Signs and Symptoms
Patients will present with a vertical ocular strabismic imbalance. There will also likely demonstrate some ocular torsion along with a head tilt. The head tilt will be in the direction of the hypotropic eye and the torsion will be in the direction of the head tilt. This combination of skew deviation, ocular torsion and head tilt has been termed ocular tilt reaction (OTR).1-4 Based upon this OTR, the patient may not complain of diplopia; when they perceive it, they often feel it is a minor issue. Disequilibrium is uncommon.
The phenomenon of OTR produces a synkinetic rotation of the eyes and head tilt designed to align with what the patient perceives as being vertical. The patient typically will not complain that the world appears tilted though what the patient perceives as being vertical is actually truly tilted.
The vertical dissociation of the eyes may be comitant, non-comitant, intermittently comitant or alternating with the hypertropia reversing in lateral gaze positions.5-7 There often will be concurrent internuclear ophthalmoplegia (INO) with an adduction deficit and an abducting nystagmus, giving a horizontal eye movement disorder compounding the vertical imbalance.3,4,8
In that skew deviation represents complex disturbance of supranuclear input from lesions in the brainstem, cerebellum and peripheral vestibular system, additional associated neurologic complications may include gaze-evoked nystagmus, gaze palsy, dysarthria, ataxia, hemiplegia and INO.3,4 Skew deviation has neither racial or gender predilection. Though skew deviation can occur at any age, the main causes of multiple sclerosis, infarction, tumor, abscess, intracranial hemorrhage and increased intracranial pressure tend to afflict an older age group.
Skew deviation is caused by a supranuclear lesion. Skew deviation can result from any insult within the posterior cranial fossa, including multiple sclerosis, ischemic infarct, tumor trauma, hemorrhage, syringobulbia and neurosurgical procedures.3,5,8,9 Unilateral vestibular lesions can cause skew deviation as well. The main function of the vestibulo-ocular system is to maintain eye position and fixation with head movement.3 This otolith-ocular response to rotation is impaired due to brainstem lesions. The majority of causes come from brainstem stroke.3
Most skew deviations arise from lesions in the posterior cranial fossa, especially those involving the brainstem tegmentum from the diencephalon to the medulla. Lesions in the utricle or vestibular nerve can also cause skew deviation. It has been suggested that disruption of the utriculo-ocular pathway is a mechanism.10,11 Focal cerebellar lesions can also cause skew deviation. Monocular or binocular imbalance of the utriculo-ocular reflex leads to cerebellar skew deviation.10,11
The medial longitudinal fasciculus (MLF) is also a prime location for a lesion producing both skew deviation as well as INO. When skew deviation accompanies INO, the hypertrophic eye is on the side of the lesion. This suggests a rostral lesion of the MLF after crossing in the pons.3 In these cases, demyelinating disease and ischemic stroke are likely causes.3
When patients present with a vertical deviation or vertical diplopia, there are several steps to ascertain the etiology. First, a mechanical restriction must be ruled out. This can be tested using the forced duction test. If mechanical restriction is suspected, orbital CT scan or MRI is recommended. Once mechanical restriction is eliminated as a possible cause, a Parks-Bielschowsky 3-Step Test should be performed. This is important because the most common differential diagnosis of skew deviation is cranial nerve (CN) IV palsy. This is especially true for non-comitant skew deviations. In fact, skew deviation can mimic CN IV palsy.4
If the 3-Step test does not indicate CN IV palsy, order MRI with gadolinium to rule out a lesion of the posterior cranial fossa as well as examine for evidence of demyelinating disease.
Other methods can further differentiate skew deviation from CN IV palsy. The patient should be examined for additional neurological signs in the form of gaze-induced nystagmus, gaze palsy, facial nerve palsy, hemiplegia, ataxia and dysarthria. These findings, common in skew deviation, are absent in cranial nerve IV palsy unless it was caused by trauma or a brainstem lesion. MRI is also recommended.4
Indirect ophthalmoscopy and double Maddox rod testing can also yield clues in differentiating skew deviation from CN IV palsy. In skew deviation, the hypertrophic eye will be incyclotorted whereas the hypertrophic eye in CN IV palsy will be excyclotorted.
Another simple test can give information to differentiate from CN IV palsy.4,12 In skew deviation, abnormal torsion and ocular vertical deviation are head position dependent but not so in CN IV palsy. After performing the 3-Step Test , the ocular torsion and vertical misalignment should be measured using a double Maddox rod and penlight and prism neutralization with alternate cover test with a 12-point font near target, both at one meter, respectively. Once done, the patient should be reclined to the supine position. The ocular torsion and vertical misalignment should again be measured in the same manner while supine.
In skew deviation, there will be a significant improvement or even complete disappearance of the torsional and vertical abnormalities; by contrast, there will be little change if CN IV palsy is the diagnosis. If there is less than a 50% improvement of the vertical deviation with supination, this part of the test is considered negative and skew deviation is the diagnosis. This upright-supine test is very sensitive and highly specific. In one report, no cases of vertical imbalance due to CN IV palsy, restrictive myopathy, childhood strabismus or myasthenia gravis demonstrated positive results (>50% improvement) with supination.12 Thus, it can be argued that the 3-Step test should now become a 4-Step test.
• In vertical diplopia and vertical ocular misalignment, the cause is CN IV palsy until proven otherwise.
• In patients with vertical imbalance, should the 3-Step test fail to identify CN IV palsy, skew deviation must be strongly considered as the likely alternate diagnosis.
• After performing the 3-Step test, recline the patient and recheck the vertical imbalance. If there is more than 50% improvement upon supination, then skew deviation is present and the patient should be referred for an MRI including the posterior cranial fossa.
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2. Sharpe JA, Kumar S, Sundaram AN. Ocular torsion and vertical misalignment. Curr Opin Neurol. 2011;24(1):18-24.
3. Brodsky MC, Donahue SP, Vaphiades M, Brandt T. Skew deviation revisited. Surv Ophthalmol. 2006;51(2):105-28.
4. Wong AM. Understanding skew deviation and a new clinical test to differentiate it from trochlear nerve palsy. J AAPOS. 2010;14(1):61-7.
5. Tsuda H, Nagamata M, Tanaka K. Alternating skew deviation due to hemorrhage in the cerebellar vermis. Intern Med. 2012;51(19):2793-6.
6. Moster ML, Schatz NJ, Savino PJ, et al. Alternating skew on lateral gaze (bilateral abducting hypertropia). Ann Neurol. 1988;23(2):190-2.
7. Zee DS. Considerations on the mechanisms of alternating skew deviation in patients with cerebellar lesions. J Vestib Res. 1996;6(6):395-401.
8. Pandey PK, Singh A, Kumar B, Ocular tilt reaction, internuclear ophthalmoplegia, and torsional nystagmus following mitral commissurotomy. J AAPOS. 2012;16(5):484-6.
9. Cosetti MK, Tawfik K, Fouladvand M, et al. Diplopia due to skew deviation following neurotologic procedures. Otol Neurotol. 2012;33(5):840-2.
10. Chandrakumar M, Blakeman A, Goltz HC, et al. Static ocular counterroll reflex in skew deviation. Neurology. 2011;77(7):638-44.
11. Wong AM, Sharpe JA. Cerebellar skew deviation and the torsional vestibuloocular reflex. Neurology. 2005;65(3):412-9.