Scleral Show Percentage for Exophthalmos
True exophthalmos is defined by exophthalmometer measurements exceeding 19 mm, not by a specific percentage of scleral show relative to corneal diameter. 1
Diagnostic Criteria for Exophthalmos
The gold standard for diagnosing exophthalmos relies on exophthalmometer measurements rather than scleral show percentages:
- Normal range: 10.5-18.8 mm (95% confidence limits in individuals without thyroid disease) 1
- Exophthalmos threshold: ≥19 mm on exophthalmometer measurement 1
- Prevalence: 21.3% of thyrotoxic patients demonstrate exophthalmos by this criterion 1
Key Clinical Distinction
The question conflates two separate anatomical findings:
- Exophthalmos = anterior displacement of the globe due to increased orbital contents in a normal bony orbit 2
- Scleral show = visible sclera exposure (typically inferior) that can occur independently of true exophthalmos 3
Scleral Show Assessment
While no specific percentage of corneal diameter defines exophthalmos, excessive inferior scleral show is assessed qualitatively rather than quantitatively:
- Scleral show represents an anatomical condition where sclera area is visibly exaggerated 3
- It can occur from constitutional, endocrine, or iatrogenic causes without true exophthalmos 3
- In long-face patients with excess scleral show, surgical correction achieved 8% and 6% decreases in scleral show relative to total eye height (right and left eyes respectively) 4
Clinical Pitfall
Do not diagnose exophthalmos based solely on scleral show appearance. Scleral show is not always linked to globe protrusion and requires differentiation from true exophthalmos through exophthalmometry 3. The mechanisms of globe protrusion must be distinguished before planning any orbital intervention 2.
Imaging Confirmation
When exophthalmos is suspected clinically:
- MRI of the orbits with and without contrast is the optimal imaging modality to localize and characterize orbital pathology causing proptosis 5
- CT imaging with multiplanar reconstructions identifies osseous orbital abnormalities 5
- Computed tomography measurements can differentiate exophthalmos (increased orbital contents) from exorbitism (decreased orbital capacity) 2
Bilateral vs Unilateral Presentation
Exophthalmos is bilateral in 85-90% of cases, particularly in thyroid eye disease 6. Unilateral or asymmetric proptosis raises concern for underlying mass lesions, vascular malformations, or carotid-cavernous fistula 5.