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categories of severity are recognized: mild, moderate-to-severe, or sight-threatening as follows:

      1.Mild: characteristics of GO have minimal impact on the patient’s life. They usually present one or more of the following signs:

      (a)minor lid retraction (<2 mm);

      (b)mild soft-tissue involvement;

      (c)exophthalmos <3 mm (above the normal range for the race and gender);

      (d)transient or no diplopia;

      (e)corneal exposure responsive to lubricants

      2.Moderate to severe: patients without sight-threatening GO whose eye disease has sufficient impact on daily life to justify the risks of immunosuppression (if active) or surgical intervention (if inactive); patients usually present one or more of the following signs:

      (a)lid retraction (>2 mm);

      (b)moderate or severe soft-tissue involvement;

      (c)exophthalmos ≥3 mm (above the normal range for the race and gender);

      (d)inconstant or constant diplopia

      3.Sight-threatening GO: patients with DON or corneal breakdown due to severe exposure; other infrequent cases are ocular globe subluxation, severe forms of frozen eye, choroidal folds, and postural visual darkening; this category warrants immediate intervention

As a rule of thumb, it is considered that all patients who do not have a mild or a sight-threatening ophthalmopathy present with moderate-to-severe disease [68].

How Do You Decide whether a Patient Has Dysthyroid Optic Neuropathy?

      There is no single test that will conclusively establish or refute the diagnosis of DON. Therefore the clinician has to be alert to the possibility of DON in all patients with active disease and look for it in particular when there are certain constellations of other GO features.

Although in theory any patient could develop DON during active GO, unless there is significant motility disturbance or extreme exophthalmos, they are essentially not at risk. To put it another way, for the optic nerve to be compromised, which is a secondary phenomenon, there has to be evidence of primary tissue involvement that could lead to this as described in the section “Can You Give Me a Short Mechanistic Explanation for All These Clinical Manifestations?” above. There are 2 scenarios: in the majority of patients DON is caused by very large muscles at the orbital apex (especially the medial rectus and inferior rectus) combined with sufficient tension in the AOS that the orbit cannot self-decompress. In such patients there may be little or no exophthalmos, although it can still be moderately severe [13, 27, 28]. However, there should always be evidence of restricted motility, very often with a vertical tropia or esotropia [27, 69]. Ballotment of the globe is a crude test, but these patients will have tense rather than soft orbits. In the alternative scenario, there is such extreme exophthalmos from self-decompression of the orbit that there is no compression of the optic nerve, but rather it is stretched, as are the muscles. In some of these patients there is global restriction of motility. Although in one series this scenario accounted for 24% of DON [28], others have found it to be much less common [13, 27].

The typical presentation of DON is of a symptomatic patient with ocular surface discomfort or aching and evidence of muscle restriction. Soft-tissue involvement is often not severe [27, 28, 70] although the CAS is often ≥4 [22]. The onset of DON is usually insidious, but symptoms of blurring, patchy visual loss or colour desaturation can be elicited from up to 80% of affected patients [27, 28]. Nevertheless, visual acuity is often well preserved, and a normal acuity does not exclude DON. DON is usually bilateral (70%) but not necessarily symmetrical. Only significant asymmetry (30%) will be detectable by a relative afferent pupil defect. Colour defects are present in most patients [13, 27], and although red-green pseudo-isochromatic colour plates (e.g., Ishihara) are thought to be less sensitive for detecting early DON, a recent study found them to be almost universally abnormal [13]. While 30–40% of eyes with DON may show disc swelling, all studies agree that 40–50% will appear normal. Visual field assessment will detect defects in most patients with other evidence of DON. These are commonly central, paracentral, and/or inferior [4]. It should be appreciated that these tests can show normal fluctuation and may be very misleading in patients with marked visual loss [71] or confounding pathology such as cataract, age-related maculopathy, or glaucoma. Unfortunately the age of patients at greatest risk of DON makes them more likely to show these confounding pathologies, and indeed a recent study showed confounding pathology in 28% [13]. This may make the signs of DON difficult to interpret.

      So, how then can the diagnosis of DON be made with confidence? Recent evidence suggests that the signs with the greatest specificity for DON are impairment of colour perception and optic disc swelling [13]. These signs are least likely to be influenced by confounding pathology, provided the patient is not colour blind. A practical approach would be to diagnose DON on disc swelling alone, provided other causes for this have been excluded. In patients without disc swelling, DON should only be diagnosed when there are at least two other features of optic neuropathy: impaired acuity or colour vision, an afferent pupil defect or abnormal perimetry [4]. Patients without significant visual loss who have inconclusive evidence of DON, may not require treatment; however, they should be monitored very carefully.

Can Dysthyroid Optic Neuropathy Ever Be Present with Normal Vision?

There is clear evidence that DON can be present with normal vision [4, 13]. In some patients, visual acuity of 1.0 may represent a reduction on their normal acuity, while others will truly have no objective reduction by the time DON is diagnosed. Indeed 50–70% of eyes with DON have acuities of 0.5 or better [28, 72, 73].

Are

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