Intraocular Muscle Function

Intraocular Muscle Function

Synonym/acronym: IOM function.

Common use

To assess the function of the extraocular muscle to assist with diagnosis of strabismus, amblyopia, and other ocular disorders.

Area of application





Evaluation of ocular motility is performed to detect and measure muscle imbalance in conditions classified as heterophorias or heterotropias. This evaluation is performed in a manner to assess fixation of each eye, alignment of both eyes in all directions, and the ability of both eyes to work together binocularly. Heterophorias are latent ocular deviations kept in check by the binocular power of fusion and made intermittent by disrupting fusion. Heterotropias are conditions that manifest constant ocular deviations. The prefixes eso- (tendency for the eye to turn in), exo- (tendency for the eye to turn out), and hyper- (tendency for one eye to turn up) indicate the direction in which the affected eye moves spontaneously. Strabismus is the failure of both eyes to spontaneously fixate on the same object because of a muscular imbalance (crossed eyes). Amblyopia, or lazy eye, is a term used for loss of vision in one or both eyes that cannot be attributed to an organic pathological condition of the eye or optic nerve. There are six extraocular muscles in each eye; their movement is controlled by three nerves. The actions of the muscles vary depending on the position of the eye when they become innervated. The cover test is commonly used because it is reliable, easy to perform, and does not require special equipment. The cover test method is described in this monograph. Another method for evaluation of ocular muscle function is the corneal light reflex test. It is useful with patients who cannot cooperate for prism cover testing or for patients who have poor fixation.

This procedure is contraindicated for



  • Detection and evaluation of extraocular muscle imbalance

Potential diagnosis

The examiner should determine the range of ocular movements in all gaze positions, usually to include up and out, in, down and out, up and in, down and in, and out. Limited movements in gaze position can be recorded semiquantitatively as −1 (minimal), −2 (moderate), −3 (severe), or −4 (total).

Normal findings

  • Normal range of ocular movements in all gaze positions.

Abnormal findings related to

  • Amblyopia
  • Heterophorias
  • Heterotropias
  • Strabismus

Critical findings


Interfering factors

  • Factors that may impair the results of the examination

    • Inability of the patient to cooperate and remain still during the test because of age, significant pain, or mental status may interfere with the test results.
    • Rubbing or squeezing the eyes may affect results.

Nursing Implications and Procedure


  • Positively identify the patient using at least two unique identifiers before providing care, treatment, or services.
  • Patient Teaching: Inform the patient this procedure can assist in evaluating eye muscle function.
  • Obtain a history of the patient’s complaints, including a list of known allergens.
  • Obtain a history of the patient’s known or suspected vision loss, changes in visual acuity, including type and cause; use of glasses or contact lenses; eye conditions with treatment regimens; eye surgery; and other tests and procedures to assess and diagnose visual deficit.
  • Obtain a history of symptoms and results of previously performed laboratory tests and diagnostic and surgical procedures.
  • Obtain a list of the patient’s current medications, including herbs, nutritional supplements, and nutraceuticals (see Effects of Natural Products on Laboratory Values online at DavisPlus).
  • Review the procedure with the patient. Address concerns about pain and explain that no discomfort will be experienced during the test. Inform the patient that a health-care provider (HCP) performs the test in a quiet room and that to evaluate both eyes, the test can take 2 to 4 min.
  • Instruct the patient to remove contact lenses or glasses, as appropriate. Instruct the patient regarding the importance of keeping the eyes open for the test.
  • Sensitivity to social and cultural issues, as well as concern for modesty, is important in providing psychological support before, during, and after the procedure.
  • Note that there are no food, fluid, or medication restrictions unless by medical direction.


  • Potential complications: N/A
  • Observe standard precautions, and follow the general guidelines in Patient Preparation and Specimen Collection. Positively identify the patient.
  • Instruct the patient to cooperate fully and to follow directions. Ask the patient to remain still during the procedure because movement produces unreliable results.
  • Note that one eye is tested at a time. The patient is given a fixation point, usually the testing personnel’s index finger. An object, such as a small toy, can be used to ensure fixation in pediatric patients. The patient is asked to follow the fixation point with his or her gaze in the direction the fixation point moves. When testing is completed, the procedure is repeated using the other eye. The procedure is performed at a distance and near, first with and then without corrective lenses. The examiner should determine the range of ocular movements in all gaze positions, usually to include up and out, in, down and out, up and in, down and in, and out.


  • Inform the patient that a report of the results will be made available to the requesting HCP, who will discuss the results with the patient.
  • Recognize anxiety related to test results, and be supportive of impaired activity related to vision loss, anticipated loss of driving privileges, or the possibility of requiring corrective lenses (self-image).
  • Reinforce information given by the patient’s HCP regarding further testing, treatment, or referral to another HCP. Educate the patient, as appropriate, that he or she may be referred for special therapy to correct the anomaly, which may include glasses, prisms, eye exercises, eye patches, or chemical patching with drugs that modify the focusing power of the eye. The patient and family should be educated that the chosen therapy involves a process of mental retraining. The mode of therapy in itself does not correct vision. It is the process by which the brain becomes readapted to accept, receive, and store visual images received by the eye that results in vision correction. Therefore, the patient must be prepared to be alert, cooperative, and properly motivated. Answer any questions or address any concerns voiced by the patient or family.
  • Depending on the results of this procedure, additional testing may be performed to evaluate or monitor progression of the disease process and determine the need for a change in therapy. Evaluate test results in relation to the patient’s symptoms and other tests performed.

Related Monographs

  • Related tests include refraction and slit-lamp biomicroscopy.
  • Refer to the Ocular System table at the end of the book for related tests by body system.
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