abnormal eye movements in dizzy patients
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Neurol Clin 23 (2005) 675703
Abnormal Eye Movements in Dizzy PatientsKi Bum Sung, MD, PhDa,*, Tae Kyeong Lee, MDa, Joseph M. Furman, MD, PhDba
Department of Neurology, College of Medicine, Soonchunhyang University, 1174 Jung-Dong, Wonmi-Gu, Bucheon, Gyeonggi 420-607, Korea b Departments of Otolaryngology and Neurology, University of Pittsburgh, EEHOS 500, Pittsburgh, PA 15260, USA
Abnormalities of eye movements frequently are observed in patients who are dizzy. Most abnormal eye movements have a specic locus but not a specic cause. Therefore, thorough knowledge of the mechanism and careful observation of the abnormal eye movements are important aspects of evaluating patients who have dizziness. This article describes basics of normal eye movements rst, followed by a discussion of abnormal eye movements occurring in peripheral and central vestibular disorders with an emphasis on their implicated sites and mechanisms.
Basics of eye movements Innervation of extraocular muscles There are six extraocular muscles that rotate each eye: four rectus (inferior, lateral, medial, and superior) and two oblique (inferior and superior). The superior oblique muscle is innervated by the trochlear nerve and the lateral rectus muscle is innervated by the abducens nerve. The other four muscles are innervated by the oculomotor nerve. The oculomotor nucleus is a paired structure lying ventral to the periaqueducal gray of the midbrain. The fascicles of the oculomotor nerve run ventrally to exit in the interpeduncular fossa as several rootlets. Each oculomotor nucleus
* Corresponding author. 1174 Jung-Dong, Wonmi-Gu, Bucheon-Si, Gyeonggi-Do 420-607, Korea. E-mail address: email@example.com (K.B. Sung). 0733-8619/05/$ - see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.ncl.2005.01.006 neurologic.theclinics.com
innervates only ipsilateral extraocular muscles except for the superior rectus muscle. Fibers to the superior rectus muscle decussate at the caudal third of the oculomotor nuclear complex and exit to join the fascicles of the other side . The fascicles of the trochlear nerve arise from a pair of grouped motor neurons in the ventral mesencephalon at the level of the inferior colliculi and course dorsally to decussate in the anterior medullary vellum, emerging from the dorsal surface immediately caudal to the inferior colliculi. Therefore, superior oblique muscles are innervated by the contralateral trochlear nucleus. The abducens nucleus lies in the oor of the fourth ventricle ventral to the genu of the facial nerve and consists of two types of neurons: motoneurons innervating the ipsilateral lateral rectus muscle and internuclear neurons supplying the contralateral medial rectus muscle. Internuclear neurons, constituting 25% to 30% of the abducens nucleus, are distributed throughout the nucleus . Axons of the internuclear neurons cross at the abducens nucleus level and in the region immediately rostral to it and then ascend in the medial part of the medial longitudinal fasciculus (MLF) [3,4]. Innervational patterns are depicted in Fig. 1. Terms for eye movements To describe the motion of the eye accurately, axes and planes must be dened. The eye rotates on three axes. These axes conventionally are referred to as X (parasagittal or nasooccipital), Y (transverse or interaural), and Z (vertical). The angular motions about these axes also are described according to the planes in which the rotations occur: horizontal (yaw, rotation about the Z axis), vertical (pitch, rotation about the Y axis), and torsional (roll, rotation about the X axis) (Fig. 2). Calling a torsional eye movement rotatory is a frequent misnomer. The primary position of the eye is the position in which pure horizontal or vertical rotation of the eye is associated with zero torsion. For clinical purposes, however, the primary position refers to the position assumed by the eye when looking straight ahead with body and head erect. A secondary position is reached by a solely horizontal or vertical rotation from the primary position. A tertiary position is reached when combined horizontal and vertical rotations take the eye away from the primary position. Torsional eye movements that rotate the upper poles of the eyes toward the subjects right (ie, in a clockwise direction from the subjects point of view) are called clockwise movements, although they seem counterclockwise to an observer. Torsional movements that rotate the upper poles of the eyes toward a certain side of the brain or toward the side of a brain lesion are called ipsiversive and ipsilesional, respectively. In addition, contrary to the usual convention for vertical tropias (misalignment of the visual axes), skew deviation (SD) is named after the side of the lower eye; for example, a left SD means a left hypotropia .
ABNORMAL EYE MOVEMENTS IN DIZZY PATIENTS
Fig. 1. Innervation of the extraocular muscles. The abducens nucleus (VI) is composed of two types of neurons: (1) motoneurons (MN) innervating the ipsilateral lateral rectus muscle (LR) and (2) internuclear neurons (IN) crossing to ascend in the MLF to innervate the contralateral subnucleus of the oculomotor nucleus (III) that contains motoneurons destined for the medial rectus muscle (MR). The superior oblique muscle (SO) is innervated by the contralateral trochlear nucleus (IV), whose axons decussate in the anterior medullary velum. Axons for the superior rectus muscle (SR) cross within the oculomotor nucleus (III). The inferior oblique muscle (IO) and the inferior rectus muscle (IR) are innervated by the ipsilateral oculomotor nucleus.
Duction means a rotation of one eye when it alone is viewing. Naming conventions are as follows: adduction (toward nose horizontally), abduction (toward ear horizontally), sursumduction (elevation), deosursumduction (depression), intorsion (upper pole of the eye moves nasalward), and extorsion (upper pole of the eye moves temporal ward). Version means a rotation of both eyes, usually with both eyes viewing. Naming conventions are as follows: dextroversion (rightward), levoversion (leftward), sursumversion (upward), deorsursumversion (downward), dextrocycloversion
Fig. 2. Axes and planes of eye movements. Linear movements of the head are called surge, sway, and heave along the X, Y, and Z axes, respectively. Angular movements (rotations) are called roll, pitch, and yaw around the X, Y, and Z axes, respectively.
(upper poles toward subjects right), and levocycloversion (upper poles to subjects left). Version also is used frequently to mean conjugate eye movements, that is, movements that rotate the eyes in the same direction by the same amount. Vergence or disjunctive movements are those that rotate the eyes simultaneously in opposite, typically horizontal, directions, such as convergence (left eye to the right and right eye to the left) and divergence (left eye to the left and right eye to the right). Actions of extraocular muscles Actions of the extraocular muscles are described in Table 1 and depicted in Fig. 3. The primary action of the extraocular muscles refers to the principal rotation of the eye when that muscle contracts; the secondary and tertiary actions mean lesser rotations.
Table 1 Actions of the extraocular muscles Muscle Medial rectus Lateral rectus Superior rectus Inferior rectus Superior oblique Inferior oblique Primary action Adduction Abduction Elevation Depression Intorsion Extorsion Secondary action Tertiary action
Intorsion Extorsion Depression Elevation
Adduction Adduction Abduction Abduction
ABNORMAL EYE MOVEMENTS IN DIZZY PATIENTS
Fig. 3. Scheme of functions of the extraocular muscles. The action of each extraocular muscle is illustrated by the direction of the arrows. IO, inferior oblique muscle, IR; inferior rectus muscle; LR, lateral rectus muscle; MR, medial rectus muscle; SO, superior oblique muscle; SR, superior rectus muscle.
Synergistic muscles in the two eyes, that is, muscles that cause the two eyes to move in the same direction, are called yoke muscles. The horizontally acting recti (ie, the medial and lateral rectus) are yoke muscles. The yoke muscle pairs for the other four muscles are more complex (see Fig. 3). Pulleys The four rectus and superior oblique muscles arise from the annulus of Zinn in the apex of the orbit, whereas the inferior oblique muscle arises from the inferior nasal aspect of the orbit. The four rectus muscles insert into the sclera anterior to the equator of the globe, whereas both oblique muscles insert into the sclera posterior to the equator of the globe. Recent studies reveal, however, that the direction of pull is not determined entirely by the origins and insertions of the extraocular muscles. Radiologic imaging using MRI and histologic examination of the orbit reveal that the four rectus  and inferior oblique muscles  pass through pulleys (ie, sleeve-like encircling collagen rings stiened by elastin and smooth muscle). They are located near the equator of the globe in the posterior Tenons capsule rmly attached to each other and to the orbital wall. All the extraocular muscles consist of orbital and global layers and each layer contains approximately equal numbers of bers . The orbital layer of each extraocular muscle inserts on its corresponding pulley, whereas the global layer inserts into the sclera as recognized classically [8,9]. The orbital layer of the inferior oblique muscle inserts on the pulleys of the lateral rectus and inferior rectus muscles and on its own . Compared with other muscles, the superior oblique
muscle is a special case, because it has the trochlea, a brous, cartilaginous rigid ring in the superior media