The response of the iris to light levels and with accommodation is a result of a neural reflex pathway that involves the iris, retina, visual pathway, midbrain, and parasympathetic and sympathetic innervation of the eye. As such, clinical assessment of the pupil response to light elicits important information about the health of all these structures.
Light reflex
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The pupillary light reflex pathway is shown in Figure 1. If a pen-torch is presented to one eye, the pupil will constrict (the direct reflex) as will that of the other eye (the consensual reflex). Both will constrict when a patient changes gaze from a distant target to a near one (the near reflex). Ambient lighting should be reduced to exaggerate the resting pupil diameter but be of sufficient levels to allow easy viewing of the pupil, particularly in patients with very dark irises (Figure 2). A different reflex pathway links pupils response with accommodation.
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Disruption of the afferent pathway, which may be caused by damage to the retina, optic nerve, chiasma, tract or superior brachium, will result in the loss or reduction of the direct and consensual reflexes. As the damage is often only to some of the pupillary fibres, there may well be a reduction in the pupil response that is only detectable when compared with the normal response; this is described as a relative afferent papillary defect (RAPD) and needs to be detected by the optometrist. This may also be found in patients with a very dense unilateral cataract as light scatter from the opacity may give an enhanced pupil response which appears as an RAPD in the contralateral eye. Some practitioners grade the RAPD by holding varying density filters before the normal eye until the contralateral direct reflex overrides the normal consensual reflex (Figure 3).
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With conditions where vision loss cannot obviously be traced to ocular signs, for example retrobulbar neuritis, it is essential to investigate anomalous pupil responses. Table 1 offers some common examples of causes of an afferent defect.
Anisocoria
Anisocoria (or unequal pupil size) may have a number of aetiologies. These can be categorised into three types:
- Physiological – this describes pupils that are unequal simply due to variation in anatomy. As such they are not assumed to be related to any disease process or tissue damage and the patient may simply need reassurance. Around one-fifth of people have different pupil sizes, usually less than 0.5mm difference in diameter. This physiological variation is confirmed by the equal response of the uneven pupils to light stimuli. An easy way to check whether anisocoria is physiological is to measure the two pupils in light and then dark conditions. The difference between each pupil diameter should stay constant whatever the ambient light level if the anisocoria is physiological. It is also worth confirming that the difference, if large enough to be noticed, has been known about for a length of time rather than only recently. Generally, physiological anisocoria tends to be small (less than 1mm), but long-standing larger differences may also be physiological.
- Efferent defects – these are due to damage to the efferent innervation of the pupil and the main causes to be aware of are listed in Table 2.
- Secondary – this blanket term is here used to describe uneven pupils due to previous damage, either pathological (for example after recurrent anterior uveitis), iatrogenic (for example as a complication of intraocular surgery), pharmacological (for example as a result of asymmetric influence of a systemic drug), or age-related (as due to asymmetric atrophy of the iris muscle fibres with age).
Damage to the efferent pathway will result in unequal pupil size (anisocoria) and needs to be distinguished from physiological anisocoria, the presence of different pupil sizes not related to any underlying disease process.
Routine assessment
A typical sequence would be to assess the direct and consensual light reflexes for each eye, then to look for an RAPD and finally to assess the near reflex. The latter need not be included in every case as there is no single disease process where the near reflex alone is lost. However, where the direct reflex has been disrupted, the presence of a near reflex is important as part of the diagnosis of Argyll-Robertson pupil. This condition is due to midbrain damage such as occurs in congenital syphilis.
It is important to establish the best background lighting to best assess pupils and this will vary from patient to patient. A dimmer switch for the consulting room main lights is useful here. The lights should be dim enough to enhance pupil dilation prior to introducing the pen-torch light, while bright enough to still allow the consensual reflex to be seen without the pen-torch. So darker irises tend to need more background lighting than paler irises.
After directing the patient to fixate on a distance letter target, a bright pen-torch or direct ophthalmoscope light (without too much spread of beam) should be moved in front of one eye from the temporal side of the patient’s head (Figure 2). A cheap pen-torch is usually all the special equipment required here, though keeping it to within a few centimetres from the target eye will help prevent light spread to the opposite eye. Once in front of one eye, a smooth and obvious constriction should be noted matched by one in the opposite eye. This should be repeated at least three times to look for any gradual reduction in reflex which may indicate an abnormality. Each time the light is removed, a direct and a consensual dilation should also be noticed. The same should be repeated for the opposite eye. As mentioned above, the absence of a direct reflex necessitates the investigation of a near reflex.
When the light is held in front of one eye for three seconds and then moved across to the opposite eye for a further three seconds, then back to the first and so on, a sequence of bilateral equal constrictions should be observed. In the case of damage to the afferent pathway (as with, for example, retrobulbar neuritis), the affected eye will show a dilation when the light is moved in front of it from the opposite healthy eye. This is because the consensual reflex from the opposite eye is stronger than the direct response in the affected eye (that is, the afferent signal is present, but defective relative to the other side). Figure 3 shows a schematic representation of the assessment of an RAPD.
Most practitioners use some form of acronym when recording the pupil responses, such as a tick next to DCN (signifying direct, consensual, near) and no RAPD. Others write PERRLA (representing pupils equal, round and respond to light and accommodation). With defective pupil responses, a description of the exact defect needs to be recorded.
Anisocoria which does not respond symmetrically to light may indicate a pathological process such as an efferent defect. Horner’s syndrome, for example, will give a unilateral miosis. In the case of a pupil defect or pathological anisocoria (in the absence of any knowledge of the cause already having been established), referral is usually appropriate.
Management of pupil anomalies
- Physiological anisocoria is usefully recorded (with measurement of the size difference in mesopic ambient light. The condition should be explained to the patient and any concerns they have be assuaged.
- Afferent defects need to be recorded as mentioned before and the nature of the defect interpreted along with other clinical information to arrive at the appropriate management option. This will differ depending on the suspected underlying cause. Occasionally an RAPD signifies a major compromise in optic nerve transmission and its presence helps diagnosis. For example, a marked RAPD in an eye with a swollen optic nerve head and reports of recent sudden vision loss might indicate an anterior ischaemic optic neuropathy. This usually would warrant immediate referral as it is, in effect, a form of arterial occlusion. If an afferent defect is found with multiple flame haemorrhages across the retina, this would suggest an ischaemic central retinal vein occlusion which requires a full cardiovascular assessment as well as an ophthalmological review of the macular oedema likely to be present. Such cases should be referred to the relevant medical centres as soon as possible, and certainly within two weeks. In other cases, as with cataract, referral might not yet be merited and the management will depend on the interpretation of the impact of the cataract and adherence to the relevant referral protocol.
- Efferent defects again vary in their management. A longstanding Horner’s (perhaps congenital, as suggested by heterochromia) is unlikely to benefit from referral. A recently acquired Horner’s warrants immediate consultation with an ophthalmologist as the compression of the long sympathetic nerve supply to the pupil can sometimes (albeit rarely) be related to an aneurysm. A third nerve palsy of traumatic origin may be best managed by immediate referral but if transient or longstanding might instead warrant communication with the clinic overseeing any existing systemic disease. Holmes-Adies pupil is often transient and does not merit medical intervention, though a routine referral for confirmation of the condition is sometimes recommended.
- Secondary anisocoria is usually confirmed by the presenting history. If thought to be linked with a systemic medication then correspondence with the prescribing practitioner might be appropriate.
Andrew Franklin is an optometrist and an examiner with the College of Optometrists
Model answers
Which of the following statements is true about physiological anisocoria?
D Pupil diameter differences of less than 2mm are typical
Which of the following is not an efferent defect?
C Iris atrophy
Which of the following is not likely to cause an afferent defect?
A Cerebrovacular accident in parietal lobe
Which of the following statements concerning an RAPD is true?
B There is an initial dilation when the light is moved from the unaffected to the affected eye
Which of the following causes a miotic pupil?
A Horner's syndrome
A patient has a pronounced RAPD, reduced acuity but apparently healthy disc and fundus. Which of the following is most likely?
D Retrobulbar neuritis