Skew deviation is characterized by vertical misalignment of the eyes that is inconsistent with the actions of the cyclovertical muscles.1,2,3
It is a common effect of brain stem dysfunction, and neurological signs and a lesion in the vestibulo-ocular pathway are frequently associated.2
Signs and symptoms of skew deviation
A 2021 clinical review of 157 cases of skew deviation at a tertiary care academic center over an 8-year period was conducted by Walter and Trobe. This study has added to our understanding of the causes, features, and management outcomes of skew deviation.2
Of these patients, 87% reported diplopia, 11% reported blur, and 2% reported no visual symptoms. Of note, 64% of those with vertical misalignment of the eyes were found to have a comitant deviation, with the overall average being an amplitude of 7 prism diopters (PD).2
Excyclo- and incyclorotation of the eyes were also noted, although to a lesser degree. Those with smaller angle deviations (< 3PD) more commonly described “blur” rather than double vision.2
It is noted that patients are not usually subjectively aware of their head tilt or the change in the subjective visual vertical. A small percentage of those with skew deviation may have no symptoms (such as in cases of pseudophakic monovision or reduced vision in one eye).2
Alternating skew deviation, where the hypertropia switched eyes on right and left gaze, can occur, and while not found in this study, intermittent skew deviation has also been reported.1,2
Understanding the ocular tilt reaction
As a basis for understanding skew deviation, optometrists are reminded of the physiological ocular tilt reaction. This reflex is demonstrated when motorcyclists and skiers reflexively tilt their heads back toward the gravitational vertical during sharp turns. It is primarily characterized by head tilt back towards the gravitational vertical, and a less prominent counterroll of the eyes.1
In cases where this otolith-ocular response is impaired due to a brainstem lesion, the result is skew deviation.1 The associated vertical deviation is part of the ocular tilt reaction to the “tilted” visual world, aiming to restore (inaccurately estimated) vertical orientation.1
Animation 1: Demonstration of normal head tilt back towards vertical on the left, and skew deviation with head tilt towards an inaccurate estimation of vertical on the right.1
Animation based on Brodsky et al.
Neurological signs associated with skew deviation
Skew deviation is usually associated with other neurological signs of brain stem dysfunction, including:2
- Dizziness
- Nystagmus
- Vertical or horizontal gaze deficits
- Internuclear ophthalmoplegia
- Ataxia
- Facial weakness
- Slow saccades
- Visual field deficits
- Horner’s syndrome
- Increased deep tendon reflexes
- Dorsal midbrain syndrome
- Homonymous hemianopia
Ataxia and other accompanying neurological signs may outlast skew deviation and be more debilitating. A small number of individuals with skew deviation may have vertical misalignment of the eyes with no additional neurological signs or associated brain imaging abnormalities.2
Underlying neuroanatomy of skew deviation
In contrast to strabismus associated with the actions of the extraocular muscles, the vertical tropia in skew deviation results from damage to prenuclear vestibular input to the oculomotor nuclei.1
Figure 1: Anatomical diagram of the possible effects of vestibular lesions.1
Figure 2: Courtesy of Brodsky et al.
The vestibulo-ocular system functions to maintain eye position and stabilize fixation during head movement. Angular and linear acceleration are sensed by the semicircular canals and otoliths, respectively, in the inner ear. Excitatory innervation is then sent to the corresponding extraocular muscles.1
Note: There are also separate effects of the vestibulospinal reflexes on spatial perception.1
Potential causes of skew deviation
Skew deviation can be caused by any injury within the posterior fossa, with a brainstem stroke being the most common association.1 Other causes include tumor, operative injury, non-tumorous cerebellar disorders, demyelination, traumatic brain injury, and posterior circulation aneurysm.2
Additional causes (including Parkinson's disease and encephalitis) have been reported; however, in certain cases, no neurological basis has been identified on imaging.2 Intracranial hypertension is a less common cause.3
Animation 2: A possible location of a brainstem stroke.
Diagnosing skew deviation
The key to the diagnosis of skew deviation is assessing whether the diplopia maps clearly to the actions of the intraocular muscles. The Park’s 3-Step Test (also known as Parks–Bielschowsky Test) is valuable in assessing this with additional steps to specifically discern cases of skew deviation.2,3
To complete the Park’s 3-Step Test, the six cardinal positions of gaze are drawn (as in Figure 4), indicating the specific muscles primarily involved in each position, for each eye.3
The possible associated extra-ocular muscles are then discerned as follows:3
- Which eye is hypertropic in the primary position of gaze? (Circle the muscles at the bottom of this eye and the top of the other eye.)
- Is this vertical deviation greater on the right or left gaze? (Circle the muscles on this side.)
- Is the vertical deviation greater with right or left head tilt? (Circle the muscles on the diagonal that correspond to this direction of head tilt.)
In cases of skew deviation, and other specific circumstances such as restrictive causes, no clear result is evident, i.e., no muscles are circled in all three instances.2
Figure 2: Illustration of the six cardinal positions of gaze for the Park’s 3-Step Test.
Should the three-step test suggest skew deviation, cyclorotation of the eye can be assessed by fundus observation or indicated by subjective feedback on the relative orientation of a double Maddox rod.1 In cases of a prenuclear lesion, intorsion of the higher eye / extorsion of the lower eye is evident. This contrasts with superior oblique palsy, where extorsion of the higher eye is evident.1
As a final step of the ocular evaluation, the deviation should be compared while they are supine to when they are upright. In cases of 4th nerve palsy, there would be no change, but in skew deviation, the amount of hypertropia is reduced in the supine position.4
Note that in trochlear nerve palsy, the head tilt is an attempt to minimize the hypertropia,5 while in skew deviation, the head tilt aims to realign the vertical axes of the eye and head to an erroneously perceived estimate of vertical.4
These findings, together with the managing neurologist’s results from brain imaging (looking for a lesion and ruling out restrictive causes), and the presence of other posterior fossa neurological signs, form the basis of the diagnosis.
For a refresher on extraocular muscle function (with animations!) and how to diagnose strabismus, check out The Simplified Guide to Understanding Eye Movements!
Differential diagnoses for skew deviation
Differential diagnoses of skew deviation fall into four key categories:1
- Neurological: Superior oblique, inferior oblique, and 3rd nerve palsies
- Neuromuscular: Myasthenia gravis and systemic botulism
- Restrictive: Thyroid eye disease, acquired Brown syndrome, and chronic progressive external ophthalmoplegia
- Congenital: Brown syndrome and monocular elevation deficiency
Management of skew deviation
Ocular tilt reactions can be transient, showing spontaneous recovery, or may persist. Surgical intervention should therefore be delayed by several months.1 Patients can be assisted with prism (ground in or Fresnel),3 particularly for smaller vertical deviations.
In cases where the angle of deviation persists, is too large to be sufficiently well-controlled with prism, or in cases of cyclotorsion, surgical intervention may be considered. Botulinum toxin administered by an ophthalmologist in strabismus practice can also give symptomatic relief of vertical diplopia in such cases.1
It is important to realize that these treatments will not eliminate the head tilt, which is secondary to a central tilt in the subjective visual vertical, and is not compensatory for the vertical diplopia. The individual is usually unaware of the head tilt, with the vertical diplopia being their main concern.1
Final thoughts
With improved understanding of the basis of, and our skill in managing the visual symptoms of skew deviation, optometrists play a valuable role in the multidisciplinary team supporting the individuals affected.
With judicious use of prism, careful monitoring, and appropriate co-management with an ophthalmologist and neurologist, the patient can be given practical assistance with visual symptoms (both during the acute phase of care and during their longer-term management).
