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Characteristics of normal and abnormal pupils--part 1.

In part 1 of a two-part series, optometrist and lecturer, Janis Orr describes the underlying pathways that serve pupil function and provides an overview of the clinical assessments that should be undertaken as a part of a routine eye examination.

Course code: C-38451 | Deadline: December 12, 2014

Learning objectives

To be able to obtain relevant detail from patients presenting with pupil abnormalities (Group 1.1.2)

To be able to evaluate pupil reactions using appropriate techniques (Group 3.1.9)

Introduction

Evaluation of pupil function is a valuable clinical test and should be assessed during every eye examination. Through careful assessment and knowledge of the underlying neuronal pathways practitioners can identify abnormalities in pupil function and determine the appropriate management strategy for the patient.

Innervation of the pupil

Parasympathetic innervation

The parasympathetic innervation of the pupil consists of four neurons comprising afferent and efferent pathways (see Figure 1).

Afferent pathway

The first neuron connects the retina with the pre-tectal nucleus at the level of the superior colliculus in the mid-brain. This reflex is mediated by the retinal photoreceptors. Fibres from the nasal and temporal retina travel along the optic nerve with the nasal fibres crossing over at the chiasm. These fibres then travel along the optic tract until they reach the pre-tectal nucleus at the level of the superior colliculus in the mid-brain. The second neuron connects the pre-tectal nucleus to the Edinger-Westphal nuclei. Internuncial neurones run to both ipsilateral and contralateral Edinger-Westphal nuclei. This explains why a unilateral light stimulus evokes a bilateral (direct and consensual) and symmetrical pupillary constriction. Damage to this part of the pathway leads to light-near dissociation (where the near response is normal but the light response is defective).

Efferent pathway

The third neuron connects the Edinger-Westphal nucleus to the ciliary ganglion. Parasympathetic fibres pass through the inferior division of the third cranial nerve. The fourth neuron leaves the ciliary ganglion and passes with the short ciliary nerves to innervate the sphincter pupillae (see Figure 2, page 54).

Sympathetic innervation

The sympathetic innervation of the pupil consists of three neurons (see Figure 3, page 55).

The first neuron starts in the posterior hypothalamus and descends down the brain stem to terminate in the cilio-spinal centre of Budge. The second neuron passes from the cilio-spinal centre to the superior cervical ganglion in the neck. The third neuron ascends along the internal carotid artery until it joins the ophthalmic division of the trigeminal nerve. Sympathetic fibres travel via the nasociliary nerve and the long ciliary nerves until they reach the ciliary body and dilator pupillae muscle.

Damage to any component of the parasympathetic or sympathetic visual pathways can cause abnormalities in the size, shape and the light reflexes of the pupil.

Factors that influence normal pupil size

Normal pupils are round in shape and equal in size. They constrict in response to light (miosis) and dilate in response to darkness (mydriasis). Pupils react briskly to changes in illumination, however, mydriasis occurs more slowly than miosis.

The mean range of pupil size across the population is 2-4mm in bright light and 4-8mm in dim light. (1) However, the size of the pupil in certain illumination levels varies considerably between individuals. (1)

Normal pupil size can be influenced by a variety of other factors:

* Hippus--this is the normal physiological fluctuation in pupil size. It is independent of eye movements or changes in illumination (2)

* Accommodation--when an individual accommodates, they also converge and undergo pupillary miosis. (3) This relationship is known as the near triad (4)

* Senile miosis--pupil size gradually decreases with age (1, 5)

* Gender--pupil diameter is not influenced (6)

* Refractive error--it is often assumed that myopic individuals have larger pupils than emmetropic and hyperopic individuals. This has been disproved by several research papers (1, 6 7)

* Iris colour--pigmentation of the iris (or race) does not influence pupil diameter (1)

* Alertness and emotion--psycho-sensory influences act on the hypothalamus (part of the sympathetic innervation of the dilator pupillae) via the limbic system. (8,9) Mydriasis can result from intense emotion, shock, pain and heightened central nervous system arousal. (8,9)

Examination of pupil function

When examining pupil function you must consider:

* The shape and position of the pupils

* The size of the pupils

* The reaction of the pupils to light (light response/reflex)

* The reaction of the pupils to accommodation (near response/reflex)

* The swinging flashlight test.

Shape and position of the pupil

An initial assessment of the shape and position of the pupils can be undertaken by general observation as the patient enters the consulting room and during history and symptoms. If the patient has a congenital iris abnormality or a history of ocular trauma, surgical damage or disease this is generally obvious unless the irises are particularly dark. If any abnormality of the pupil or a marked difference in pupil size between the eyes (anisocoria) is detected, it is worth asking the patient a few more questions about their general health and medical history. If the patient is aware of the abnormality and it is longstanding, this is much less of a concern than if it is a novel finding. It is important to ask about visual and neurological symptoms, for example, blurred vision, diplopia, visual field loss, numbness and headaches, in order to perform an effective differential diagnosis.

The size of the pupil

Observe the size of the pupils using a direct ophthalmoscope (set at +2.00DS with a large aperture of medium brightness at a distance of 50cm). This is more accurate than simply observing the pupil with the naked eye and makes it much easier to detect anisocoria and other pupil abnormalities (particularly in patients with dark irises). Ensure that the beam illuminates both pupils. Pay attention to the size, shape and centration of the pupils.

It is important to measure pupil diameter in dim and bright light, especially in the presence of anisocoria, as the difference between the eyes can change; this is an important factor when determining whether anisocoria is pathological or physiological.

The light response test (direct and consensual)

The light response test is important as it examines the integrity of the pupillary light reflex pathway.

The room light should be slightly dimmed for this test, but the test must not be performed in complete darkness, as both pupils need to be observed. Ask the patient to fixate on a non-accommodative distant target, for example the duochrome. Use a pen-torch or direct ophthalmoscope as the light source and illuminate the pupil from below (or temporally) at a distance of 5-10cm. Shine the light on each pupil at least twice. First, check the direct light response (the reaction of the pupil you are stimulating) and then the consensual light response (the reaction of the contralateral pupil).

It is good practise to repeat this test two or three times in each eye in order to confirm the direct and consensual response and to assess the influence of fatigue.

The near response test

After performing the light response test ask the patient to shift their gaze from the distant target towards a near target, for example, a budgie stick or pen tip, and then back to the distant target again.

Constriction of the pupil to near fixation but not to direct light is called light-near dissociation. This can be caused by viral infection, as in the case of Adie's tonic pupil, damage to the pre-tectal area (Parinaud's syndrome), or damage in the rostral midbrain (Argyll-Robertson pupil).

There aren't any known conditions where the near reflex is defective or lost when the light reflex is normal. It could be argued, therefore, that you need only check the near response if the light response is abnormal.

The swinging flashlight test

The swinging flashlight test is performed in order to detect relative afferent pupillary defect (RAPD). It is one of the most important objective tests for the detection of abnormalities in the afferent visual pathway. The swinging flashlight test is often confused with the Marcus Gunn test (which examines re-dilation under sustained illumination).

It is vital that this technique is performed carefully in order to detect RAPD using a pen-torch/direct ophthalmoscope in dim room lighting. It should be mentioned that ophthalmologists often use head-mounted binocular indirect units to examine pupils as they get a direct view of the pupil (that is, they are not looking from the side of the instrument).

Shine the pen-torch on the right eye (from below) for two to three seconds and watch the right pupil constrict. Quickly move the light to the left pupil (within one second) and hold the light over the left eye for two to three seconds. The left pupil should stay the same size or dilate and quickly constrict again if RAPD is absent.

An eye with an RAPD will dilate when the light is shining on it as the consensual dilation response due to the light moving away from the normal eye overpowers the poor direct constriction response of the affected eye. Repeat this alternation several times to check that RAPD is definitely absent. It should be noted that the most common error is not shining the light on each pupil for long enough (ie less than two to three seconds).

Clinical interpretation

Firstly, record the diameter of the pupils in bright and dim light (the size of the pupil can be measured using a ruler or a circular scale).

Secondly, the shape, symmetry, reaction to light and accommodation must be recorded. The acronym PERRL(A) can be used: Pupils Equal Round Reactive to Light (and Accommodation). (10) Please ensure if you use acronyms that you know what they stand for and remember that you must describe anomalies in detail. You must also indicate that you checked the direct (D) and consensual light reflex (C) (light response test).

If the pupil responses are sluggish this must be noted (as normal pupil responses should be brisk, simultaneous and equal). If the pupil reflex is absent (in one or both eyes), record which eye is affected and which response is defective.

Thereafter, you must record the result of the swinging flashlight test. If RAPD is present, record which side is affected.

RAPD can be quantified by placing neutral density (ND) filters in front of the normal eye until the swinging-flashlight test result appears normal (unit = log units). This is not routinely measured in optometric practice but is worth mentioning. RAPD is always quantified in the Hospital Eye Service (HES) as this allows the afferent input from the afferent visual pathway of each eye to be compared. Since the light reflex represents the total neuronal input, damage anywhere along this part of the visual pathway reduces the amplitude of the pupil response to light.

Figure 4 Correct recording of a) normal
pupils, b) physiological anisocoria, c)
RAPD

Reflective learning

Having completed this CET exam, consider whether
you feel more confident in your clinical skills--how will
you change the way you practice? How will you use this
information to improve your work for patient benefit?

A Normal pupils eg PERRL(A)
D+C No RAPD
(brisk response).

Diameter = 4mm (R+L)

B Physiological anisocoria eg
pupils round, reactive to light
(D+C) and accommodation.
No RAPD (brisk response).
Anisocoria (R>L) Diameter:
R=4mm, L=3.5mm.

C RAPD present (in left eye) eg
pupils round, reactive to light
(D+C) and accommodation.
RAPD + ve LE 0.4log.
Diameter = 4mm (R+L)


A summary of appropriate recording of pupil assessment is detailed in Figure 4.

Conclusion

Pupil assessment is a vital part of every routine eye examination. It is simple to perform and effective in the differential diagnosis of disorders of the anterior visual pathways or the autonomic nervous system, which can be sight--or even life--threatening. A pupil abnormality may be the only sign of these problems and accurate detection can facilitate early diagnosis and treatment.

Dr Janis Orr PhD, MCOptom

Dr Janis Orr is a lecturer in optometry at Aston University where she is the leader of the further investigative techniques module. Her research interests include refractive error development and control, optical and biometric characteristics of the eye and corneal reshaping therapy.
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Title Annotation:CONTINUING EDUCATION & TRAINING: 1 CET POINT
Author:Orr, Janis
Publication:Optometry Today
Geographic Code:4EUUK
Date:Nov 14, 2014
Words:2022
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