Thyroid Eye Disease (TED) is an autoimmune condition where the immune system mistakenly attacks the tissues and muscles surrounding the eyes. This response leads to inflammation and swelling within the eye socket, causing symptoms like eye bulging and double vision. To manage this complex condition, clinicians rely on a standardized tool called the Clinical Activity Score (CAS). The CAS provides an objective, numerical assessment of the disease’s current inflammatory status, which is fundamental for guiding treatment decisions.
What the Clinical Activity Score Measures
The primary purpose of the CAS is to accurately distinguish between the two main phases of TED: the active inflammatory phase and the stable, inactive phase. Thyroid Eye Disease is considered a biphasic condition, meaning its clinical course involves an initial period of inflammation that eventually subsides and leads to a quiescent state. The CAS specifically measures the current level of inflammation, swelling, and pain, which are the hallmarks of active disease.
This distinction is fundamental because the disease is only responsive to anti-inflammatory medical treatments during its active phase. The score does not primarily measure chronic, structural changes, such as fixed eye misalignment or permanent eye bulging, which are residual effects of past inflammation. By providing a snapshot of the disease’s current activity, the CAS helps physicians determine if the immune-driven process is still attacking the orbital tissues.
The Specific Criteria Used for Calculation
The CAS is typically calculated using a 7-point scale, where a clinician assigns one point for the presence of each specific sign or symptom, resulting in a maximum score of seven. These seven criteria are all direct indicators of active orbital inflammation. The first criterion is spontaneous retrobulbar pain, which is discomfort felt deep within the eye socket without any external trigger. A second point is given for pain experienced when the patient attempts to move their eye in different directions.
The remaining five criteria focus on visible signs of inflammation in the surrounding tissues:
- Redness of the eyelids, medically termed eyelid erythema.
- Visible swelling or fullness of the eyelids.
- Diffuse conjunctival injection, which is redness of the white part of the eye.
- Swelling of the conjunctiva, known as chemosis.
- Inflammation or swelling of the caruncle or plica, the small structures in the inner corner of the eye.
While the 7-point scale is used for initial diagnosis, a 10-point scale may be utilized during follow-up visits to monitor progression. The three additional points on the expanded scale include an increase of 2 millimeters or more in proptosis (eye bulging) over a defined period. A decrease in eye movement greater than eight degrees in any direction also counts as a point, as does a reduction in visual acuity by at least one line on a standard eye chart.
Interpreting the Score and Monitoring Disease Status
The resulting numerical score from the CAS directly guides the assessment of disease activity. A score of three or more out of the maximum seven points is widely accepted as the threshold for defining active, inflammatory TED. This high score indicates that the autoimmune process is ongoing and that the patient’s symptoms are likely to respond to medical treatment aimed at suppressing the immune system. Conversely, a low score, such as zero, one, or two, suggests that the disease is in its inactive, or stable, phase.
Monitoring the CAS over time is a crucial component of managing the disease, as the score tracks whether the active inflammation is progressing, stabilizing, or resolving. Regular assessment of the CAS helps physicians confirm when the inflammatory period has ended, which is a necessary step before considering surgical correction of any residual structural changes. A consistently low score over several months provides confidence that the disease has entered the stable phase.
How the CAS Influences Treatment Pathways
The CAS acts as the principal determinant for choosing the appropriate type and timing of therapeutic intervention. A high score, typically three or higher, dictates a treatment path focused on reducing inflammation and halting the progression of the disease. This often involves medical therapies such as high-dose corticosteroids or other immunosuppressive agents. Targeted biological therapies may also be considered in patients with high CAS, as they specifically aim to neutralize the inflammatory drivers of the condition.
In contrast, a consistently low CAS indicates that the inflammatory phase has passed, and the disease is now stable. At this point, the focus of treatment shifts away from anti-inflammatory drugs, which are generally ineffective in the inactive phase, toward rehabilitative surgery. Surgical interventions, such as orbital decompression to create more space in the eye socket, eye muscle surgery to correct double vision, or eyelid surgery to improve appearance and function, are performed to correct the chronic structural damage caused by the earlier inflammation. The CAS, therefore, provides a clear, binary guide, ensuring that medical resources are directed toward controlling inflammation during the active phase and that structural corrections are safely reserved for the stable phase.