Clinical Activity Score (CAS) for Exophthalmos
The Clinical Activity Score (CAS) is calculated by assigning 1 point for each of the following 7 clinical signs of inflammation present: spontaneous orbital pain, gaze-evoked orbital pain, eyelid erythema, eyelid edema, conjunctival redness, chemosis, and caruncle/plica swelling, with a score ≥3 indicating active disease requiring immunosuppressive treatment. 1
How to Calculate CAS
The CAS assessment involves evaluating seven specific inflammatory signs, with each present sign receiving 1 point 1:
The 7 Components (1 point each if present):
- Spontaneous orbital pain - pain behind the globe at rest 1
- Gaze-evoked orbital pain - pain with eye movement 1
- Eyelid erythema - redness of the eyelids 1
- Eyelid edema - swelling of the eyelids 1
- Conjunctival redness (injection) - visible redness of the conjunctiva 1
- Chemosis - conjunctival edema 1
- Caruncle or plica swelling 1
Total possible score: 0-7 points
Interpretation and Clinical Significance
Activity Threshold:
Predictive Value for Treatment Response:
The CAS is primarily useful for predicting response to immunosuppressive therapy, with patients having CAS ≥4 showing significantly better outcomes 1:
- Specificity: 86% for predicting treatment response 1
- Sensitivity: 55% 1
- Positive predictive value: 80% 1
- Patients with CAS ≥4 had an 80% response rate to immunosuppression versus only 14% in those with CAS <4 1
Critical Limitations and Pitfalls
Major Caveat - The "Low-CAS Progressive Disease" Phenomenon:
CAS has significant limitations, particularly in Asian populations and patients with moderate-to-severe disease who may have low inflammatory scores. This is a critical pitfall that can lead to undertreatment:
- 38% of patients with moderate-to-severe Graves' orbitopathy had CAS <3 3
- Patients with vision-threatening disease had mean CAS of only 2.6±1.5 3
- 44.4% of patients with progressive diplopia had CAS <3 4
- In a large Chinese cohort, 55.6% of patients were in the inactive phase (CAS <3) despite significant disease burden 5
What This Means Clinically:
Do not rely solely on CAS to determine disease severity or need for treatment. The score reflects inflammatory activity but does not capture:
- Fibrotic changes in extraocular muscles causing diplopia progression 4
- Compressive optic neuropathy which can occur with minimal inflammation 3
- Degree of exophthalmos - CAS does not measure proptosis severity 4
- Functional impairment from muscle restriction 4
Treatment Decision Algorithm Based on CAS
For Active Disease (CAS ≥3):
- First-line: Intravenous methylprednisolone for moderate-to-severe active disease 2
- Alternative: Mycophenolate mofetil showed 91% response rate versus 68% for glucocorticoids in patients with CAS ≥4 2
- Selenium: Recommended for mild active orbitopathy 2
For Low-CAS Disease (CAS <3) with Progression:
This requires careful clinical judgment beyond the CAS score:
- Monitor for objective diplopia progression using standardized scoring 4
- Assess for optic neuropathy regardless of CAS 3
- Measure exophthalmos progression - not reflected in CAS 4
- Consider modified NOSPECS scoring for comprehensive severity assessment 4
For Sight-Threatening Disease:
Emergency referral to specialized ophthalmology regardless of CAS 2:
Additional Clinical Context
Disease activity (CAS), not disease duration, is the prime determinant of immunosuppressive treatment outcome 1. However, the composite endpoint used in clinical trials (≥2 point reduction in CAS AND ≥2mm reduction in exophthalmos) demonstrates that CAS alone is insufficient for comprehensive disease assessment 2.
Male sex and TED onset after age 40 are associated with higher CAS and more severe disease 5, which should inform your clinical suspicion even when initial CAS appears low.