Secondary effects of Gb3 accumulation which might be responsible for disease pathology include also inflammatory processes [
7]. Recently, Gb3 has been shown to be identical with membranous CD77 [
8] which is supposed to play an important role in apoptosis and necrosis [
9]. In addition, Rozenfeld and co-workers reported perturbed leukocyte function in Fabry disease compared to healthy controls and abnormal numbers of different immune cells, including lymphocytes, monocytes, CD8+ cells, B cells and dendritic cells [
10]. Noteworthy, in another study no correlation was found between CRP-levels as a global inflammatory marker and the Mainz-Severity-Score-Index (MSSI) as a marker for disease severity [
11].
Finally, Gb3-accumulation has been reported to induce oxidative stress and/or the formation reactive oxygen species (ROS) [
13]. Lipids and proteins may be oxidised and may be unable to function. Of note, ERT increased the generation of ROS
in vitro, and up-regulated the intracellular adhesion molecule ICAM 1. The authors hypothesized that ERT may enhance endothelial damage, allowing to understand continuously occurring strokes in patients on treatment.
Heart
Despite the high frequency of cardiomyopathy in Fabry disease [
16], only little is known on the mechanisms leading to functional impairment and on the early cardiac affection.
To identify early cardiac abnormalities Toro and colleagues performed Tissue Doppler Imaging (TDI)-studies in 59 adult patients and compared these results with that of 24 healthy controls [
17]. The authors demonstrated significant abnormalities of systolic and diastolic ventricular function with TDI even in patients with normal conventional echocardiography. In addition, isovolumetric contraction time was found to be the most sensitive parameter for detecting these functional changes.
Consistent with this, Kampmann et al. reported on cardiac manifestations of Fabry disease in 20 children below the age of 18 years prior to ERT [
18]. Thirty-five percent of the children already had left ventricular hypertrophy, and the remainder subjects had a LVM-index above the 75th percentile of healthy controls. During the 26-months follow-up period the LVM-index increased in more than 85% of the patients, while the remainder patients entered the echocardiographic criteria for LVH. Noteworthy, the increase in LVM was not followed by abnormalities in either systolic or diastolic function, and in general there were no signs of LVH seen in ECG. Another warranting observation of this study was the demonstration of reduced heart rate variability (HRV) in boys with Fabry disease suggestive of an impaired autonomic control of the heart which may be responsible for the increased cardiac morbidity. Palecek et al. demonstrated that also affection of the right ventricle is a common finding in both, males and females with Fabry disease [
19]. Impaired diastolic function was observed in about 50% of the patients, and the authors speculated that involvement of both ventricles may reflect the progressive character of the disease.
Weidemann et al. stressed the involvement of the heart valves in particular in patients with advanced cardiomyopathy, where insufficiencies of the mitral, aortic and tricuspid valve were found [
20]. It is likely that the altered myocardial morphology in advanced cardiomyopathy maybe together with fibrosis of cardiomyocytes may indirectly lead to heart valve regurgitation.
Apart from fibrosis of the myocytes, disturbances of the myofilamental structure has been observed in isolated cardiomyocytes of male patients with Fabry disease and LVH [
21]. A high percentage of Gb3 was found widely distributed over the cells, and it is not unlikely that the storage material directly impairs ventricle function (e.g. by impaired relaxation or stiffening of cardiomyocytes). The authors hypothesised that such changes may be responsible for diastolic dysfunction in Fabry disease. On a more functional level, Chimenti and co-workers found that the maximal isometric tension of a single cardiomyocyte was reduced [
21]. This reduction correlated with the decreased systolic velocities from TDI and with the degree of myofibrillolysis. In addition, myofibrillolysis was associated with degradation of myofilamental proteins suggesting that cardiomyocyte impairment may in part be explained by proteolysis. Progression of Fabry disease cardiomyopathy may be related to fibrosis of endomyocardial tissue, but was not correlated with TDI measurements of systolic or diastolic LV function in this study.
Cardiac involvement in Fabry disease is one of the major causes for premature death in Fabry disease [
4]. However, it is generally accepted that classic myocardial infarction as a result of atherosclerotic coronary heart disease is rare. Coronary angiographic properties were determined by intravascular ultrasound in nine patients and ten atherosclerotic controls [
22]. Atherosclerotic plaques in Fabry disease were suspected to have a larger number of lipid cores, and the pattern of the lesions was more echogenic. The authors speculated that Gb3 and disease-specific trophic alterations may explain the differences to regular atherosclerotic formations.
In a double-blind, randomized, placebo-controlled study [
23] sixteen adults were followed over a period of six months before 10 of them entered a two years extension study. The authors found a mean decrease in myocardial Gb3-content of ~20% in the treated group whereas the placebo-group showed a mean increase in Gb3 of ~10% [
23]. Though reduction of myocardial Gb3-content did not reach significance, LVM as measured by cardiac MRI and echocardiography was significantly decreased in the patients treated with Agalsidase alfa compared to an increase in the placebo group. After two years of treatment these results were confirmed by repeated cardiac MRI. Ejection fraction as a measure of cardiac function remained normal or hypernormal in the entire cohort. Shortening of QRS-duration confirmed previous reports on normalized conduction [
2]. The authors indicated that the significant reduction in LVM may be clinically relevant since LVM has been demonstrated to be an independent risk factor for premature mortality in other conditions.
From an observational study it has been reported that ERT with Agalsidase beta may reduce LVM and interventricular septum thickness [
24]. In contrast to this Koskenvuo et al. reported only minimal effects of Agalsidase beta on cardiac symptoms of nine patients with Fabry disease who were enrolled in an open-lable study [
25]. Only resting heart rate was significantly decreased over a follow-up period of 2 years, but no improvement in echocardiographic measures, ECG or exercise capacity was observed. Improvements seen in stroke volume on cardiac MRI after 12 months ERT were not retained after two years, and no other parameter reached significance.
Echocardiographic follow-up over more than three years in 29 males and females with Fabry disease under ERT with either of the available Agalsidase formulations failed to prove a decrease in left ventricular wall thickness, or left atrial size though improvement of diastolic function was observed [
26]. However, it has to be noted that the age of the study population was relatively young (37 ± 12 years) and that no further progression of the disease was observed in respect to cardiac manifestations under ERT. This observation is supported by a study from Kampmann et al. on the natural course of cardiomyopathy in male and female patients with Fabry disease [
27]. Repeated echocardiography was performed over an average time of 4.5 years in 76 patients. There, left ventricular mass index increased with age in both, males and females, respectively. Median time of onset of left ventricular hypertrophy was 44 years in males and 55 years in females. As expected, LVH was more frequently found in male patients and was also related to age. No functional measure was related to left ventricular mass index or left ventricular hypertrophy. Of note, left ventricular mass index increased by some 4 g/m
2.7*year in men and slightly more than 2 g/m
2.7*year in women. Following the authors, this difference may be explained by the mosaic-like distribution of Gb3 in female cardiomyocytes compared to male patients. In summary, these studies recommend initiation of ERT at an early stage of the disease since at least some of the cardiac manifestations may be irreversible [
26,
27].
Recent data showed that left ventricular wall thickness showed significant improvement over a period of eight years in patients under ERT with Agalsidase alfa [
28].
Apart from morphologic improvement of LVH and cardiac mass, Lobo and co-workers tried to evaluate a clinical effect of ERT on cardiac function by performing a 6-minute-walk-test and bicycle stress testing [
29]. However, the authors mixed male and female patients despite the different clinical affection. Moreover, females were on ERT only one third of the time males were treated with one of the enzyme formulations. In addition, both tests used may be significantly biased, e.g. by motivation, neuropathic pain in feet or aggravation of pain under physical exercise.
Neurology
Probably the most debilitating symptom in Fabry disease is neuropathic pain. Though often labelled as "acroparaesthesia", it is well known today that virtually the whole body can become place of painful experiences [
30].
The typical neurophysiologic pattern of pain in Fabry disease allows discrimination from other sensory neuropathies. Standardized quantitative sensory testing demonstrated severe impairment of thermal discrimination compared to patients with sensory neuropathies of large nerve fibres and to patients with other small fibre neuropathies [
31].
Apart from the peripheral nervous system, the central nervous system is also known to be affected in Fabry disease, too. Cerebrovascular events significantly contribute to the pre-term mortality of Fabry disease with a median cumulative survival time of 50 years [
4]. Based on clinical-neurological and MRI evaluation Buechner et al. reported on a prevalence for CNS-involvement in up to 72% of patients with Fabry disease [
32]. Cerebrovascular disease occurred at a mean age of 42 ± 11 years in males and approximately 10 years later in females. In contrast to other reports, the authors found a nearly equal distribution of cerebrovascular involvement between vertebrobasilar and carotid districts.
Alari et al. observed changes in cerebral blood flow velocities (CBFV) in patients with Fabry disease who were treated with ERT [
33]. At baseline, there was a tendency towards higher CBFV in patients with Fabry disease compared to age-matched healthy controls. After 30 months ERT CBFV decreased significantly, which is in good agreement with previous observations over a shorter period of time [
34].
Neuropsychiatric symptoms in Fabry disease were evaluated by Schermuly et al. together with MRI and diffusion tensor imaging [
35]. Patients with Fabry disease were significantly more depressed and showed a lower cognitive performance than healthy controls. Interestingly, pain interference was significantly correlated to white matter lesions supporting previous observations by Cole et al. [
36].
Kidney
Close examination and laboratory analyzes is important to detect also pre-clinical affection of the kidney in Fabry disease. Tøndel et al. [
37] performed kidney biopsies in nine children, including two boys under ERT. Accumulation of Gb3 was found irregardless of clinical nephropathy, and structural changes of the kidney were observed in nearly 80% of the patients including glomerulosclerosis and interstitial fibrosis already in these young patients. Moreover, changes were seen in the glomerulum, the tubulo-interstitial space and small renal vessels. Noteworthy, the two boys receiving ERT developed de novo albuminuria despite treatment. The authors suggested kidney biopsies in all children and adolescents with Fabry disease and albuminuria in order to evaluate the extent of renal damage.
Erroneously, for decades females with Fabry disease were considered as carriers only. Meanwhile, there is no doubt that they are in fact patients who may develop the whole spectrum of the disease [
5,
38].
A cross-sectional analyzes of the Fabry Registry focussing on kidney involvement in females revealed that renal impairment occurs in all classes of the chronic kidney disease classification including end-stage renal disease [
39]. Consistent with this other authors found proteinuria in one third of the female and nearly two third of the male patients, increasing with age in both genders. The latter finding is in contrast to a previous report by Deegan et al. [
40], but was confirmed by other authors [
6]. Kidney affection was clearly evident by Gb3-accumulation in every glomerular cell type, and accumulation in peritubular capillaries was grossly related to the severity of nephropathy. These changes were observed in an early stage of the disease, and the authors speculated on its value as an indicator for ERT efficacy. In addition, non-specific glomerular and tubulo-interstitial lesions of degenerative origin were also seen. The authors concluded that kidney involvement in females with Fabry disease is not different from findings in male patients.
Rather seeking for morphologic but functional alterations in patients with Fabry disease, Vylet'al and co-workers reported on quantitative and qualitative changes in the excretion of uromodelin (UMOD) [
41]. Attenuated UMOD-expression was suspected to be directly linked to lysosomal storage processes as both, UMOD and accumulated Gb3 meet in the Henle-Loop of the kidney. Deficient UMOD-expression has been reported in association with impaired tubular function, in particular with impaired urine-concentration. Hence, the reported observations may explain the changes in tubular function seen in Fabry disease, and in particular the phenomenon of hyperfiltration reported in some patients.
Treatment effects have been analyzed based on data from 165 patients with Fabry disease enrolled in the Fabry Outcome Survey [
42]. Feriozzi et al. demonstrated that ERT with Agalsidase alfa may slow the decline in renal function of male and female patients. Moreover, this effect was also seen in patients with advanced renal failure (eGFR ≤ 59 ml/min*1.73 m
2), and it was sustained over a period of three years.
Both, diagnosis and monitoring of kidney involvement in Fabry disease is hindered by the repeated discrepancies between estimated glomerular filtration rate (eGFR), 24-hour urine creatinin clearance (Cr) and isotopic GFR measurement.