What is the definition and interpretation of the test of skew in medical research?

Test of Skew: Definition and Interpretation in Medical Research

The test of skew is a diagnostic examination used to detect vertical misalignment of the eyes (skew deviation), which can indicate central nervous system pathology affecting vestibular pathways in the brainstem and cerebellum. 1

Definition of Test of Skew

The test of skew is one component of the three-part HINTS examination (Head Impulse, Nystagmus, Test of Skew), which is used to differentiate between peripheral and central causes of acute vestibular syndrome. 2

The test specifically:

• Assesses whether the eyes are properly aligned vertically
• Evaluates for skew deviation, a vertical strabismus associated with disorders of vestibular pathways
• Is performed by alternately covering each eye while the patient fixates on a target, observing for vertical movement of the uncovered eye

Clinical Interpretation

Positive Test Result

• A positive test of skew shows vertical misalignment of the eyes when alternately covering each eye
• Clinically significant skew deviation is typically defined as vertical misalignment greater than 3 degrees 3
• Large skew deviations (>3.3 degrees) strongly suggest a central lesion 3

Diagnostic Significance

• Sensitivity: Research shows the clinical test of skew has a sensitivity of only 15% for detecting central causes 3
• Specificity: The test demonstrates high specificity of 98.2% for central pathology 3
• When combined with other HINTS components, the complete examination has 94.0% sensitivity and 86.9% specificity for central causes 2

Clinical Applications

The test of skew is particularly valuable in:

1. Differentiating causes of acute vestibular syndrome:

   • Central causes (stroke, demyelination, mass lesions)
   • Peripheral causes (vestibular neuronitis)

2. Distinguishing skew deviation from fourth nerve palsy:

   • In skew deviation: The hypertropic eye shows fundus incyclotorsion and the hypotropic eye shows fundus excyclotorsion 1
   • In fourth nerve palsy: The hypertropic eye typically shows excyclotorsion 1

3. Identifying patients requiring urgent neuroimaging:

   • A positive test of skew may indicate need for urgent MRI with contrast 1

Advanced Testing Considerations

Upright-Supine Test

• An extension of the test of skew (sometimes called the "fourth step")
• In true skew deviation, the hypertropia typically reduces by 50% when measured in the supine position (sensitivity 80%, specificity 100%) 1
• However, this reduction may not be reliable in acute skew deviation (onset within 2 months) 1

Video Test of Skew

• Video-assisted assessment offers more precise measurement of vertical misalignment
• Has slightly higher sensitivity (29.2%) but lower specificity (75.5%) than the clinical test 3

Clinical Pitfalls and Caveats

1. Prevalence misconception:

   • Contrary to traditional teaching, skew deviation is present in approximately 24% of peripheral vestibular disorders and 29% of central causes 3
   • This challenges the traditional view that skew deviation is exclusively a central sign

2. Diagnostic limitations:

   • The test of skew alone has limited diagnostic value and should be interpreted as part of the complete HINTS examination 2
   • False negatives are common due to low sensitivity

3. Timing considerations:

   • The upright-supine test may be less reliable in acute-onset skew deviation (within 2 months) 1

4. Interpretation challenges:

   • Small degrees of vertical misalignment can be difficult to detect clinically
   • Proper technique requires careful observation and experience

When evaluating patients with acute vestibular symptoms, the test of skew should be performed as part of a complete neurological and ophthalmological examination, with particular attention to other brainstem signs that may indicate central pathology requiring urgent intervention.