FUNCTION -- Control Extra- and Intraocular Muscles

TEST I -- Extra- and Intraocular Muscles
The test should begin with simple observation of the eyes. When gazing at distant objects, the eyes of the patient should operate in tandem. At rest, neither of the eyes should deviate from the midline horizontal position. The clinician should also ensure that neither of the eyelids is drooping (i.e., exhibiting ptosis) and that the pupils are equal in size but neither markedly dilated nor constricted. Following the initial observations, the clinician should ensure that the head of the patient is oriented vertically and in direct opposition to the head of the clinician. The clinician should then place his/her finger approximately 45 cm from a point directly between the eyes of the patient. Subsequently, the clinician should move the tip of the finger through space, forming a large capital-H. The patient should attempt to follow the movements of the finger with the eyes, while keeping the head fixed. The H should be sufficiently large as to force the eyeballs to their extremes of motion. At the completion of the H, the clinician should return the finger to the distant central location (i.e., directly in front of the patient). Then the finger should be moved towards the patient, with the target being directly between the eyes. This causes the eyes to cross, perhaps culminating in bilateral constriction of the pupils (i.e., accommodation-related pupillary constriction). Click here to view pursuit eye movements more closely.

INTERPRETATION

Resting deviation of the eyes
Problems with movements of the eyes can reflect damage anywhere between cortical motor centers (frontal or occipital) and the rostral medulla (including the cerebellum). First the persistent tendency for the eyes to drift as a pair to one side may indicate either ipsilateral frontal (or occipital) damage or contralateral pontine damage. The tendency for the eyes to drift and remain down commonly reflects damage to diencephalon or midbrain. Patients exhibiting a persistent upward gaze are less commonly seen, and damage in such patients tends to involve the midbrain. Substantial numbers of patients exhibit problems with both upward and downward gaze, potentially implicating telencephalic, diencephalic, or mesencephalic pathology.

Outward and slightly downward deviation of the eye often reflects damage to cranial nerve III. Frequently seen along with this sign is an enlarged pupil (which responds poorly to light) and ptosis. Isolated ptosis or pupillary dilation may indicate partial dysfunction of cranial nerve III. Damage to cranial nerve IV tends to cause a subtle upward and inward repositioning of the eye. Defects affecting cranial nerve VI commonly cause the affected eye to drift medially.

Drooping Eyelids
Unilateral or bilateral ptosis may reflect defects of cranial nerve III or central or peripheral portions of the sympathetic nervous system. Disease processes such as myasthenia gravis or multiple sclerosis may be implicated.

Pursuit Movements
Even in the absence of immediately obvious defects of cranial nerve III, IV, and VI, the H-test may prove revealing. To explain, some pathology may only become apparent by virtue of a failure of the eyes to move fully into extreme positions while following moving objects. The linked figures demonstrate the clinically informative positions of the eyes in relation to the mediating cranial nerves and muscles. At the end of the H-test, note that both the crossing of the eyes and the contemporaneous constriction of the pupils depends bilaterally on cranial nerve III. One should also note, however, that irregular movement of the eyes either during the H-test or at rest can reflect interrupted occipital connections, particularly those involving midbrain and pontine structures. Damage to cerebellar and vestibular systems can additionally provoke serious oculomotor deficits, including ocular ataxia (i.e., nystagmus).

PITFALLS AND SAFEGUARDS
When visually assessing the eyes of the patient at rest, ensure that the head of the patient is vertical with the face oriented anteriorly. In many instances, individuals with strabismus (eyes that are pathologically misaligned in relation to each other) may mask the deviating eye with postural adjustments of the head. Proper positioning of the head is also of vital importance for valid assessment of the cardinal movements of the eyes (i.e., movements exhibited during the H-test). In addition to the static positioning and pursuit movements of the eyes, the clinician should carefully assess for either induced or spontaneous nystagmus (unsteady or ataxic movements of the eyes). It is not uncommon, however, for neurologically normal individuals to exhibit some nystagmus, particularly when the eyes are forced far laterally in the horizontal plane (as would be done during the H-test). Extra care must also be taken to ensure that the head of the patient is immobile during the H-test. The inexperienced or highly cautious clinicians may wisely perform the H-test twice to provide ample opportunity to observe irregularities from either eye.

TEST II -- Light Reflexes and Intraocular Muscles
The testing of pupillary responses necessarily involves the shining of light (usually via an ophthalmoscope or a small flash- or pen-light) obliquely into one of the eyes. The clinician should observe that, in response to the light, both pupils will constrict followed by a partial return to pretest diameter. The response of the pupil ipsilateral to the light is called "direct," and the response of the pupil contralateral to the light is called "consensual." The test should be repeated such that the clinician may observe for direct and consensual responses from both pupils.

INTERPRETATION
Responses to light are varied, with the full expression of light-induced pupillary constriction indicating a functional reflex arc involving cranial nerve II (afferent) and cranial nerve III, which constitutes the critical efferent component of the parasympathetic nervous system. Deficits of light-induced pupillary constriction may thus reflect defects located within the following pathway: Cornea -- retina -- cranial nerve II -- optic chiasm -- optic tract -- pretectal area (mesencephalon) -- posterior commissure (diencephalon) -- nucleus of Edinger-Westphal -- superficial cranial nerve III -- constrictor pupillae muscle.