10 Ocular Motility C. Denise Pensyl, William J. Benjamin linicians are faced with the challenge of differenti­ approximately 2 rnm, below which the effects of Cating the etiologies of asthenopia, blur, diplopia, reduced retinal illuminance and diffraction outweigh and headaches associated with the use of the eyes by the beneficial aspects ofan increase in depth offield and their patients. Oculomotor deficiencies can be one of reduction of ocular spherical aberration. The entrance several possible causes of such symptoms and are the pupil also controls blur circle size at the retina for object result of defects in the central nervous system, afferent rays not originating from the far point plane of the eye. or efferent nerve pathways, or local conditions of a The entrance pupil averages 3.5 mm in diameter in nature so as to impede appropriate oculomotor func­ adults under normal illumination but can range from tion. Oculomotor function will be extensively analyzed 1.3 mm to 10 mm. It is usually centered on the optic during phorometry (see Chapter 21) in terms of binoc­ axis of the eye but is displaced temporally away from ularity and muscle balance after the subjective refraction the visual axis or line of sight an average of 5 degrees. has been completed. This chapter focuses on clinical The entrance pupil is decentered approximately procedures that are typically used to analyze oculomo­ 0.15 mm nasally and 0.1 mm inferior to the geometric tor function before the subjective refraction is per­ center ofthe cornea. J This amount ofdecentration is not formed, though in some cases the practitioner may distinguished in casual observation or by the clinician's decide to use a few of these tests after the refraction is normal examination ofthe pupils. In general, the diam­ determined. eter of the pupil gradually becomes smaller after about By using primarily handheld devices or instruments, the age of 12 to 18 years. This appears to be a linear the normality or abnormality of the functions of the relationship in which pupil size for light-adapted and ocular muscles are estimated: the iris sphincter and dark-adapted eyes at age 20 (means nearly 5 mm and dilator muscles, the eyelid muscles involved in mainte­ 8 mrn, respectively) both diminish to about 2 mm and nance ofthe palpebral aperture, the extraocular muscles, 2.5 mrn, respectively, at age 802.3 (see Figure 25-4). The and the ciliary muscles. Hence, ocular motility is progressive change in size is known as senile miosis and assessed by observation of (1) the pupils and pupillary is shown in Table 10-1. It is not totally explained by reflexes, (2) the palpebral apertures and eyelid move­ increased iris rigidity or loss of muscle fibers in the iris, ments, (3) monocular and binocular eye movements, but apparently also includes a progressive delayed (4) monocular and binocular eye alignment, and (5) latency of response time, indicating some neurological accommodative amplitudes and facility. involvement. 2 Aging is presumed to cause a reduction in sympathetic tone." Because it is only the entrance pupil that can be examined noninvasively by the prac­ THE PUPILS AND PUPILLARY titioner, in clinical parlance the word "entrance" is REFLEXES dropped from the term and the single word "pupil" is meant to apply to that which is observed, which is the The pupil controls retinal illumination and determines entrance pupil. retinal image quality. The entrance pupil of the eye is Pupil size is always changing in the normal eye formed by refraction of the real pupil by the cornea and because of convergence/accommodation (near or triad is just over 3 mm behind the anterior corneal surface. response), pupillary responses to light and small, Retinal illumination is proportional to the square ofthe regular oscillations from fluctuations in the sympathetic pupillary diameter. Depths of field and focus for clear and parasympathetic nervous systems. Pupil size can be vision are inversely proportional to pupil diameter. The influenced by drugs and medications (see Chapter 12) lower limit of pupil size for optimal visual acuity is and is slightly larger in persons with light irises 356 Ocular Motility Chapter 10 357 Pupil size is controlled by smooth muscles inner­ TABLE 10-1 Approximate Pupil Diameters vated by the autonomic nervous system. An effective According to Age competition between the radial dilator muscle, which is sympathetically innervated and acts to dilate the pupil, Photopic Scotopic and the annular sphincter muscle, which is parasympa­ Age (years) Diameter [mm] Diameter (mm) thetically innervated and acts to constrict the pupil, 20 5.0 8.0 determines the pupil size. The parasympathetic inner­ 40 4.0 6.0 vation has more control over the pupil size, because the 50 3.5 5.5 sphincter muscle is the stronger of the two. The dilator 60 3+ 4.25 muscle acts in opposition such that mydriasis occurs 70 2.5 3.0 when the sphincter tone is released. As a result, con­ 80 2+ 2.5 striction of the pupil to light (miosis) is slightly faster than dilation (mydriasis) when the light is extinguished. The unstable equilibrium reached between the sphinc­ ter and dilator muscles creates small continuous varia­ compared with dark irises. Pupils become mydriatic in tions in the pupil size, which are normal and called response to large sensory, emotional, or psychological pupillary unrest or hippus," The normal function of the stimuli. Hyperthyroidism and ingestion of lead can pupil and its innervation are extensively covered in produce mydriasis. Various medications, including anti­ Chapter 4, and pharmacological manipulation of the histamines, over-the-counter decongestants, anticholin­ pupil is covered in Chapter 12. The reflexes controlling ergics, phenothiazines, amphetamines, cocaine, and pupil size are important in the diagnosis of neuro­ antianxiety agents, may also enlarge the pupils.v" The ophthalmic conditions. pupils become miotic in response to pain or irritation The principal sensor for the pupillary light response within the globe as a result of the oculopupillary reflex is the photopic system (the cones); therefore, illumina­ during keratitis, iritis, or trauma." However, pain else­ tion of the fovea is the primary determinant of the where in the body tends to cause pupil dilation. Long­ pupillary light reflex. The retinal nerve fibers that relay standing diabetes, sleep, and intraocular inflammation information for pupillary control travel through the produce miosis. The pupils ofinfants and the elderly are optic nerve to the optic chiasm. Here, approximately small." Medications, including cholinergic agents used half of the afferent (sensory) fibers decussate to the to treat glaucoma, chlorpromazine, cholinesterase contralateral optic tract, and the remaining half con­ inhibitors (found in insecticides and toxic nerve gases), tinue in the ipsilateral tract. The afferent fibers follow and morphine derivatives, constrict the pupil.t" Under the brachium ofthe respective superior colliculus to the ordinary circumstances, the pupils of females may be midbrain, where they synapse with a pretectal nucleus larger than those of males, and the pupils of myopes in the hypothalamus. Here, the fibers undergo semide­ larger than those ofhyperopes. However, the pupil sizes cussation. Each ofthe pretectal nuclei sends crossed and of females and males are no different, and of myopes uncrossed intercalated neurons through the posterior and hyperopes no different, when specialized condi­ commissure to the Edinger-Westphal nucleus, the tions ensure that accommodative differences have been origin of the efferent (motor) fibers that control pupil accounted for." size." Therefore, the photopic information given to one The pupils should appear round, roughly centered eye is normally transmitted to both pupils, creating within the iris, and of equal size in the normal patient. direct and consensual (indirect) responses. Both of The irises should be of the same coloration. The clini­ these responses, the direct pupillary light reflex and the cian should be aware of deviations from this norm, consensual pupillary light reflex, should be evaluated which could appear in the form of a unilateral or bilat­ when the pupillary responses are examined. eral irregular pupil (a nonround pupil), ectopic pupil The parasympathetic innervation to the iris, starting (significantly decentered pupil), polycoria (more than in the Edinger-Westphal nucleus, travels with the third one pupil in an eye), anisocoria (pupils ofdifferent sizes cranial nerve, emerging from the brain ventrally between in the two eyes), or heterochromia (irises of different the cerebral peduncles. The nerve then follows a course color or lightness/darkness in the two eyes). In ocular along the posterior communicating arteries to pierce the albinism and oculocutaneous albinism, the irises of the wall of the cavernous sinus. Here it is close to the first two eyes appear to contain little or no color and scat­ and second divisions ofthe fifth nerve. The nerve enters tered light enters the eye through the iris, such that the the orbit via the superior orbital fissure. The pregan­ pupil is not allowed to perform its optical functions. glionic parasympathetic fibers deviate from the third In aniridia, the iris is absent or only partially present; cranial nerve and synapse at the ciliary ganglion. therefore, the pupil does not exist or is considered to be Postganglionic fibers reach the iris sphincter muscle via expanded to the ciliary body. the short ciliary nerves.' The overwhelming majority of 358 BENJAMIN Borishs Clinical Refraction these parasympathetic fibers innervate the ciliary muscle mination are made, is an ideal time to make the assess­ for control of accommodation, whereas only 3% of the ment. The room should be illuminated well enough to fibers innervate the iris sphincter muscle. II clearly see the pupils, but light should not be beamed The iris dilator muscle receives innervation from the directly at the eyes. The patient is asked to fixate a dis­ sympathetic system, which begins in the posterior hypo­ tance target, and the diameter of each pupil is measured thalamus.