called transposition. In addition, when a simplified or averaged correction is needed, especially for contact lens fitting or low vision care, the spherical equivalent is used. This section explains both concepts in detail.

Transposition of Prescriptions

Transposition involves rewriting a lens prescription from plus cylinder form to minus cylinder form, or vice versa, without altering its optical effect. This is important for communication between different eye care professionals and when prescribing different optical devices.

Steps for Transposition

1. Add the sphere and cylinder powers algebraically → this becomes the new sphere.
2. Change the sign of the cylinder (plus to minus or minus to plus).
3. Rotate the axis by 90° (e.g., axis 180 becomes axis 90).

Example 1: Plus-to-Minus Cylinder Transposition

Prescription: +2.00 DS / +1.00 DC × 90 Step 1: Sphere + Cylinder = +2.00 + (+1.00) = +3.00 (new sphere) Step 2: Change sign of cylinder → −1.00 DC Step 3: Rotate axis → 180° Final Prescription: +3.00 DS / −1.00 DC × 180

Example 2: Minus-to-Plus Cylinder Transposition

Prescription: −2.50 DS / −0.75 DC × 45 Step 1: Sphere + Cylinder = −2.50 + (−0.75) = −3.25 (new sphere) Step 2: Change sign of cylinder → +0.75 DC Step 3: Rotate axis → 135° Final Prescription: −3.25 DS / +0.75 DC × 135

Spherical Equivalent

The spherical equivalent (SE) is the single spherical power that best represents the overall refractive effect of a spherocylindrical prescription. It is calculated by adding half of the cylinder power to the sphere.

Formula: Spherical Equivalent = Sphere + (½ × Cylinder)

Example 1

Prescription: +2.00 DS / −1.00 DC × 180 SE = +2.00 + (½ × −1.00) = +2.00 − 0.50 = +1.50 D

Example 2

Prescription: −3.00 DS / +2.00 DC × 90 SE = −3.00 + (½ × +2.00) = −3.00 + 1.00 = −2.00 D

Clinical Uses of Spherical Equivalent

• Contact lens fitting: Soft contact lenses correct only spherical error. Cylindrical prescriptions are often converted to SE to approximate vision correction.
• Low vision aids: Simplified prescriptions improve comfort when fine correction of astigmatism is not critical.
• Screening and research: SE values provide a single numerical representation of refractive error distribution in populations.
• Refractive surgery: SE is often used to calculate laser correction targets.

Limitations

• SE does not fully correct astigmatism, so visual acuity may be reduced compared to full spherocylindrical correction.
• Best suited for low astigmatism (<0.75 D); higher astigmatism requires full correction.
• Not a substitute for precise refraction in routine spectacles.

Dynamic Retinoscopy – Various Methods

Dynamic retinoscopy is an objective technique used to evaluate the accuracy and behavior of the accommodative response while the patient is actively focusing on a near target. Unlike static retinoscopy, which is performed with accommodation relaxed, dynamic retinoscopy measures how well the eye adjusts its refractive state to meet near visual demands. It is particularly valuable in diagnosing accommodative anomalies such as lag, lead, insufficiency, or excess of accommodation.

Principle

In dynamic retinoscopy, a near target is presented to stimulate accommodation. The examiner observes the reflex with a retinoscope while the patient maintains fixation. Any discrepancy between the accommodative demand and the actual accommodative response results in either a lag (under-accommodation) or a lead (over-accommodation). Trial lenses are used or examiner’s working distance is adjusted to neutralize the reflex and quantify the response.

Methods of Dynamic Retinoscopy

1. MEM (Monocular Estimation Method)

• The patient reads aloud or identifies symbols from a near card (typically at 40 cm) attached to the retinoscope.
• The examiner observes the reflex monocularly while the patient continues reading.
• Trial lenses are briefly introduced (flashed) before the eye until neutrality is achieved.
• Findings are usually expressed in diopters.

Interpretation:

• Normal lag: +0.25 D to +0.75 D.
• Lag >+1.00 D: Suggests accommodative insufficiency or uncorrected hyperopia.
• Lead (−0.25 D or more): Indicates accommodative excess or spasm.

2. Nott Retinoscopy

• A near fixation target is placed at 40 cm.
• No lenses are introduced; instead, the examiner moves closer or farther with the retinoscope to find neutrality.
• The accommodative response is calculated from the distance where neutrality occurs relative to the fixation target.

Interpretation:

• If neutrality is behind the target → lag of accommodation.
• If neutrality is in front of the target → lead of accommodation.

3. Radical Retinoscopy

• Used to measure amplitude of accommodation.
• The near target is gradually brought closer to the patient’s eye while the examiner monitors the reflex.
• The point where neutrality is achieved corresponds to the patient’s near point of accommodation.

Clinical Use: Provides objective measurement of accommodative amplitude, especially useful in children or non-verbal patients.

4. Bell Retinoscopy

• A fixation target (e.g., a small bell) is moved toward and away from the patient at a fixed working distance.
• The examiner notes where the reflex changes direction from “with” to “against.”
• Distance from the fixation plane provides information on accommodative posture.

Interpretation: Normal lag of accommodation is about 1–2 cm behind the fixation target.

5. Book Retinoscopy

• The patient reads from a book held at 40 cm while retinoscopy is performed.
• The reflex behavior is graded qualitatively (bright/dull, fast/slow, with/against) rather than quantified.
• It provides insight into functional accommodative performance during sustained reading tasks.

Clinical Significance of Dynamic Retinoscopy

• Detects accommodative lag: Common in children with convergence insufficiency or uncorrected hyperopia, leading to near blur and fatigue.
• Identifies accommodative excess: Over-accommodation may cause distance blur after near work, pseudomyopia, or headaches.
• Evaluates therapy outcomes: Used to monitor improvement after vision therapy for accommodative or binocular vision disorders.
• Guides lens prescribing: MEM findings help determine the need for near plus lenses in symptomatic children.

Advantages and Limitations

• Advantages: Quick, objective, suitable for children and non-verbal patients, provides direct measure of accommodative accuracy.
• Limitations: Requires examiner skill, proximal cues may influence accommodation, results can vary with target type and patient cooperation.

Radical Retinoscopy and Near Retinoscopy

While static retinoscopy provides an objective assessment of refractive error with accommodation relaxed, and dynamic retinoscopy assesses accommodative response during near fixation, there are specialized forms of dynamic testing such as radical retinoscopy and near retinoscopy. These methods extend the usefulness of retinoscopy in measuring amplitude and accuracy of accommodation, particularly in pediatric optometry, binocular vision testing, and cases where subjective feedback is unreliable.

Radical Retinoscopy

Definition

Radical retinoscopy is a technique used to objectively determine the amplitude of accommodation. Unlike standard static or MEM retinoscopy, which evaluate accommodative accuracy at fixed distances, radical retinoscopy measures how much the eye can increase its optical power as a near target is brought closer.

Principle

As the fixation target is moved closer to the patient’s eye, the accommodative demand increases. The examiner observes the retinoscopic reflex. The point of neutrality corresponds to the patient’s near point of accommodation. The reciprocal of this distance (in meters) gives the amplitude of accommodation in diopters.

Procedure

1. The patient is asked to fixate on a small, accommodative near target (letters, symbols, or picture) placed on the retinoscope.
2. The examiner begins at a distance (e.g., 40 cm) and gradually moves the target closer toward the patient’s eye while observing the reflex.
3. At the point where the reflex neutralizes, the distance from the spectacle plane to the target is noted.
4. Amplitude of accommodation is calculated as: Amplitude (D) = 100 / Near Point (cm)

Clinical Uses

• Provides objective measurement of accommodative amplitude in children or non-verbal patients.
• Useful in diagnosing accommodative insufficiency or spasm.
• Helps confirm presbyopia in older adults by demonstrating reduced amplitude.

Advantages and Limitations

• Advantages: Simple, objective, does not depend on subjective blur perception.
• Limitations: Reflex interpretation may be difficult in small pupils or low cooperation; overestimation may occur due to proximal cues.

Near Retinoscopy

Definition

Near retinoscopy refers to retinoscopic techniques performed while the patient fixates on a near target at a fixed distance, commonly 40 cm. Unlike radical retinoscopy, it does not measure maximum amplitude but evaluates accommodative accuracy and posture (lag or lead of accommodation).

Procedure

1. Place an accommodative target (letters, pictures, or MEM cards) at a fixed near distance (typically 40 cm = 2.5 D demand).
2. Perform retinoscopy while the patient continues to fixate and respond to the near task.
3. Observe the reflex: “with” motion suggests lag of accommodation; “against” motion suggests lead of accommodation.
4. Neutralize by introducing plus (for lag) or minus (for lead) lenses briefly before the eye.

Interpretation

• Normal finding: A small lag of +0.25 to +0.75 D.
• High lag (>+1.00 D): Indicates accommodative insufficiency or uncorrected hyperopia.
• Lead (negative finding): Suggests accommodative spasm or excess.

Clinical Uses

• Assessment of functional accommodation in children with reading or learning difficulties.
• Evaluation of accommodative lag in digital eye strain.
• Guides prescribing near addition lenses in cases of binocular vision dysfunction.

Comparison of Radical vs. Near Retinoscopy

Aspect	Radical Retinoscopy	Near Retinoscopy
Purpose	Measures amplitude of accommodation	Measures accommodative posture (lag/lead)
Setup	Target gradually moved closer	Target fixed at 40 cm
Findings	Near point and amplitude in diopters	Lag or lead values in diopters
Clinical use	Diagnosis of insufficiency/spasm, presbyopia	Guides near correction, evaluates accommodative accuracy

Cycloplegic Refraction

Cycloplegic refraction is an essential procedure in objective refraction where pharmacological agents are used to temporarily paralyze the ciliary muscle and thus eliminate accommodation. This allows the examiner to reveal the full refractive error of the eye, particularly the latent hyperopia that is otherwise hidden by accommodative effort. It is especially critical in pediatric optometry, strabismus evaluation, and in cases of suspected accommodative dysfunction.

Principle

Cycloplegia is achieved using drugs that block the parasympathetic supply to the ciliary muscle (muscarinic antagonists). This prevents accommodation, forcing the crystalline lens to remain in its relaxed, least convex state. By immobilizing accommodation, cycloplegic refraction ensures that the measured refractive error reflects the eye’s true optical state rather than an accommodative response.

Indications

Cycloplegic refraction is indicated in cases where accommodation interferes with accurate refraction:

• Children: High accommodative tonus may mask hyperopia or produce variable findings.
• Strabismus: Particularly esotropia, where uncorrected hyperopia may drive accommodative convergence.
• Latent Hyperopia: Accommodation conceals part of the hyperopia unless paralyzed.
• Inconsistent subjective responses: Patients with poor cooperation, communication difficulties, or malingering.
• Accommodative anomalies: To rule out spasm or pseudomyopia.

Common Cycloplegic Agents

Drug	Onset	Duration of Cycloplegia	Comments
Atropine (1%)	30–40 min	7–14 days	Strongest, used in suspected amblyopia or high hyperopia; not practical for routine refraction.
Cyclopentolate (1%)	20–30 min	6–24 hours	Most commonly used for children; safe and effective.
Tropicamide (0.5%–1%)	15–20 min	4–6 hours	Weaker effect, used in adults; may not fully eliminate accommodation in children.
Homatropine (2%)	30–40 min	1–2 days	Intermediate strength; rarely used today due to poor cycloplegia compared with cyclopentolate.

Procedure

1. Baseline visual acuity and preliminary refraction are recorded.
2. One to two drops of a cycloplegic agent are instilled in each eye, sometimes repeated after 5–10 minutes for stronger effect.
3. Pupillary dilation occurs along with cycloplegia.
4. Retinoscopy is performed after sufficient time has elapsed for maximal effect (varies with drug).
5. The objective refraction obtained represents total hyperopia and true refractive error.

Interpretation of Findings

The difference between non-cycloplegic (manifest) and cycloplegic refraction reveals the amount of latent hyperopia. For example:

• Manifest hyperopia: +1.50 D
• Cycloplegic hyperopia: +3.50 D
• Latent hyperopia = +2.00 D (compensated by accommodation before cycloplegia)

This information is crucial in prescribing the correct correction for children and in managing accommodative esotropia.

Clinical Applications

• Pediatric refraction: Detects full hyperopia and anisometropia to prevent amblyopia.
• Strabismus evaluation: Determines accommodative component of esotropia and guides spectacle prescription.
• Diagnosis of pseudomyopia: Differentiates true myopia from accommodative spasm.
• Baseline prescription: Provides reliable starting point for subjective refinement in adults with unstable responses.

Precautions and Contraindications

• Use cautiously in patients with narrow angles (risk of angle closure).
• Atropine may cause systemic side effects (flushing, dry mouth, tachycardia); avoid in very young infants unless necessary.
• Parents must be educated about blurred near vision and photophobia following dilation.
• Protective sunglasses should be advised after cycloplegic drops due to light sensitivity.

For more units of "Visual Optics-II" click below on text 👇 

✅ Unit 1

✅ Unit 2

✅ Unit 4

✅ Unit 5