Three Step Test for Cyclovertical Muscle Palsy

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Diagnosis

Diagnostic procedures

The three-step test, also known as the Parks-Bielschowsky three-step test and the Parks-Helveston three-step test, is a diagnostic test used to identify which muscle is paretic in the case of a acquired hypertropia ( vertical misalignment) using the cover-uncover or the Maddox rod to measure the amount of deviation in different head positions . It was first described by Bielschowsky in 1935 and later popularized by Parks. This test is most useful for diagnosing superior oblique palsies in clinical practice. The three-step test may also be used to diagnose the less common inferior oblique or vertical rectus muscle palsy, and in differentiating DVD from other vertical strabismus. It is designed for the diagnosis of a single paretic vertical muscle, and is unreliable when there are multiple paretic muscles or in restrictive strabismus.

It can also be useful to determine whether a superior rectus palsy is true or simulated in a patient with an inhibitional palsy of the contralateral antagonist. This condition is characterized by a simulated superior rectus palsy in a patient with contralateral superior oblique paralysis who fixates with the paretic eye. It is important that the results of the three-step test must be interpreted together in context of the patient’s history and associated neuro-ophthalmologic findings.

  1. The first step in the three-step test is to determine which eye is hypertropic in primary position.
  2. The second step is to determine whether the hypertropia increases in right or left gaze.
  3. The third step is determination if the hypertropia increases upon left head tilt or right head tilt. For this test, the ophthalmologist or orthoptist uses prisms to quantify the hypertropia in primary position, side gazes, and head tilts. Alternatively, it can be helpful to quantify smaller deviations using prisms and a Maddox rod or red filter.

Example:

Measurements:

Left Gaze Primary Position Right Gaze
RHT 25 RHT 10 RHT 2
Left Head Tilt Right Head tilt
RHT 5 RHT 30

Step 1. Which eye is hypertropic?

In this patient, the right eye is hypertropic. When trying to isolate which muscle is paretic in a right hypertropia, it must be either one of the depressors of the right eye or one of the elevators of the left eye. Therefore, we have deduced from step one that the paretic muscle must be one of the following cyclovertical muscles:

  • Right superior oblique
  • Right inferior rectus
  • Left inferior oblique
  • Left superior rectus

Step 2. Does the hypertropia increase in right gaze or left gaze?

In this patient, the hypertropia increases in left gaze. Understanding the field of action of each cyclovertical muscle is critical for further deduction of which muscle could be paretic in step two. The field of action of an extraocular muscle is that direction in which the muscle’s primary action is the greatest. In each of the cardinal positions of gaze, there is always one muscle in each eye which is the prime mover. When measuring a deviation in side gazes for step two of the three-step test, the examiner is attempting to isolate specific muscles which are the prime movers in those gazes.

Eye Muscle Primary action in left gaze Primary action in right gaze

Right inferior oblique

(in adduction) elevation

(in abduction) excyclotorsion

Right superior oblique

(in adduction) depression

(in abduction) incyclotorsion

Left superior rectus

(in abduction) elevation

(in adduction) incyclotorsion and adduction

Left inferior rectus

(in abduction) depression

(in adduction) excyclotorsion and adduction

In this example patient, the right hypertropia increases in left gaze. Therefore, the paretic muscle must be either (1) one of the muscles whose primary action is elevating the left eye on left gaze, or (2) one of the muscles whose primary action is depressing the right eye in left gaze.

The right inferior oblique and the left superior rectus both have a primary action of elevation in left gaze. Therefore, one of these muscles could be the paretic muscle. Likewise, the right superior oblique and the left inferior rectus have a primary action of depression in left gaze and could be paretic.

Because the right inferior oblique and the left inferior rectus did not meet Step 1, they can be eliminated from the list of possible paretic muscles. This leaves the right superior oblique and the left superior rectus as the two cyclovertical muscles left which meet criteria for both steps 1 and 2.

Step 3. Does the hypertropia increase in right head tilt or left head tilt?

Step three is the final step in determining which of the two remaining muscles is paretic. In this example, the right hypertropia is worse in left gaze.

If the deviation increases on right head tilt and decreases on left head tilt, as it does in this example patient, then the right superior oblique is implicated as the paretic culprit. The reason for this is because the right superior oblique and right superior rectus muscles work together to incyclotort the right eye in right head tilt. Usually they offset each other in terms of vertical movement, because the superior oblique depresses and the superior rectus elevates. However, in the case of the paretic right superior oblique, the elevating action of the right superior rectus is unopposed and causes an increase in the right hypertropia.

During head tilt to the right, the otolith system sends impulses to the extraocular muscles that assist in torsion, because the eyes need to adjust to the right head tilting by a compensatory rotation to the left around the anteroposterior (Y) axis of Fick. On right head tilt, the left eye excyclotorts due to the actions of the left inferior oblique and the left inferior rectus. Likewise, the right eye incyclotorts due to the actions of the right superior oblique and the right superior rectus.

Since the right superior oblique is paretic and unable to counteract the elevation and adduction actions of the right superior rectus in the right head tilt, the right eye moves upward.

Upon head tilt to the left side, the right hypertropia in this example decreases significantly. In some patients, the right hypertropia may be completely eliminated in left head tilt. When the head is tilted to the left, the cyclorotation of both eyes to the right (incyclotorsion of the left eye and excyclotorsion of the right eye) do not require the right superior oblique. Therefore, this example of a right hypertropia which increases in left gaze and right head tilt is a positive three-step test for a right superior oblique palsy.

Step 4.

A fourth step can be added to 3-step test to quantitate the torsional component of trochlear nerve palsy by using double Maddox rod. More than10 degree of excyclotorsion suggests bilateral trochlear nerve palsies.

Step 5.

Wong et al recommended adding a fifth step to the 3- step test called upright-supine test. [1]This test is used to differentiate skew deviation, a vertical misalignment with or without head tilt or fundus torsion of supranuclear origin (brainstem or cerebellum) from other causes of vertical strabismus including trochlear nerve palsy.

Additional Resources

References

  1. Wong AM, Colpa L, Chandrakumar M. Ability of an upright-supine test to differentiate skew deviation from other vertical strabismus causes. Arch Ophthalmol. 2011 Dec;129(12):1570-5.
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  3. "Head Tilt Test." Head Tilt Test. Orbis Telemedicine, 2014. Web. 08 May 2015. <http://telemedicine.orbis.org/bins/content_page.asp?cid=735-2858-4397-2804-3110-3021-3037>.
  4. Kushner BJ. Errors in the three-step test in the diagnosis of vertical strabismus. Ophthalmology 1989;96: 127-132.
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