Near Refraction and Presbyopia: Complete Guide to Near Addition, NPA, NRA, PRA and Final Near Prescription
Title: Near Refraction and Presbyopia | Near Addition, NPA, NRA & PRA
Description: Learn near refraction and presbyopia, including determining near addition, near point of accommodation, NRA, PRA, final near prescription, and distance and near vision assessment.
Focus Keyword: Near Refraction and Presbyopia
Related Keywords: near addition, presbyopia, near point of accommodation, NPA, NRA, PRA, near vision testing, near prescription, subjective refraction
Introduction
Near refraction is an important part of a comprehensive optometric examination, particularly for patients who experience difficulty with reading, smartphone use, computer work, sewing, writing, or other near tasks.
As people age, the eye gradually loses its ability to increase its optical power for near vision. This age-related reduction in accommodation is known as presbyopia.
The optometrist may prescribe an additional plus power, called a near addition (ADD), to help the patient focus comfortably at the required working distance.
Near assessment involves more than simply adding an arbitrary +1.00 D or +2.00 D. The optometrist should consider:
- Distance prescription
- Patient's age
- Working distance
- Near visual acuity
- Accommodation
- Near symptoms
- Binocular vision
- NRA
- PRA
- Patient's occupational and lifestyle requirements
What Is Presbyopia?
Presbyopia is an age-related reduction in the ability of the eye to accommodate sufficiently for clear near vision.
The crystalline lens gradually becomes less flexible with age, and changes also occur in the accommodative system.
As a result, a person who previously could read comfortably at a close distance may begin to experience difficulty with near tasks.
Common symptoms of presbyopia
- Difficulty reading small print
- Holding reading material farther away
- Eye strain during near work
- Headache after reading
- Difficulty reading in dim illumination
- Needing brighter light
- Blur at normal reading distance
- Difficulty changing focus between distance and near
What Is Near Refraction?
Near refraction is the process of determining the appropriate optical correction for near visual tasks.
It may involve determining:
- Near sphere
- Near cylinder when required
- Near addition
- Near visual acuity
- Working distance
- Accommodative status
- Binocular comfort
For a presbyopic patient, the near prescription is commonly expressed as:
Distance prescription + Near Addition
Example
Distance:
OD: −2.00 DS
OS: −1.50 DS
Near addition:
+1.50 D
Near prescription:
OD: −0.50 DS
OS: Plano
This is an illustrative example; the actual near prescription must be determined clinically.
What Is Near Addition?
Near addition, commonly written as ADD, is the additional plus power placed over the distance prescription to provide appropriate near focusing assistance.
For example:
Distance: −2.00 DS
ADD: +1.50 D
The resulting near spherical power is:
−2.00 + (+1.50) = −0.50 DS
Therefore:
Distance: −2.00 DS
Near: −0.50 DS
Why Is Near Addition Required?
When accommodation becomes insufficient for the patient's desired working distance, additional plus power reduces the amount of accommodation the patient needs to provide.
For example, a patient who needs approximately 2.50 D of accommodative demand for a particular working distance may not be able to comfortably provide the entire demand.
A suitable near addition supplies part of the required optical power.
Working Distance and Accommodation
Working distance is extremely important when determining near addition.
The accommodative demand for a working distance can be estimated using:
Accommodative demand = 1 / working distance in metres
For example:
At 40 cm
40 cm = 0.40 m
1 ÷ 0.40 = 2.50 D
Therefore, the accommodative demand at 40 cm is approximately:
2.50 D
At 50 cm
50 cm = 0.50 m
1 ÷ 0.50 = 2.00 D
Therefore:
50 cm → 2.00 D
At 33 cm
33 cm ≈ 0.33 m
1 ÷ 0.33 ≈ 3.00 D
Therefore:
33 cm → approximately 3.00 D
Determining Near Addition
There is no single method that should be used for every patient.
The clinician considers the patient's:
- Age
- Distance prescription
- Working distance
- Near visual acuity
- Accommodation
- Binocular status
- Occupational requirements
- Symptoms
- Previous near prescription
Several clinical approaches may be used.
Method 1: Age-Based Estimation
Age can provide a rough starting estimate for near addition.
However, age alone should not determine the final prescription.
Two patients of the same age may have different:
- Accommodative abilities
- Working distances
- Visual demands
- Near symptoms
- Binocular requirements
Therefore, age-based values should be treated as an initial guide, followed by clinical assessment.
Method 2: Working-Distance Method
The desired working distance can be used to estimate the accommodative demand.
For example:
Working distance = 40 cm
Accommodative demand:
1 ÷ 0.40 = 2.50 D
The patient's available accommodation and comfortable reserve are then considered when determining the appropriate addition.
Method 3: Trial Addition
A tentative plus addition can be introduced over the distance prescription.
The patient is then asked to read an appropriate near target.
The clinician assesses:
- Clarity
- Comfort
- Reading ability
- Working distance
- Binocular response
The addition can then be refined.
Method 4: Binocular Accommodative Testing
Additional information may be obtained from tests such as:
- Near point of accommodation
- NRA
- PRA
- Amplitude of accommodation
- Binocular accommodative tests
These findings help the clinician understand the patient's accommodative function.
Near Point of Accommodation
The Near Point of Accommodation (NPA) is the closest point at which the patient can maintain clear vision using accommodation.
It is usually measured using a near target.
The patient focuses on a small target while it is gradually moved closer.
The examiner observes when the target becomes continuously blurred.
Procedure for Measuring NPA
Step 1: Provide Appropriate Correction
The patient should wear the appropriate distance correction.
Step 2: Select a Near Target
A small, detailed target is commonly used.
Step 3: Position the Target
The target is initially placed at a comfortable near distance.
Step 4: Move the Target Slowly Toward the Patient
The target is gradually moved closer.
Step 5: Ask the Patient
The patient is instructed:
"Tell me when the target first becomes continuously blurred."
Step 6: Record the Distance
Measure the distance from the appropriate reference point of the eye to the target, according to the clinical method being used.
What Is Amplitude of Accommodation?
The Amplitude of Accommodation (AA) represents the maximum accommodative ability of the eye.
It can be estimated from the near point of accommodation.
A commonly used relationship is:
AA = 1 / NPA in metres
For example:
If NPA = 10 cm:
10 cm = 0.10 m
AA = 1 ÷ 0.10 = 10 D
Therefore, the amplitude of accommodation is approximately:
10 D
NRA: Negative Relative Accommodation
NRA stands for Negative Relative Accommodation.
It measures the amount of additional plus power a patient can accept while maintaining binocular single vision and an appropriate near target, under the conditions of the test.
NRA is useful in evaluating accommodative function and can provide information relevant to near prescription determination.
NRA Procedure
A typical clinical procedure involves:
- Establishing the appropriate near prescription.
- Presenting a suitable near target.
- Adding plus power binocularly in small increments.
- Asking the patient to report when the target first becomes continuously blurred.
- Recording the total plus power accepted.
For example:
If the patient accepts:
+2.00 D
before blur occurs, the NRA is recorded as:
+2.00 D
The exact test procedure should follow the protocol used in the clinical setting.
What Does NRA Tell Us?
NRA provides information about the patient's ability to:
- Relax accommodation
- Accept additional plus power
- Maintain binocular single vision during the test
An unusually high or low value should not be interpreted in isolation. It should be considered along with other accommodative and binocular findings.
PRA: Positive Relative Accommodation
PRA stands for Positive Relative Accommodation.
It measures the amount of additional minus power a patient can accept while maintaining binocular single vision and a near target before blur occurs.
PRA Procedure
A typical procedure involves:
- Establishing the appropriate near prescription.
- Presenting the near target.
- Adding minus power binocularly in small increments.
- Asking the patient to report when the target becomes continuously blurred.
- Recording the total additional minus power accepted.
For example:
If the patient accepts:
−2.50 D
before blur occurs:
PRA = −2.50 D
Again, the exact clinical procedure should follow the protocol being used.
NRA and PRA can provide useful information about the patient's accommodative and binocular function.
They may help the clinician evaluate:
- Accommodative flexibility
- Ability to relax accommodation
- Ability to stimulate accommodation
- Near visual symptoms
- Binocular accommodative relationships
However, they should not be used alone to prescribe an addition.
Determining the Final Near Prescription
The final near prescription should be based on the complete clinical picture.
The clinician considers:
1. Distance Prescription
Start with the patient's finalized distance correction.
2. Near Addition
Determine an appropriate plus addition.
3. Working Distance
Consider the patient's actual reading or working distance.
4. Near Visual Acuity
Confirm that the patient can achieve the desired near acuity.
5. Comfort
The patient should be able to maintain near work comfortably.
6. Binocular Vision
Assess binocular function where clinically appropriate.
Example of Calculating a Near Prescription
Suppose:
Distance prescription:
OD: −2.00 DS
OS: −1.50 DS
Suppose the clinically determined addition is:
+1.50 D
Then:
Right eye
−2.00 + 1.50 = −0.50 DS
Left eye
−1.50 + 1.50 = Plano
Therefore, the near prescription would be:
OD: −0.50 DS
OS: Plano
This is only a mathematical example. The actual addition must be determined from the patient's examination.
Checking Distance Vision
Before finalizing a near prescription, distance vision should be checked with the finalized distance correction.
Assess:
- OD
- OS
- OU
Record the visual acuity accurately.
For example:
OD: 6/6
OS: 6/6
OU: 6/6
The exact acuity will vary between patients.
Checking Near Vision
Near vision can be assessed using an appropriate near-vision chart or reading material.
The patient should hold the target at the required working distance.
Common working distances include:
- 40 cm
- 50 cm
- Patient-specific occupational distance
The patient should be tested with the proposed near correction.
Why Working Distance Matters
A reading prescription should not be determined without considering how the patient actually works.
For example:
Patient A
Reads at 40 cm
Demand ≈ 2.50 D
Patient B
Uses a computer at 67 cm
Demand ≈ 1.50 D
These patients may have different near visual requirements.
Therefore, a patient's actual working distance should be discussed during the examination.
Checking Near Comfort
After placing the tentative near prescription, ask the patient to perform a realistic near task.
For example:
"Please read this paragraph as you normally would."
Observe:
- Reading speed
- Head posture
- Working distance
- Ability to maintain clarity
- Symptoms
- Binocular comfort
Ask:
"Is this comfortable for you?"
Distance and Near Vision: Final Check
Before prescribing, perform a final assessment.
Distance
Check:
Unaided/presenting VA → Refraction → Best corrected VA
Near
Check:
Near VA without addition → Trial addition → Near VA with final addition
Then assess comfort.
Common Mistakes in Near Refraction
1. Prescribing Addition Based Only on Age
Age is only a guide.
The patient's actual visual requirements must be considered.
2. Ignoring Working Distance
A patient who works at 50 cm has different accommodative demand from someone working at 30 cm.
3. Giving Too Much Plus
Excessive plus may produce blur at the intended working distance or reduce the patient's ability to accommodate appropriately.
4. Giving Too Little Plus
Insufficient addition may leave the patient struggling with near tasks.
5. Ignoring Binocular Vision
Near symptoms can sometimes be related to binocular or accommodative problems rather than simply presbyopia.
6. Not Checking Near Visual Acuity
The addition should be verified using an appropriate near target.
Practical Clinical Sequence
A simplified sequence for near refraction is:
Finalize Distance Prescription
↓
Determine Patient's Working Distance
↓
Assess Near Vision
↓
Assess Accommodation When Indicated
↓
Determine Tentative Near Addition
↓
Trial the Addition
↓
Check Near Visual Acuity
↓
Assess Binocular Comfort
↓
Refine Addition if Required
↓
Final Distance + Near Prescription
Simple Example for Students
A 50-year-old patient reports difficulty reading small print.
Distance prescription:
OD: −1.00 DS
OS: −0.75 DS
Suppose the clinically determined addition is:
+1.75 D
Near powers would be:
OD:
−1.00 + 1.75 = +0.75 DS
OS:
−0.75 + 1.75 = +1.00 DS
Therefore:
Near:
OD: +0.75 DS
OS: +1.00 DS
The patient should then be checked at the intended working distance and the prescription adjusted if clinically necessary.
Important Formulae
Working-distance demand
Accommodation demand = 1 / working distance in metres
Example
40 cm:
1 / 0.40 = 2.50 D
Amplitude of accommodation
AA = 1 / NPA in metres
Example:
NPA = 20 cm
AA = 1 / 0.20 = 5 D
Near prescription
Near power = Distance sphere + Near addition
Example:
Distance = −2.00 D
Addition = +1.50 D
Near = −0.50 D
Clinical Tips for Optometry Students
Remember:
1. Always finalize the distance prescription first.
2. Ask the patient's actual working distance.
3. Do not prescribe addition based only on age.
4. Understand the relationship between working distance and accommodative demand.
5. NPA provides information about accommodative ability.
6. NRA tests the ability to accept additional plus.
7. PRA tests the ability to accept additional minus.
8. NRA and PRA should not be interpreted in isolation.
9. Check near visual acuity with the proposed addition.
10. Always assess comfort before finalizing the near prescription.
Questions
1. What is presbyopia?
Presbyopia is the age-related reduction in the ability to accommodate adequately for near vision.
2. What is near addition?
Near addition is the additional plus power prescribed over the distance correction to assist near vision.
3. What is NPA?
NPA stands for Near Point of Accommodation, the closest point at which clear vision can be maintained using accommodation.
4. What is NRA?
NRA stands for Negative Relative Accommodation and involves adding plus power binocularly during near testing.
5. What is PRA?
PRA stands for Positive Relative Accommodation and involves adding minus power binocularly during near testing.
6. What is the accommodative demand at 40 cm?
Approximately 2.50 D.
7. What is the formula for accommodative demand?
1 ÷ working distance in metres.
8. What is the formula for amplitude of accommodation from NPA?
AA = 1 / NPA in metres.
9. Should age alone determine near addition?
No.
10. Why is working distance important?
Because accommodative demand depends on working distance.
Frequently Asked Questions
Can presbyopia be prevented?
Normal age-related loss of accommodation cannot currently be prevented by ordinary exercises or lifestyle measures. Appropriate optical correction can help manage the resulting near-vision difficulty.
Is reading glasses power the same for everyone?
No. Near addition depends on factors such as distance prescription, age, working distance, accommodative ability, visual demands, and binocular status.
What is the difference between NPA and NRA?
NPA measures the closest point at which accommodation can maintain clear vision. NRA measures the amount of additional plus power accepted during a specific binocular near test.
What is the difference between NRA and PRA?
NRA uses plus lenses, while PRA uses minus lenses during binocular near testing.
Can a patient have different near powers in the two eyes?
Yes. The near prescription is calculated from each eye's distance prescription and the clinically appropriate addition; it does not necessarily result in identical powers.
Conclusion
Near refraction and presbyopia assessment are essential components of comprehensive optometric practice. Determining the correct near prescription requires more than simply selecting a plus lens based on the patient's age.
The optometrist should consider:
Distance prescription → Working distance → Accommodation → Near addition → Near visual acuity → Binocular comfort → Final prescription
The Near Point of Accommodation (NPA) provides information about accommodative ability, while NRA and PRA provide additional information about the patient's ability to relax and stimulate accommodation under binocular testing conditions.
The ultimate goal is simple: