Evans' Sign (Evans Index)
Evans' sign, more accurately termed the Evans Index (EI), is a radiological measurement defined as the ratio of the maximal width of the frontal horns of the lateral ventricles to the maximum inner skull diameter on axial imaging, with values >0.3 traditionally indicating ventricular enlargement and serving as a key diagnostic criterion for Normal Pressure Hydrocephalus (NPH). 1, 2, 3
Definition and Measurement
- The Evans Index is calculated by dividing the maximal width of the frontal horns by the maximum inner skull diameter (biparietal diameter) on axial CT or MRI imaging. 1, 3
- This measurement is also referred to as the ventricular/biparietal (V/BP) ratio in the context of posthemorrhagic hydrocephalus in premature infants. 1
- The measurement is simple, fast, and does not require specialized software, making it practical for routine clinical use. 3
Clinical Application in Normal Pressure Hydrocephalus
- An Evans Index >0.3 is the established criterion for ventriculomegaly and is required by guidelines for diagnosing idiopathic NPH. 2, 3, 4
- MRI without IV contrast is the preferred imaging modality for NPH diagnosis, which can identify ventriculomegaly along with other classic NPH features including narrowed posterior callosal angle, effaced sulci along high convexities, and widened sylvian fissures. 2
- The disproportionately enlarged subarachnoid-space hydrocephalus (DESH) pattern, characterized by tight high-convexity spaces, enlarged Sylvian fissures, and ventriculomegaly with Evans index >0.3, is particularly characteristic of NPH. 2
Grading of Hydrocephalus Severity
In posthemorrhagic hydrocephalus, the Evans Index (V/BP ratio) can stratify severity: 1
- Mild hydrocephalus: V/BP ratio 0.26–0.40
- Moderate hydrocephalus: V/BP ratio 0.40–0.60
- Severe hydrocephalus: V/BP ratio 0.60–0.90
- Extreme hydrocephalus: V/BP ratio 0.91–1.0
Age-Related Considerations and Normal Values
- In population-based studies of adults ≥70 years, the mean Evans Index was 0.28 (SD 0.04), with 20.6% having values >0.3, indicating that the traditional cutoff may require age-specific adjustment. 4
- Among men aged ≥80 years, the mean Evans Index was 0.30 (SD 0.03), approaching the pathological threshold. 4
- Despite age-related ventricular enlargement, normal aging never causes the ventricular system to attain truly pathological Evans Index values when properly interpreted in clinical context. 5
- The risk of having an Evans Index >0.3 increases by 7.8% per year of age, and males have 83.9% greater risk than females. 5
Important Clinical Caveats
- Clinicians must interpret Evans Index values >0.3 in older adults with caution, as a substantial proportion of elderly individuals without NPH may exceed this threshold due to age-related changes. 4, 5
- For patients aged <70 years, the cutoff of 0.3 remains appropriate, but for older populations (especially >80 years), consideration should be given to raising the threshold to 0.34. 6
- Evans Index values >0.3 should still prompt evaluation for underlying neurological conditions, as normal aging alone does not typically cause abnormal ventricular enlargement. 5
- The Evans Index should always be evaluated alongside other NPH imaging features (callosal angle <90°, cerebral aqueduct flow void, periventricular white matter changes) and the clinical triad of gait disturbance, cognitive impairment, and urinary incontinence. 2
Alternative Imaging When MRI Unavailable
- CT head without IV contrast can identify ventriculomegaly and calculate Evans Index when MRI is contraindicated, though it has lower sensitivity for periventricular white matter changes and cannot detect cerebral aqueduct flow void. 2
Note on Evans Syndrome
Evans syndrome is an entirely separate entity—an autoimmune disorder characterized by concurrent autoimmune hemolytic anemia and immune thrombocytopenia—and should not be confused with Evans Index/sign. 1, 7 Direct antiglobulin testing should be performed to rule out concurrent Evans syndrome in patients with immune thrombocytopenic purpura. 1