RAF RULE FOR CALCULATING NEAR POINT OF ACCOMMODATION AND NEAR POINT OF CONVERGENCE

Royal Air Force for measuring the Near Point of Accommodation and Near Point of Convergence
HISTORY
MATERIALS
NORMAL RANGE
PROCEDURES
CRITERIA

1. RAF RULE ASHISH NEUPANE NEH, B.OPTOMETRY

2. INTRODUCTION RAF rule is a routinely employed instrument in ophthalmology and optometry practices to measure Near Point of Convergence ( NPC ) and Near Point of Accomodation ( NPA ) Uses as standard tool for research purpose and to provide therapeutic home based orthoptic exercises.

3. A Brief History The RNPR was first mentioned in the literature by J.C. Neely in 1956. It stated that the RNPR was originally developed in the UK in the mid-1950s to assess the visual abilities of the individuals conscripted into the US military to carry out routine clerical responsibilities.

4. Instrument was used to assess the visual functions like convergence and accomodation, as well as used to establish the correlation between distance and near visual acuity. Apart from the UK and Ireland, The RNPR is marked by Haag-Streit as RAF binocular gauge.

5. THE DESIGN OF RNPR TARGET DRUMS RNPR consists of a rotating four-sided cubical drum held on a slider. The slider is attached to a square section metal rule. Each of the four sides has different accomodative targets with black prints on a white background which includes:

6. TARGET DRUMS A REDUCED SNELLEN CHART SECTION OF GENERAL POST OFFICE (G.P.O) TELEPHONE DIRECTORY TIMES ROMAN TYPE FACE A DOT ON A LINE

7. Side 1: a reduced snellen chart Distance snellen chart is reduced to one-seventeenth of its actual size and is accurate at 35 cm. Reduced snellen chart substends an angle of 5 min of arc at 35 cm, the same that a distance snellen chart substends at 6m Fig A

8. Side 2 : a section of the General Post Office (G.P.O) telephone directory One face consist of a photographed section of G.P.O telephone directory Fig B

9. Side 3 : Times Roman Typeface The faculty of Ophthalmologists (1951) suggested that the standard reading types should be, I. in “Times Roman’’ II. with “standard” spacing III. using printing sizes of 5, 6, 8, 10,12,14,18,24,36 and 48 points IV. recorded for near as S5, S8 and so on. Based on these recommendations one face consists of four lines : N5, N8, N10 and N12.

10. Side 4 : a dot on a line The fixation target to assess NPC is a small black dot in the centre of a vertical line. Sides 1,2 and 3 are used for measurements of NPA and side 4 is used for NPC.

11. SQUARE RULE The square rule which holds the drum is 50 cm long. This distance allows the measurement of NPC and NPA of presbyopes who prefers to read at 40 cm or more. Furthermore , it is also easier to determine the angle of 25 degrees from the eyes by a point at the end of a rule 23.3 cm in length (tan 250 x 50) held at 90 degrees to the end of the rule

12. Four sides of the square rule are marked as : a centimeter scale in 1 cm increment a corresponding equivalent dioptric scale expected age scale scale indicating the positions of normal and abnormal convergence

13. Cheek rest The cheek rest of the RNPR (6cm in length) is attached to one end of the rule. Made of plastic and has a V- shaped notch in the center to fit the nose. The cheek rest allows the device to be comfortably placed on the subject’s cheek.

14. Near Point of Convergence It is the closest point at which an object can be seen single during bifoveal vision. In other words, it is the point at which the two foveal lines of sight intersect, when maximum convergence is exerted.

15. Procedure of NPC RNPR measures both the subjective and objective NPC . Prerequisite for measuring NPC is the binocularity of a patient and a normal room illumination Made the examiner holds the ruler and gently places the cheek rest on the inferior orbital margin. NPC is most accurately measured bu the RAF rule in the depressed position of 45 degrees. Ask the patient to focus on the black dot and slowly moves the target towards the patient’s eye at a constant and linear rate of about 1 + 2cm/sec

16. Objective Methods: The examiner should look at patient’s eye movement. The patient is asked to fix steadily on the central dot on the line and the slider is moved slowly towards the patient’s eye. The NPC is the distance where one eye is seen to diverge The test is repeated 2 to 3 times.

17. Subjective Methods Test should be performed with best correction. Instruct the patient to inform you when the target becomes double or when he/she sees the line move to one or the other side. this happens when the image is suppressed, the line moves to the side of the dominant eye. Ask the patient to blink several times when the target becomes blur or double.

18. Now move the target back until the vertical line again becomes single. Record the values in cm or diopters from the square rule.

19. Normal Range Normal near point of convergence is about 6-10 cms. NPC less than 5 cm indicates convergence excess. If the NPC is more than 10 cm there is sign of poor convergence.

20. Break Point and Recovery Point The subjective break point is indicated when the patient either reports diplopia or until the slider is stopped by the cheek rest. The recovery is noted when the patient reports single target when the slider is slowly moved back.

21. Near Point of Accomodation It is the closest point at which small objects can be seen clearly. It is also called ‘near point’ or ‘punctum proximum’. The normal near point depends on the age of the person tested.

22. Procedure Of NRA The standard target for the measurement of NPA is Times Roman Typeface (NV chart) The top smallest line which is N5 We have to do the NPA measurement both monocularly and binocularly.

23. To carry out binocularly, We have to put the cheek rest on the inferior orbital margin of the patient. Position of RAF rule should be slightly depressed at an angle of 45 degrees. Now ask the patient if he/she can see the word. Now move the target slowly towards the patient at a constant rate of 1 to 2 cm/sec Instruct the patient to inform you when the word gets blurred.

24. Put the target back and note the point (cm/diopters) at which the word again looks clear. The same test is performed for monocular RE and LE. Repeat the test 2 to 3 times.

25. REFERENCES Theory and practice of Squint and Orthoptics, A.K khurana 2nd edition Primary care optometry, Theodore Grosvenor 3rd edition Article = Sharma IP. RAF near point rule for near point of convergence -a short review. Ann Eye Sci 2017;2:16.