Why do you close one eye at a time when measuring PD?

Alternating eye closure (the occlude-fixate technique) eliminates parallax error. When your right eye is closed and the patient looks at your left eye, the patient's right eye is fixating straight ahead without convergence. Then when you switch, the left eye fixates straight ahead. This gives you the true distance PD without the narrowing effect of convergence that would occur if the patient fixated on a single near target.

What is parallax in PD measurement?

Parallax is the apparent shift in the perceived position of an object when viewed from different angles. In PD measurement, parallax error occurs when the examiner's eye and the patient's fixating eye are not properly aligned, causing the measurement to be inaccurate. The most common parallax error is measuring a near PD when a distance PD is intended, because the patient converges on a single fixation point.

How is near PD measured with a ruler?

For near PD, hold a fixation target at the patient's reading distance (about 40 cm). With both eyes open, have the patient look at the target. Their eyes will converge naturally. Measure directly from the center of one pupil to the center of the other. The near PD will be approximately 3-4 mm less than the distance PD.

Is a ruler or pupilometer more accurate for PD?

A pupilometer is more accurate because it eliminates parallax error entirely, provides monocular measurements automatically, and uses corneal reflexes for precise alignment. A ruler measurement is limited to approximately 1mm precision and is more susceptible to technique errors. However, the ruler technique is tested on the ABO exam and is an essential backup skill.

Manual PD Measurement: Ruler Technique and Parallax Avoidance

Manual PD Measurement Fundamentals

Manual pupillary distance (PD) measurement uses a millimeter ruler to determine the distance between the centers of the patient's pupils. While less precise than a pupilometer, manual measurement is a fundamental skill tested on the ABO exam and serves as a backup when instruments are unavailable.

The Occlude-Fixate Technique

The standard method for manual PD measurement is the occlude-fixate technique, which eliminates parallax error by having the patient fixate on the examiner's corresponding eye.

Distance PD Measurement

Stand at arm's length (approximately 40 cm) from the patient, with your eyes at the same height as theirs
Close your right eye and instruct the patient to look at your open left eye
Align the zero mark of the ruler with the center of the patient's right pupil (or the nasal limbus for an alternative reference)
Without moving the ruler, close your left eye and open your right eye
Instruct the patient to shift fixation to your now-open right eye
Read the ruler measurement at the center of the patient's left pupil
The result is the binocular distance PD

The alternating eye closure (occlude-fixate) is not arbitrary. When you close your right eye and the patient looks at your left eye, the patient's right eye is fixating straight ahead (not converging). When you then switch eyes, the patient's left eye fixates straight ahead. This sequential measurement eliminates the parallax error that would occur if both your eyes were open and the patient was fixating on a point between them.

Understanding Parallax

Parallax is the apparent shift in an object's position when viewed from different angles. When measuring PD, parallax error occurs if the examiner's viewing eye and the patient's fixating eye are not properly aligned.

If both of your eyes are open and the patient fixates on your nose (a single target), the patient's eyes converge slightly. This convergence narrows the apparent PD, producing a reading that is smaller than the true distance PD. The occlude-fixate technique avoids this by measuring each eye separately while it fixates in the straight-ahead position.

Having both eyes open while measuring and asking the patient to look at your nose or a single target. This causes the patient to converge, measuring a near PD when a distance PD is intended. Always alternate eye closure using the occlude-fixate technique for distance PD.

Near PD Measurement

For near PD, the technique is simpler:

Hold a fixation target (pen tip, small letter) at the patient's normal reading distance (typically 40 cm)
Keep both eyes open
Have the patient look at the target
The patient's eyes naturally converge to the near target
Measure directly from the center of one pupil to the center of the other

Near PD is typically 3 to 4 mm less than distance PD due to convergence.

Monocular PD with a Ruler

While pupilometers easily provide monocular PD, the ruler method can approximate monocular values:

Identify the center of the nose bridge (anatomical midpoint)
Measure from the center of each pupil to this midpoint independently
The two monocular values should sum to approximately the binocular PD

This method is less precise than a pupilometer for monocular PD but is adequate when instruments are unavailable.

Reference Points

The most common measurement reference point is the center of the pupil. An alternative reference is the nasal limbus (the nasal edge of the iris), measured from the nasal limbus of one eye to the nasal limbus of the other. Some practitioners find the limbus easier to identify consistently, especially in patients with dark irides where the pupil edge is hard to see.

Reference	Advantage	Disadvantage
Pupil center	Most anatomically accurate	Hard to see in dark irides
Nasal limbus to nasal limbus	Easier to see consistently	Assumes symmetrical iris diameters

Sources of Error

Parallax: Not using the occlude-fixate technique for distance PD
Ruler alignment: Not keeping the ruler level and parallel to the patient's face
Head tilt: Either the patient or examiner tilting their head
Eye height: Examiner not at the same height as the patient
Pupil identification: Difficulty seeing the pupil center in patients with dark eyes
Ruler precision: Most rulers have 1mm graduations, limiting precision

For patients with very dark eyes where the pupil is hard to distinguish, use a penlight held below the ruler to create a small corneal reflex. Measure from reflex to reflex instead of pupil edge to pupil edge. This technique mirrors what the pupilometer does internally.

Clinical Relevance

Manual PD measurement is a backup skill and a foundational learning exercise. Understanding the occlude-fixate technique and parallax avoidance demonstrates mastery of the optical principles underlying PD measurement, even if a pupilometer is your primary tool.

Key Takeaways

Manual PD uses a millimeter ruler with the occlude-fixate technique
The occlude-fixate technique eliminates parallax by measuring each eye sequentially
Distance PD requires alternating eye closure; near PD is measured with both eyes open
Near PD is typically 3-4 mm less than distance PD due to convergence
Pupil center is the standard reference point; nasal limbus is an alternative
Proper head alignment and ruler positioning minimize measurement errors