Skew Deviation. For more information about this entity at AAO click here: https://www.aao.org/bcscsnippetdetail.aspx?id=daa80517-a4c1-4ef2-842e-59dd8e3666ce
Skew deviation is characterized as an acquired vertical misalignment of the eyes that is not due to any single muscle or ocular motor nerve. It is typically a comitant hypertropia but can be incomitant. Skew is caused by damage to the prenuclear vestibular input (before it reaches the ocular motor nuclei) and can be caused by damage to the brainstem or cerebellar region. The inner ear is composed by the utricle and saccule that contain otoliths that detect linear and tilt motions and transmit this information to the interstitial nucleus of Cajal in the midbrain. Therefore any lesion disconnecting this input may produce a skew deviation.
Skew deviation does not demonstrate a preference for any age, race, ethnicity, or gender because the etiologies for skew are multiple. Patients that develop skew deviation usually have damage to the brainstem (most common) or cerebellum (less common). In a review of 157 cases with skew deviation, the most common areas of stroke were thalamus, brainstem, cerebellum in ap 52% of the cases. There are, however, few cases that may develop even after blunt trauma  The presumed mechanism is damage to the vestibular nerve inputs to the brainstem.    In older patients skew is predominantly caused by a stroke, but lesions secondary to brainstem demyelinating lesions, inflammation, trauma, tumor, abscess, and surgical procedures can also result in skew deviation. Other rare causes of skew deviation include Arnold-Chiari malformation and Creutzfelt-Jakob disease. 
Common vasculopathic causes of strokes are the main risk factors for skew deviation in older patients. These include hypertension, diabetes, hyperlipidemia, smoking, and atherosclerosis. Multiple sclerosis, or demyelinating lesions form NMOSD and trauma are risk factors for younger patients.
The exact pathophysiology of skew deviation remains incompletely understood. However, it is believed to be caused by damage to the prenuclear vestibular input prior to its destination on the ocular motor nuclei. Specifically, the neural route begins at the vestibular nucleus and ends at the interstitial nucleus of Cajal (rostral midbrain). Any lesion along this pathway has the potential to promote skew deviation, and findings may differ depending on the origin of the lesion in the pathway. These insults can be divided into peripheral (utricle, vestibular nerve) and central (medulla, cerebellum, medial longitudinal fasciculus, midbrain, thalamus, and vestibular cortex).
The diagnosis of skew deviation is usually a diagnosis of exclusion. It is generally considered when ocular findings do not fit a cranial nerve insult, or when a brainstem or cerebellar injury is expected. Skew is typically accompanied by other neurologic signs and symptoms (i.e., defined by the “company it keeps”) involving the brainstem and/or cerebellum. Skew deviation can be further divided into multiple clinical subtypes that describe the clinical findings:
- Comitant skew deviation
- Incomitant skew deviation
- Paroxysmal (intermittent) skew deviation
- Periodic or slowly alternating skew deviation
- Lateral alternating skew deviation
- Transient neonatal skew deviation
Skew deviation was first recognized in 1824 by Francois Magendie, an experimental physiologist.  Since this time, the subdivision of skew deviation has increased further (see diagnosis section). Westheimer and Blair coined the phrase ocular tilt reaction in 1975 after a series of experiments of brainstem stimulation on monkeys and determined that the ocular tilt reaction and skew deviation described a singular pathology. Shortly thereafter, in 1977, the first verified case of a human ocular tilt reaction was described by Rabinovitch.The increased understanding of skew deviation over time has moved it from being solely nonspecific to sometimes focal, and as a result can be seen not only in neurologically debilitated patients, but also in mostly neurologically intact and ambulatory patients.
Skew deviation is normally accompanied by a myriad of findings which include:
- Binocular torsion
- Perceived tilting of the visual field
This quad of symptoms has been termed the ocular tilt reaction. The perceived tilting of the visual field is a trademark symptom. It can be confusing because the patient often does not complain of a tilted visual field, even when prompted. It is defined as a “tilt in the subjective visual vertical”. Their entire visual field is tilted together, but due to the visual orientation cues present in everyday life, they can maintain accuracy of the true vertical orientation.
In a recent review of Skew deviation, most of the patients who complained from diplopia or blurry vision also, also presented with nystagmus, while some of them had vertical or horizontal gaze deficits , internuclear ophthalmoplegia, slow saccades, visual field defect, Horner syndrome and dorsal midbrain syndrome.
In the same study, in 91 /136 patients who complained from diplopia, the most common accompanied neurological symptoms, included: imbalance, dizziness, headache, weakness, slurred speech, nausea , numbness, extremity weakness, oscillopsia , hearing loss and tinnitus, drowsiness, facial weakness, facial numbness, facial pain and visual field loss .
As detailed in the symptoms section, the subjective tilting can be a subtle symptom that can be hard to bring out. In these patients, a thorough physical exam is essential to elicit this obscured deficit. Additionally, it is important to know that skew deviation and trochlear nerve palsy can present very similarly. Being aware of this and knowing the clinical tests available to differentiate the two is an important ophthalmic skill. To measure ocular torsion, the double Maddox rod test may be used to differentiate trochlear palsy versus skew deviation. For this test, a Maddox rod (specialized device that contains a series of parallel cylinders that produce a line image from a point source of light) is placed in either a phoropter or trial frame. One Maddox rod is used for each eye. One axis is rotated until two parallel lines are perceived by the patient. The degree of cyclodeviation is determined by measuring the angle of rotation that is required to cause a parallel and horizontal image. This test is important to quantify the degree of ocular torsion, and it can also be used to differentiate skew deviation and trochlear nerve palsy (see upright-supine test below).
In 2010, Wong et al recommended an algorithm to follow if a patient comes in with vertical strabismus. It is especially useful when abnormal torsion is absent. The algorithm goes as follows:
- First rule out a pseudostrabismus or dissociated vertical deviation. This can be performed by looking at the light reflection bilaterally and the cover-uncover test respectively. Also rule out a physically restrictive cause.
- a. If ruled out, move to step 2.
- Park’s three-step test. This test is classically performed to identify trochlear nerve palsy.
- a. If the test localizes to any other muscle other than the superior oblique,think of alternatives, i.e skew deviation, Thyroid eye disease, Myasthenia gravis etc . Perform MRI with contrast, as skew deviation can mimic this presentation.
- b. If positive, this is suggestive of trochlear nerve palsy. Although, skew deviation can result in a positive step. If positive, move on to step 3. (Which can be the 4th step extension of the Park’s 3 step test.)
- Upright-supine test.  This test was added to differentiate trochlear palsy and skew deviation. During the test, an object is positioned 1/3 of a meter away from the patient, and they are instructed to look at it. A positive test is defined as a >50% decrease in vertical deviation when the patient is in the supine position when compared to the upright position. Again, the vertical deviation is measured by double Maddox rods.
- a. If positive, suggestive of skew deviation. Perform MRI with contrast.
- b. If negative, this is suggestive of trochlear nerve palsy because a paralysed 4th nerve does not change the deviation based on patient position and stays the same. We cannot however completely exclude a Skew deviation from this test as the test could be false negative . (Move to step 4)
- Check for other neurologic signs. If present, perform an MRI with contrast.
As a side note, is also necessary to know that myasthenia gravis can have an overlap in presentation with either skew deviation or trochlear nerve palsy.
On physical exam, you may also view a comitant hypertropia that may alternate in opposite gaze directions, torticollis (turning of the head in one direction) and torsion of one or both eyes. The ocular tilt reaction in skew deviation is typically characterized by torsion of the eyes towards the hypotropic eye which is again towards the direction of the head tilt. This is converse of the normal tilt reaction where the counter roll is opposite to the direction of the head tilt. For example, in a left head tilt,in a skew deviation, the left eye will be excyclotorted and the right eye will be incyclotorted. This could also be assessed on fundus examination to look for torsion. In a trochlear nerve palsy, the hypertropic eye will be excyclotorted, whereas in a skew deviation the hypotropic eye would be excyclotorted.
Prognosis and Treatment
The prognosis depends on the etiology. Most skew deviation cases which are demyelinating or ischemic are transient and spontaneous recovery is common. Recovery usually occurs in months, and botulinum injections or prismatic therapy may be used to alleviate symptoms during this recovery period.  Additionally, botulinum injections, prismatic therapy, and vertical muscle resection can be used for persistent cases.  These treatments are very effective for treating all the symptoms of the ocular tilt reaction except for the head tilt itself.
- YouTube. Neuro-Ophthalmology with Dr. Andrew G. Lee. SKEW DEVIATION. https://youtu.be/MqSEAuWHk28 Accessed July 11, 2022.
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