Clinical Optometric Procedures: Interpupillary Distance Measurement (Ipd)

Clinical Optometric Procedures: Interpupillary Distance Measurement (Ipd)

CHAPTER 9 – INTERPUPILLARY DISTANCE MEASUREMENT (IPD)

• This chapter will include a review of:
• Distance PD measurements
• Near PD measurements
• Monocular PD measurements

INTRODUCTION

The Interpupillary distance (IPD) measure is also referred to as the pupillary distance (PD). The determination of this measurement is an important part of the ophthalmic prescription and the refraction routine. The use of the PD measurement begins when refracting the patient with the trial frame or phoropter. Thereafter, it is indicated on a prescription form for the ordering of spectacle lenses. The specification of this measurement is essential in order to place the optical centers of lenses in their required position, which in most cases lies in front of the patient’s line of sight or pupil centers.

The entrance pupil of the eye determines the size and location of the bundle of light that enters the eye to stimulate the retina. By definition, the PD is the horizontal distance in millimeters between the entrance pupils of the 2 eyes for a given viewing distance (Fig. 9.1). Practically, it is the distance between the centre of the pupil of one eye and the centre of the pupil of the other eye. The PD will not be the same for all people and is measured for distance and near fixation.

Figure 9.1 Interpupillary distance of a patient’s eyes.

The distance PD is placed at the optical centers of the spectacle lenses before the entrance pupils of the eyes when patients are looking in primary gaze. The distance PD is related to the amount of binocular convergence required by a patient for bi-foveal fixation. Patients with larger distance PDs have greater demands for convergence to a near target than patients with smaller PDs.

There are several factors one must take into consideration when measuring the PD of a patient. They include:

• Binocularity of the patient (is strabismus/tropia present?)
• Is there any facial asymmetry
• Are the irises too dark to determine the pupil center International Centre for Eyecare Education

These factors would contribute to the method that is used to determine the PD measurement. For example, if the pupils are too dark, and there is no facial asymmetry and the patient is binocular, one may use other reference points as opposed to using the center of the pupil. One may in this case, use the limbal edges as the reference points to take the measurements. If there is facial asymmetry or a strabismus present, the lenses will be placed such that the optical center of the lens is coincident with the entrance pupil of the eye. In this case, the practitioner would need to take a monocular PD measurement.

DISTANCE PD MEASUREMENTS

• Instrumentation
• Millimeter ruler (accurate)

Procedure

1. The practitioner and the patient must be positioned directly in front of each other (at eye level) at an arm’s length away (± 40cm).

2. The patient is directed to fixate the practitioner’s left open eye, while the examiners right eye is closed.

3. Align the zero mark on the millimeter rule with the temporal limbus of the patient’s right eye (Fig. 9.2a) (The limbal reference point is used when it is difficult to precisely locate the center of the entrance pupil in a clinical situation). Some refraction manuals suggest that the practitioner use the pupillary margins as the reference points, however, this is provided that the pupils are symmetric (Fig. 9.2b). The practitioner can also align the PD rule with the pupil center of the patient’s right eye to pupil center of the patient’s left eye (Fig. 9.2c).

Figure 9.2 (a) IPD temporal limbal alignment; (b) IPD temporal pupil margin alignment; IPD pupil center alignment

4. The practitioner then closes his left eye and opens his right eye and directs the patient to bi-foveally fixate the practitioner’s open right eye.

5. The examiner then notes the position on the ruler that corresponds to the nasal limbus of the patient’s left eye (or pupil center of the patient’s left eye).

6. This measurement is noted as the distance PD (Fig. 9.3). International Centre for Eyecare Education Figure 9.3 IPD measurement using pupil center alignment.

7. The procedure may be repeated to ensure alignment of the ruler and accuracy of the measurement.

8. While the patient is not actually directed to a distance target, it should be noted that the patient’s fixation of the practitioner’s eye is not enough to deviate the patient’s eyes from the straight ahead position by an amount that would significantly alter the measurement.

9. In cases where a patient has an alternating strabismus, the measurement is taken in a similar manner, except that the patient’s non-fixating eye is occluded during the alignment process. In this way, the practitioner is confident that the patient will be viewing through the optical center of the lens if that particular eye is fixating a distance target (Fig. 9.4).

Figure 9.4 PD measurement in a case of an alternating strabismus, each eye fixating at a time with the non-fixating eye being occluded.

NEAR PD MEASUREMENTS

• Procedure

1. The position of the patient and the examiner is as per the distance measurement.

2. The patient is directed to view the tip of the examiner’s nose (or some other near target), thereby causing the patient to converge slightly.

3. Align the zero mark on the millimeter rule with the temporal limbus of the patient’s right eye (or pupil center of the patient’s right eye) while the examiner’s right eye is closed.

4. The examiner closes his left eye and opens his right eye and notes the position on the ruler that corresponds to the nasal limbus of the patient’s left eye (or pupil center of the patient’s left eye).

• 5. This measurement is noted as the near PD. International Centre for Eyecare Education
• Recording
• The values are normally recorded as distance PD/near PD (in millimeters). e.g. PD: 63/60.

Interpretation

• According to Benjamin (2006), the mean PD for adults is 64mm and for children in the range between 50 and

60mm. It is important to note these norms vary in different ethnic groups.

• The distance PD for women is most commonly in the range of 55-65mm, men 59-70 and in children may be
• as low as 45mm.
• The distance PD is usually 3-4mm greater than the near PD.

Common errors in the procedure

Moving the rule during the measurement and losing the alignment. The hand holding the ruler must therefore be very steady. To achieve this, it is recommended that the examiner grasp the millimeter rule between the thumb and forefinger and rest the remaining fingers of his hand on the patient’s cheek.

MONOCULAR PD MEASUREMENTS

Since very few eyes are actually symmetrically located relative to the nose, take a binocular PD measurement may be somewhat inaccurate in determining the placement of the optical centers of the lenses before each eye. It is more useful for the practitioner to determine a monocular PD (Fig. 9.5).

• Figure 9.5 Monocular PD in relation to a binocular PD measurement
• Indications
• Unilateral strabismus
• Facial asymmetry
• High refractive error
• Multifocal dispensing measurements

Procedure

1. Monocular PDs are measured just as the distance and near PDs but with slight variations in procedure.

2. The patient fixates the practitioner’s open left eye while his right eye remains closed.

3. The PD rule is aligned at the zero mark with the center of the patient’s right pupil.

4. The practitioner notes the measurement from the center of the patient’s right pupil center to the center of the bridge of the patient’s nose. This measurement is the patient’s right monocular PD.

5. The patient is then directed to fixate the practitioner’s open right eye while his left eye remains closed.

6. The practitioner notes the measurement from the center of the bridge of the patient’s nose to the center of the patient’s left pupil. This measurement is noted as the left monocular PD. Sometimes, the practitioner may move the zero mark from the patients right pupil to the center of the bridge of the patient’s nose and then note the measurement from the center of the bridge to the patients left pupil.

Recording

Monocular PD measurements are recorded in millimeters with the right monocular PD preceding the left monocular PD, i.e. OD/OS. e.g. 31/30.

Difficulties with taking a monocular PD

• It is sometimes difficult to detect the center of the patient’s pupil in patients with darkly pigmented irides. In

these cases, many practitioners prefer to use the corneal reflexes as a landmark to take the measurements.

In this case, instead of the patient using their finger to direct the patient to fixate their open eye, they hold a penlight directly below their open eye for the patient to fixate.

• It must be noted that this measurement may tend to underestimate the monocular PD since the

corneal reflexes tend to be located 0.4mm nasal to the center of the entrance pupil. International Centre for Eyecare Education Section A – PRELIMINARY EXAMINATION