Neutral density filters are available in a variety of densities, with 0.3, 0.6, 0.9 and 1.2 log units being most helpful in grading an RAPD ( Figure 1 ).
RAPDs can also be assigned a grade using a neutral density filter over the good eye to quantify the defect. 4 Visual acuity does not necessarily correlate with an RAPD however, clinicians should always look carefully for one in cases of significantly reduced acuity in one eye. In addition, an RAPD cannot be caused by disorders of ocular media or refraction, even if extreme. Severe but bilaterally equal disease will not result in an RAPD, as a bilateral APD does not exist. 4 To cause an RAPD, the damage must be unilateral or asymmetric, such as in severe retinal disease, optic nerve disease or compromise, or a lesion behind the eye. When the consensual response is greater than the direct response in the affected eye, the patient has a relative afferent pupillary defect (RAPD), also known as an APD or Marcus Gunn pupil, signifying damage at or anterior to the LGN. The strength of the direct pupillary response is compared with that of the consensual pupillary response in the same eye.
2,3 Pharmacologic Testing for Horner’s Syndrome 15Ĭlinicians use the swinging flashlight test to detect an afferent pupillary defect and should conduct the test in a dark room with a transilluminator or the light from the binocular indirect ophthalmoscope, which are preferred over a handheld penlight due to the intensity of the light. Neurons traveling with the ophthalmic artery go on to innervate Mueller’s muscle for eyelid control, whereas those traveling with V1 pass through the ciliary ganglion to innervate the iris dilator muscle, which allows for mydriasis. Third-order neurons give rise to post-ganglionic axons, which leave the superior cervical ganglion and run along the course of the internal carotid artery through the cavernous sinus, where they meet up with the ophthalmic division of the trigeminal nerve (V1) and ophthalmic artery to travel to the eye. The second-order neuron leaves the spinal cord and passes over the apex of the lung to synapse at the superior cervical ganglion. Originating in the posterior hypothalamus, the first-order neuron descends through the brainstem to synapse in the ciliospinal center of Budge between the levels of the eighth cervical and fourth thoracic vertebrae (C8-T4). Oculo-sympathetic innervation to the eye consists of a three-neuron arc. Efferent pupil fibers then travel with CN III back towards the orbit, where they synapse in the ciliary ganglion, with 3% of post-ganglionic fibers innervating the iris sphincter muscle (which allows for miosis) and the remaining 97% innervating the ciliary body (which allows for accommodation). This is also the reason why a lesion of the optic nerve or optic tract does not result in anisocoria, or difference in pupil size between the two eyes. For example, the direct response of the right eye (and consensual response of the left eye) indicates the integrity of the afferent pathway on the right side. Neutral density filters can be useful in grading relative afferent pupillary defects.īecause of this neuroanatomy, we are able to objectively measure the integrity of the afferent pathway by observing the direct and consensual light responses. 1 Pupil fibers synapse in the pretectal nuclei of the midbrain and travel to the two Edinger-Westphal nuclei of the oculomotor nerve (CN III), beginning the efferent pathway. The afferent pathway is responsible for transmitting the impulse of the incoming light via the photoreceptors of the retina, through the optic nerve to the chiasm and optic tract, then separate from the tract just anteriorly to the lateral geniculate body (LGN) before traveling to the mid-brain to bilaterally project to the pretectal nuclei. The pupillary light response consists of both an afferent and efferent pathway. The pupillary light and near responses are under parasympathetic innervation. Meaningful interpretation of pupillary findings requires a solid working knowledge of the anatomy of the light reflex and the autonomic innervation of pupillary responses. This article addresses the more commonly encountered pupil disorders and how clinicians can detect them through routine pupil testing. With careful clinical examination, this test can aid in the diagnosis and management of many of these conditions at the primary care level. Pupil testing can reveal serious retinal and neuro-ophthalmic disease and therefore should be incorporated into every comprehensive eye examination.
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