We report ocular involvement and saccadometric parameters in this to date largest monocentric ophthalmologic cohort of GD3.
Ophthalmologist’s findings
Diffuse corneal opacities have been infrequently reported in GD 3 patients [
24]. They occurred in D409H homozygous patients [
35‐
38] in association with cardiac valve calcification, while we found corneal haze in a heterozygous patient as did Inui et al. in toddler Gaucher type 2 [
39]. Our heterozygous patient with corneal opacity was significantly older (44 years) than other patients. If the corneal opacity is due to the presence of the D409H mutation (albeit in the heterozygous compound state), it could be hypothesized that the absence of this finding in the other carriers of the same mutation (L444P/D409H) might be due to their still young age. In another case of Gaucher type 1 disease (F216Y/L444P), corneal abnormality preceded the diagnosis almost 15 years [
40].
First descriptions on posterior segment abnormalities in Gaucher patients, before enzyme replacement therapy was established, focused on vitreous opacities [
41,
42]. Recent case reports confirmed vitreous opacities, condensations, preretinal hyperreflective dots and posterior vitreous detachment by OCT [
21,
23] [
24]. Sheck et al. [
23] located a hyperreflective preretinal accumulation between the posterior hyaloid interface and the nerve fiber layer of the retina in the mid peripheral and perimacular area (as also shown in Fig.
3). An additional sign in this 14-year-old girl was partial posterior vitreous detachment [
23]. One year later in 2013, Coussa showed comparable OCT recordings in a 13-year-old girl –
preretinal deposits - with slight progression after a follow up period of 5 years [
21]. These are in accordance with our experiences in 16 GD3 patients, in which we found epiretinal particles in the macula, epiretinal and vitreous opacities in the mid periphery, and partial detachment of the posterior vitreous. However, in the present study, we did not only see
vitreoretinal lesions (at the interface
), but also
subretinal lesions. Their occurrence seems more typical with severe phenotype and longer disease duration. The OCT characteristics seem similar to drusen but unlike drusen, the color was whitish instead of yeallowish, similar to the pre-retinal presumed Gaucher cell bodies. One could speculate that the drusen-like deposits are Gaucher cell collections. These lesions located at the peripapillary region, reaching the macula were accompanied by retinal atrophy per loss of photoreceptors and retinal pigment epithelium. We were unable to find comparable findings in a computerized literature search. While chorioretinal atrophy is described in other storage diseases [
43], their appearance in these diseases is considerably less marked and without deposits.
Likewise, tortuosity of the retinal vessels is known from other lysosomal storage diseases such as Fabry disease and alpha-mannosidosis. Unlike the isolated finding of tortuosity in our cohort, a previously reported case of GD showed severe tortuosity with extensive opacities in the vitreous body accompanied by visual impairment [
44]. Red cherry spot is no specific sign for Gaucher disease [
24], which we could confirm. In our cohort, we could not detect any nerve fiber layer deposits as described by Sawicka-Gutaj et al. [
22].
At the time being, we cannot answer the crucial question whether the occurence of subretinal (or vitreoretinal) affection indicates a neuronopathic (or visceral) manifestation.
Watanabe [
25] and Zhao [
26] described tractional retinal detachment due to strong vitreoretinal adhesions and massive vitreous opacities in GD3. In teenagers, the underlying mechanism might include early development of liquefied cavity that exercises traction by the vitreous [
25]. If retinal detachment develops, surgical treatment will be essential to preserve vision [
25], otherwise permanent vision loss might occur at a young age [
26].
Therefore, from the ophthalmologist’s point of view, Gaucher patients whether they are diagnosed type 1 or 3, should undergo the following diagnostic procedures:
-
Macula and papillary dense optical coherence tomography (the resolution of OCT is 7–25 μm, which makes it a sensitive technique) helps to localize subretinal and vitreoretinal lesions.
-
Fundoscopy (with dilated pupils) and fundus pictures enable to document the extent of the lesions in selected cases for follow-up. Differential diagnosis of these lesions – especially the distinction from intraocular lymphoma – is challenging, as GD patients are known to have an increased risk of haematological malignancies [
24]. Ideally, fundus photography should be performed by obtaining coloured and red-free recordings, as the case may be.
-
Electroretinogram and visual field investigation might be informative to detect retinal involvement and optic disc anomalies or in case of otherwise unexplained visual impairment.
Neuroophthalmologic aspects
GD3 patients develop a progressive horizontal supranuclear gaze palsy [
15,
45]. Motility restriction, namely bilateral abduction deficits, indicate abducens motoneuron/nucleus affection [
12], esotropia being the likely result of this N. abducens involvement. We found esotropia or decompensated esophoria in 8/18 patients (44%), which is in a comparable range as described earlier [
11], and considerably more frequent than its prevalence in healthy subjects (2–3.5%). We could additionally demonstrate motility restriction involving all directions of gaze in 6/16 patients (38%). Oculomotor disturbances follow a typical pattern in Gaucher type 3, in accordance to the topo-anatomical areas (PPRF (paramedian pontine reticular formation), riMLF (rostral interstitial nuclei of the medial longitudinal fasciculus), motoneurons of the abducens nucleus, flocculus/cerebellum and vestibular system [
12,
18]. Besides slowed saccadic velocity, the initiation of saccades is delayed and [
10] and saccadic gain (accuracy) is reduced, horizontally more severely than vertically [
15]. Horizontal gaze may be affected so severely that technical measurement is impossible. Therefore, being a more sensitive measurement, the investigation of the less impaired vertical saccades has gained much more attention in saccadometric studies or possible effects of medical treatment [
15,
17]. 2/16 of our patients exhibited saccades that were not reliably measurable due to motility restriction.
Saccade parameters: peak velocity, latency and gain
Peak velocity: The relation between increasing peak velocity by increasing amplitude, termed main sequence in healthy subjects [
46], is preserved in GD3 patients despite abnormal saccades. The overall saccadometric results of decelerated horizontal and secondary vertical saccades confirms previous data on slowed saccades in GD3 patients [
8,
10,
18].
Latency: As in previous reports, latency was found to be prolonged [
10,
12,
15,
18], except for upwards saccades, which impair lastly. The results show enlarged interquartile distances in the boxplots, which in our view rather reflects the susceptibility for confounders such as age than for disease severity.
Gain: Saccadic accuracy (gain) received relatively little attention in studies on oculomotor involvement in GD3 patients. A study on Norbottnian GD3 has recently reported normal accuracy [
18], while a previous report described it to be reduced [
15]. In our cohort, horizontal saccades were less precise and rather hypometric. The same held true for vertical ones, although to a lesser extent. One of the reasons for low gain values in general is motility restriction, especially regarding the maximal target eccentricity. Anticonvulsive medication might be a confounder resulting in slowed and hypometric saccades [
45]. We figured out the patients without anticonvulsive medication (12/16) to have slowed saccades (just like the patients with anticonvulsive medication (4/16)) with one exception corresponding to a mild genotype (N188/−).
Saccadometry is useful
Saccadometry was feasible in all Gaucher type 3 patients (14/16 performed saccadometry with sufficient data quality). We also recommend performing OCT as it is a widely used, expeditious, and informative investigation method feasible in less than 10 min in Gaucher patients, while saccadometry rather takes 10 to 30 min depending on the protocol used and the necessity to retake a test. The additional, typical signs of oculomotor abnormalities such as synkinetic blinking [
8] are assessable by clinical evaluation. Another technique is an upwards curvature to horizontal saccades likely a Bell’s phenomenon (but keeping the lids open). Both are felt to inhibit the omnipause region allowing the saccade to begin. The latter phenomenon is noticeable during saccadometry and may disturb measurements by producing artefacts. Clinical assessment is difficult and less sensitive: While clinical assessment could not reveal differences between upwards and downward saccades, saccadometry was able to measure significant differences regarding this measurement. This is why we recommend to screen Gaucher type 1 patients and to monitor Gaucher type 3 patients for their neurologic manifestation by meticulous motility evaluation, ideally by video-oculography. We suggest that GD1 patients should be assessed for individual follow-up and that a normative GD1 cohort data, which does not exist to the best of our knowledge, should be established.
Infrared video-oculography is non-invasive, not stressful and easy to understand. It is well tolerated by children, however case-by-case-decision is surely advocated. The measurements are time consuming and editing of the raw data is needed to ensure optimum data acquisition. The quality of measurements depends primarily on the patient’s cooperation of the investigator’s experience.
Correlation between peak velocity and neurologic status (adjusted SARA, disease duration, IQ and to a slightly less extend the mSST) was greatest vertically. We were unable to reproduce the strongest correlation for downwards saccades as reported by Bremova (2018) [
12]. However, the better preserved vertical spectrum of velocity allows stronger correlations. This fact has been explained by the horizontal floor effect [
15] or ceiling effect [
12]. Vertical saccadic peak velocity therefore functions as indicator for neurologic manifestation [
12,
15,
17]. Saccadometry has the potential to qualify for those diagnostic procedures allowing quantification of neurologic manifestation such as the mSST. It might replace the less objective clinical saccade assessment and optimize the scoring system. Although correlation between clinical assessment of saccadic impairment and adjusted mSST, SARA and IQ were demonstrated, saccadometry is definitely more objective.
Strengths and limitations of this study
Despite the low prevalence of all GD-phenotypes of 0.7 to 1.75 per 100.000 inhabitants [
47] and an amount of 5% GD3 [
48] within these, we were able to investigate a group of 16 GD3 patients. Most studies reporting on oculomotor disorders in GD3 are based on smaller samples [
8,
10,
14,
15,
18] or display a multicenter study design [
12]. Schiffmann et al. performed saccadometry on 30 GD3 patients in two study centers as part of a randomized controlled study on miglustat [
17]. One older report exists on clinical oculomotor and ophthalmological findings in 22 Norbottnian GD3 patients [
11]. Statistical analysis of the limited samples encountered in rare diseases is challenging. In this explorative study, the significance level has not been adjusted for multiple testing, which may result in an excess of false positive results. Confirmatory studies are required to verify the results.
Another problem is a possible selection bias as the included patients mostly presented mild or intermediate phenotype severity. For patients with severely affected oculomotor disturbances, a favorable atmosphere for the measurement was created and the eyelid was fixed to minimize blinking artefacts.
A potential flaw of our investigation is the fact that in presbyopic study participants, no near correction was used. Quite interestingly, up to now there is no data on how near correction influences saccadometric precision [
49]. Myopes and emmetropes show similar saccadic eye movements [
50].
Regarding the pre-set program of the video oculography system we used, it would be useful to provide a shorter protocol to maintain attention and thus to reduce artefacts during saccadometry in patients with abnormal eye movements. As patients with restricted motility are not able to reach large target eccentricities, smaller maximal target displacements should be considered in those cases. Longitudinal saccadometry might be appropriate for monitoring and seems to be more sensitive than the clinical testing used in the scoring system.