SCAN0146

192 Clinical Anatomy of the Visual System

Clinical Comment: Extraocular Muscle Assessment

Assessment of eye position and movements can be an important tool in determining the integrity of the extraocular muscles and associated nerves. The practitioner first notes the position of each eye while directing the patient to fixate on a target straight ahead. An eye that is deviated toward the nose would indicate an underactive lateral rectus muscle; the medial rectus is unopposed by the lateral rectus. Figurę 10-17, A, shows the direction of puli of each muscle when the eye is in primary position.

Ocular motility testing provides further 'Information on the contractile abilities of the muscles. Evaluation of horizontal eye movement is straightforward. If the eye cannot adduct, the problem lies with the medial rectus; if the eye cannot move into the abducted position, the problem lies with the lateral rectus muscle.

With the morę complex movements of the other muscles, the most reliable way to determine a dysfunctional muscle is to put the eye into a position in which one muscle is the primary actor. In the adducted position the oblique muscles are the primary elevator and depressor; in the abducted position the vertical rectus muscles are the primary elevator and depressor. This arrangement can be

represented by the "H" diagrams in Figurę 10-17, B. Thus when doing ocular motility testing it is important to mo\e the eyes to such a position as to isolate the vertical ab : 'es of these muscles. The usual manner of performing ocu/o^r motility testing follows:

1.    Using a smali target, usually a bead, the patient is instructed to follow the target.

2.    The horizontal ability is determined first by moving the bead to the far right and to the far left, noting any inability of either eye to follow.

3.    In left gazę the bead is elevated to determine the ab^; cy of the left superior rectus (left eye is abducted) and the right interior oblique (right eye is adducted). The bea: : depressed to determine the ability of the left interior rectus and the right superior oblique muscles.

4.    In right gazę the bead is elevated to determine the ability of the right superior rectus (right eye is abdur.e: and the left interior oblique (left eye is adducted). The bead is depressed to determine the ability of the rignr interior rectus and the left superior oblique muscles.

The extensive network of connective tissue septa and

fibroelastic pulley system associated with the extraocukr muscle may affect eye movement, and MRI may be ar additional tool to determine the involved muscle once a strabismic condition is diagnosed.

FIGURĘ 10-17

A,    Direction of eye movemen: cr contraction of each muscle, v £ in primary position. For examp e f superior rectus muscle contracrs e will move up and in and intort C_r.' arrows represent torsional moverre^—

B,    Muscles that cause vertical movement when eye is either adducted or abducted. In add_:::^_ for example, muscle that causes elevation is interior obliąue musae 10, Interior oblique; IR, interior rectus; LR, lateral rectus; MR ~ez i rectus; 50, superior obliąue;

SR, superior rectus.