NPA & NPC_2023.pdf

Near Point of Convergence and Near Point of Accommodation (NPC & NPA) Ariette Acevedo, O.D. PPO1 Accommodation • Process in which there is a change in dioptric power of the crystalline lens to have and maintain an in-focus retinal image of an object in the fovea. • The focusing ability of the eye...

Near Point of Convergence and Near Point of Accommodation (NPC & NPA) Ariette Acevedo, O.D. PPO1 Accommodation • Process in which there is a change in dioptric power of the crystalline lens to have and maintain an in-focus retinal image of an object in the fovea. • The focusing ability of the eye that allows targets to be kept clear over a range of distances. “Fathers of Accommodation” • Thomas Young (1801) was the first to demonstrate that changes in the crystalline lens were responsible for the changes in focus. • During Young’s time the ocular structures has not been studied in detail, thus he attributed the accommodative power to the lens fibers. • Bowman & Burke (1847) discovered the ciliary muscle and identified it as responsible for the lens power changes during accommodation. “Fathers of Accommodation” • Finchman (1937) described the ciliary muscle as occupying most of the ciliary body and having the shape of a “prismatic ring”. • The ciliary muscle consists of 3 types of fibers: • Longitudinal • Radial • Circular Ciliary Muscle • Longitudinal (meridional): • Long fibers originating in the scleral spur and inserting into the equatorial portion of the suprachoroid. • Radial: • Short fibers, also apparently originating in the scleral spur, going backward and giving rise to the circular fibers. • Circular: • Innermost fibers and have the action of a sphincter muscle. Ciliary Muscle: Radial Fibers • The radial portion is also referred as the reticulated portion, referring to it as an open network of muscle bundles. L R C Zonular Fibers • The main group of zonular fibers (suspensory ligament) inserts over the lens capsule. • Consists of 3 bundles: • Anterior bundle: the strongest and thickest fibers, inserting into the anterior lens capsule and having the greatest effect for holding the lens in the unaccommodated (flattest) position. • Equatorial fibers: relatively small in number. • Posterior fibers: large in number, but relatively thin. Schlemm’s Canal TM Scleral Spur Zonular Fibers • Moses also describes a second group of fibers composed of 2 subgroups of fibers: • Fibers that form a dense meshwork on the inner surface of the ciliary body. • Fibers extending from the pars plana of the ciliary body into the vitreous, forming the vitreous base. • The vitreous base is the strongest adhesion site of the vitreous to the retina. Changes during Accommodation • Hermann von Helmoltz (1855) used the Purkinje images to determine the changes occurring during accommodation. • Purkinje images: reflected images formed by the cornea and lens. • Range from P1 to P4 • P1 and P2: are on the cornea • P3 and P4: are on the lens Changes during Accommodation • Von Helmoltz described the changes occurring during the accommodation process: 1. Pupil constriction 2. Pupillary margin of the iris and the anterior surface of the lens move forward. 3. The anterior surface of the lens becomes more convex. 4. The posterior surface of the lens becomes slightly more convex. 5. Lens sinks, due to gravity.* 6. Choroid moves froward. * Changes during Accommodation • Finchman went on to describe how the lens capsule is thicker anteriorly and, on the equator, than posterior and near the poles. • This variation allows the anterior lens surface to become highly curved in accommodative state. • Thus, leading to more power. • https://www.youtube.com/watch?v=1y Ipyitm6eE Range and Amplitude of Accommodation • We lose accommodation beginning at a young age, but it does not affect us until a certain age range. • In young patients, some may present with a decreased amplitude of accommodation for their age. • Accommodation can be specified in terms of range and amplitude. • Both are based on the position of the far point and near point of accommodation. • From the farthest point to the nearest point at which the eye can form a sharp image on the retina. Range and Amplitude of Accommodation • In a young emmetrope (or fully corrected), the range of accommodation they have, if they have clear vision from infinity up to 10cm from the nose, the range of accommodation will be infinity minus 10 cm. • 𝑅𝑎𝑛𝑔𝑒 𝑜𝑓 𝑎𝑐𝑐𝑜𝑚𝑚𝑜𝑑𝑎𝑡𝑖𝑜𝑛: ∞ − 10𝑐𝑚 = ∞ Amplitude of Accommodation • Amplitude of Accommodation (AoA) is the dioptric value of the near point of accommodation (NPA) minus the dioptric value of the far point of accommodation (FPA). • NPA – FPA = AoA • Ex: a patient with correction has a FPA at infinity and a NPA at 10cm. 1 𝑥 100 10𝑐𝑚 1 = 0.00𝐷 ∞ • NPA dioptric value = • FPA dioptric value= • AoA: 𝑁𝑃𝐴 − 𝐹𝑃𝐴 • 10𝐷 − 0𝐷 = 10𝐷 • AoA = 10D = 10.00𝐷 Near Point of Accommodation (NPA) • Used to evaluate the amplitude of accommodation. • This procedure is performed monocularly. • Amplitudes may vary by eye • Procedure: • Monocularly and with habitual correction for near. • Exception: presbyopes who have no accommodation at all. If wearing correction, you are not testing AoA but instead near add power. • Full room illumination and Illuminate the chart but not too bright • Constriction of pupils, increasing depth of focus • Target: 1-2 lines above near best VA • Ex: NVA 20/20 target: 20/25-20/30 Near Point of Accommodation (NPA) • Slowly bring the target closer to the eye until the letters cannot be focused. • Ask the patient when they cannot see the letters, ask them to try to focus. • A single letter target may be used but a paragraph may be preferred. • Record the distance in centimeters (cm) from the test card to the spectacle plane. • Recommended to use a transparent ruler, but do not move the target towards the ruler. • Calculate the AoA with the inverse of the NPA • Donder’s AoA • Example: NPA = 25cm, then AoA is 4.00D • 𝐴𝑜𝐴 = 1 25 = 4.00𝐷 Instructions • ”Read me this line, while bringing it closer until you cannot read it anymore. Blink as necessary.” • Ask them to blink and try to focus until they can no longer focus the target. • “Léame esta línea, acercándola a su ojo, hasta que no la pueda leer mas. Pestañee o parpadee si es necesario.” Recording • When recording results must include: • Method used, with (cc) or without (sc) correction • Ex: NPA cc OD 5cm OS 4cm • Or AoA Donder’s in Diopters • NPA sc OD 12cm OS 13cm • AoA Donder’s sc OD 8.3D OS 7.7D • Record OD and OS, this is a monocular test. • Compare to the expected findings according to Hofstetter's formula. Hofstetter’s Formula • Minimum Expected • 15 − 0.25 𝑎𝑔𝑒 𝑜𝑟 15 − 𝑎𝑔𝑒 4 • Average Expected • 18.5 − 0.3(𝑎𝑔𝑒) • Maximum Expected • 25 − 0.4(𝑎𝑔𝑒) • AoA of both eyes should be within 1D of each other and no more than 2D below the minimum expected. Donder’s Table for Age Referenced AoA Age Amplitude (D) 10 14.00 15 12.00 20 10.00 25 8.50 30 7.00 35 5.50 40 4.50 45 3.50 50 2.50 55 1.75 60 1.00 65 0.50 70 0.25 75 0.00 Convergence • Movement of the eyes turning inward. • Usually in an effort to maintain single binocular vision when viewing an object. • Mediated by the MR muscle, innervated by the Oculomotor Nerve (CN III), inferior division. • It is a type of vergence movement. Convergence and Lenses • Minus lenses (concave) cause more convergence and require more accommodation. • Plus lenses (convex) cause more divergence and require less accommodation. Light bends toward the base and the image bends toward the apex Convergence and Prisms • Prisms only affect the vergence of the eye, never affect accommodation. • Base Out (BO) prism produces convergence. • Positive convergence or positive relative convergence Positive convergence is the normal Convergence and Prisms • Base In (BI) prisms will produce divergence. • Negative convergence Our eyes have good convergence but poor divergence, so negative convergence is not the normal Convergence • The amplitude of convergence is equal to the reciprocal (in meters) of the near point of convergence (NPC) multiplied by the patients interpupillary distance (IPD) in centimeters. • 𝐴𝑚𝑝𝑙𝑖𝑡𝑢𝑑𝑒 𝑜𝑓 𝐶𝑜𝑛𝑣𝑒𝑟𝑔𝑒𝑛𝑐𝑒 = 1 𝑁𝑃𝐶 𝑚 𝑥 𝐼𝑃𝐷 𝑐𝑚 • The NPC is specified in terms of the line joining the centers of rotation of both eyes. • Has 4 components: • • • • Tonic Accommodative Fusional Proximal We have to take ALL in consideration for measuring convergence Tonic Convergence When we sleep our eye goes usually up and out • If all innervation of the ocular muscles cease, the anatomical position of rest of the eyes would be divergence. • Tonic convergence is responsible for moving the eyes from the anatomical position of rest to a more convergent position. • Deficiencies in tonic convergence will result in: • Excessive tonic convergence will produce esophoria. • Affected by sleep, drowsiness, alcohol and anesthesia. • If deficient will produce exophoria. • Tonic convergence is measured indirectly with phoria position. Convergence 3 things that happen when we look at near: -Accomodation -Convergence -Miosis (Pupils constrict) • Fusional Convergence: • Compensates for any excess or deficit in the tonic convergence by identifying retinal disparity as the stimulus. • This will help in preventing diplopia (double vision) • May be stimulated by prisms by inducing retinal disparity. • Accommodative Convergence: • Convergence associated with accommodation. • Can be measured with the use of near targets and minus lenses. Minus lenses induce accomodation and convergence Proximal Convergence • A voluntary convergence that takes place when it is known that an object is nearby. • Known as “psychic convergence” due to the awareness of nearness. • Only directly inferred, cannot be measured directly. Near Point of Convergence You should do it monocular, determine distance with a ruler to see when patient see clears or not(in cm). We measure the amplitude convergence(Diopters). • This is a test used to determine the ability to converge while maintaining fusion. • This is a binocular test that provides both objective and subjective results • Illumination: full room lights and illuminating the target. • Habitual correction to be worn • Accommodative Target (acc.): one or two lines above best near VA in the worse eye • Non-Accommodative Target: light • Can be used in conjunction with a R/G glasses or Red Lens red/green glasses Near Point of Convergence Some patients do not report blur but does report double. You must move back. BUT if patient does not report either, you have to see eye deviated (supressing) • Placing the target ~50cm from the patient, directly in front and slightly below midline. • Ask the patient to report when the target is blurry, breaks into two, and appears single again. When they say in turns blurry turn it back to see when they see well • Blur/Break/Recovery • “Dígame cuando se ponga borroso, doble y sencillo.” • Slowly move the target toward the patient until they report blurry, continue moving towards the nose until they report double vision • If the patient does not report double but you notice an eye turning outward (nondominant eye) this is reported as a break TTN? UTN? • The patient is suppressing • Now move the target away until the patient reports seeing single. Near Point of Convergence • Objectively you can notice when the eye turns out (breaks) and when it regains alignment (recovery) • Very useful for infants or non-verbal patients • Record the distances in centimeters • Always observe which eye deviates and make a recording of it. • Always record the dominant (non-deviating) eye. Near Point of Convergence • If you notice one eye deviating out, but the patient never reported diplopia, record the distance of the break and the deviated eye (nondominant). • Do not perform recovery • The eye that maintains fixation is the dominant eye, the deviating eye is the non-dominant eye. • If the target reaches the nose and the patient never reported diplopia and no eye was seen deviating, record: “Up to the Nose (UTN)” or “To The Nose (TTN)”. • Do not perform recovery Recording • Must include test, target used, with (cc) or without (sc) correction, blur/break/recovery (cm) and dominant eye if noted. You do not write the WORD just the NUMBER • Ex: • • • • Blur and recovery should be bigger than break NPC cc (acc.) TTN NPC cc (acc.) 9cm/5cm/12cm diplopia (OD out) NPC cc (acc.) X/6cm/10 cm, suppression, OS out NPC cc (RG) X/9cm OS out or OD dominant • This means the patient never saw blurry, at 9cm doctor noticed that OS lost fixation (break), but patient never reported diplopia. • If this happens do not record a recovery. Expected Values Kids who have the values over 7 you must watch because they cannot read or write correctly (prescribe therapy) -presbyopes will suffer of convergence insufficieny. Cuz they cannot accomodate well meaning not converging well. More esophoric. They can develop diplopia • Acommodative target: 5cm break/7cm recovery • Non-Accommodative target: 7cm break/10cm recovery • Recovery: 12 ±2cm • If break values are over 7cm, consider Convergence Insufficiency (CI). • Ex: 16/12/15 • But keep in mind that presbyopes will suffer from CI, which can sometimes cause diplopia. • If up to the nose (UTN/TTN) consider Convergence Excess • Accommodative dysfunctions may mimic CI/CE, full binocular vision evaluation is required before a diagnosis. Most Common NPC Errors • Relying on subjective NPC measurements. • Correlate what you see with what the patient is reporting. • Performing the test only once. • Test should be performed at least twice to gain an impression of sustained and repeated convergence ability. • Moving the target too quickly • Can lead to overestimation • Moving the target too lowly • May cause loss of attention especially in children