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A precise and consistent method is required when assessing the various signs of severity. One such method is described in principle below but can be found in more detail at www.eugogo.eu. The order of NOSPECS has been used.

1.Palpebral aperture (Fig. 1): The vertical height of the eyelid in the mid-pupil position is noted after first stabilizing the patient’s head position and fixation to reduce artefacts, and occluding the opposite eye if vertical strabismus is present. Both upper and lower eyelid positions are recorded relative to the respective limbus. Lateral flare is disregarded.

2.Soft-tissue involvement: Although soft-tissue involvement indicates activity, the degree of soft-tissue swelling also describes severity. The signs are assessed as described in “How Are These Signs Assessed?” and Figure 5.

Fig. 10. Measurement of exophthalmos. A Hertel exophthalmometer, ideally with a single mirror and straight foot plates, is chosen, and the fixed (left) side is positioned fairly firmly against the orbital rim (1) before sliding the other (right) side into a similar position (2). The reference points in red (3 and 4) are kept aligned while the position of the corneal surface is read off from the ruler (5).

Table 3. Scheme for subjectively scoring diplopia after Bahn and Gorman [55]

Grade I	Intermittent diplopia, present only when patient fatigued
Grade II	Inconstant diplopia, present only on lateral or upward gaze
Grade III	Constant diplopia, present in primary gaze but correctable with prisms
Grade IV	Constant diplopia, not correctable by prisms

3.Exophthalmos: This is usually measured clinically using a Hertel exophthalmometer. Unfortunately the numerous models available give significantly different readings, and accuracy will depend on using the same instrument, and ideally the same observer [50]. An intercanthal distance is chosen to fit the instrument snugly against the lateral orbital margins at the level of the lateral canthi and prevent horizontal rotation, and the patient looks at the examiner’s eye being used to record the position of the corneal apex, i.e., the examiner’s right eye for the patient’s left eye, etc. The measurement is taken after aligning the reference points on the instrument (Fig. 10). Exophthalmos is defined as a reading 2 mm greater than the upper limit of normal for that patient’s gender, age, and race; however, despite many publications reporting normal ranges, the instruments on which they are based are not always described, and meaningful calibration has yet to be achieved [51–53]. It appears true that women have lower measurements than men, and children have lower measurements than adults, although these decline again with age. Asians have lower measurements than Caucasians who have lower measurements than Black people. Until normal ranges are reported for specified and calibrated instruments, the measured change in exophthalmometry is of the greatest relevance to monitoring [54].

4.Extraocular muscle dysfunction: There are numerous ways of assessing the extraocular muscles; indeed, the lack of a standardized system makes for real difficulties when comparing patient cohorts and surgical outcomes. Some assessments are more relevant to quantifying the severity of GO than others. Subjective diplopia scores [55] are simple and reasonably helpful (Table 3); however, significant changes in limitation of motility will go unrecorded. Additionally, it could be argued that grade II may be less severe than grade I. For example, a patient may have severe but asymmetrical bilateral inferior rectus restriction to which they have adapted well owing to a good prism fusion range, but their fusion may break down daily when tired, leading to intermittent diplopia. By contrast a car driver may be very aware of a much smaller restriction in one medial rectus which is evident daily on lateral gaze. Hence, objective assessments are required to assess therapeutic interventions, and these should include assessment of the capacity for fusion.

The extraocular muscles may behave quite differently over the course of GO. Hence, uniocular fields of fixation (UFOFs) are of value as they independently assess the limitation of excursions of each eye [32, 56–58]. The prism cover test and the field of binocular single vision (BSV) reflect changes in both eyes; however, each retains a valuable place in assessment, the first in planning for strabismus surgery and the second as a useful way to monitor change. They remain useful when both eyes are abnormal, unlike the Hess-Lees screen [4]. BSV has been shown to be quantifiable and reproducible [59], and to correlate well with the functional deficit from the patient’s perspective [60]. UFOFs are quantified in either 4 or 6 directions of gaze [56, 57] by using a bowl or arc perimeter, with mean excursions shown in Table 4. An age-related decline in excursions, especially elevation, has been noted by some but not all studies [56, 57, 61]. What does not appear to be useful in assessing or monitoring muscle function is imaging. Neither MRI volumes nor ultrasound correlate with muscle function, clinical course, or subjective diplopia [62–65].

5.Corneal pathology: While minor corneal pathology requires slit-lamp examination to detect punctate fluorescein staining, sight-threatening pathology is evident with simple torch examination. In this situation, the eyelids do not close gently to cover the cornea, which remains visible. The lower conjunctiva is generally red, and if ulceration has developed, a grey opacity or even an abscess will be seen in the inferior cornea. This constitutes an emergency.

6.Visual disturbance: Clinical assessments for DON comprise the following:

(a)best-corrected visual acuity of each eye, which is most accurately measured with a logMAR chart, although Snellen charts are more widely available;

(b)colour vision testing in the blue/yellow axis is most likely to pick up early defects of DON; however, red-green pseudo-isochromatic charts (e.g., Ishihara) are more readily available and remain very useful in this context (see the section “How Do You Decide whether a Patient Has Dysthyroid Optic Neuropathy?”). Each eye is tested separately using a reading correction as required;

Table 4. Mean ocular excursions at all ages in degrees, after Mourits et al.^a [57] and Haggerty et al.^b [56]

Lateral rectus (0°) (abduction)	46.2a to 52b
Superior rectus (67°)	43b
Elevation centrally (90°)	33.8a
Inferior oblique (141°)	46b
Medial rectus (180°) (abduction)	47.5a to 51b
Superior oblique (216°)	49b
Depression centrally (270°)	58.4a
Inferior rectus (293°)	62b
By convention, ocular excursions away from primary fixation (gaze straight ahead) are designated as 0° for pure abduction, i.e., right gaze for the right eye and left gaze for the left eye. The degrees for each gaze direction follow from this reference point such that they increase in a clockwise direction for the right eye and an anticlockwise direction for the left.

(c)pupil responses are assessed by the swinging flashlight test for a relative afferent pupil defect; artefacts can easily be produced if a consistent method is not followed, particularly in patients with manifest strabismus; the patient fixates on a distant target, and care is taken to give both eyes equal stimulation with the same alignment to the visual axis while the light is moved between alternate eyes;

(d)funduscopy will detect abnormal swelling or pallor of the optic disc and the presence of choroidal folds (Fig. 11) as well as giving valuable information on confounding pathology such as cataract and glaucoma; choroidal folds are thought to develop when the eyeball is mechanically deformed by the secondary effects of enlarged rectus muscles in a restricted space; the folds are horizontal and generally pass straight through the macula unlike retinal folds;

(e)perimetry is reserved for eyes with suspicion of DON; automated perimetry is most commonly used.

Fig. 11. Fundus showing choroidal folds.

In addition to the above, the intraocular pressure is measured. High readings are commonly found in patients with orbital congestion [66], and although these may increase on upgaze in relation to a tight inferior rectus muscle, the reliability of this sign is poor [4].