Introduction
Mucopolysaccharidoses (MPS) are a group of inborn errors of metabolism. These lysosomal storage disorders result from the dysfunction of specific enzymes responsible for the degradation of glycosaminoglycans (GAGs) or mucopolysaccharides [1, 2]. MPS is a group of heterogeneous disorders with different types and manifestations due to impaired dysfunction of 11 enzymes related to GAGs catabolism [1, 2].
Impaired catabolism of GAGs and their accumulation in different cells and tissues leads to severe multisystem dysfunction such as cardiac defects, facial feature abnormalities, hepatosplenomegaly, corneal clouding, hearing impairment, respiratory disease, skeletal deformities, hydrocephalus, central nervous system dysfunction, spinal cord compression, growth and developmental delay, and short stature [3, 4].
Different types of MPS exhibit various clinical manifestations, age of onset, and rate of progression [3]. The most common and key manifestation of all types of MPS is growth impairment and short stature. The underlying mechanisms responsible for this manifestation are still unclear. It is suggested that deposition of GAGs in the growth plate, bone, and cartilage, osteoblast dysfunction, reduced matrix deposition, and hypertrophic chondrocytes contribute to the growth abnormalities [4, 5].
The main and well-tolerated treatments that have been used currently are enzyme replacement therapy (ERT) and hematopoietic stem cell transplantation (HSCT). These treatments could improve or slow the progression of the disease without significant adverse effects [6].
Growth measurements are essential for monitoring disease progression and treatment efficacy [5]. Some therapeutic options, including substrate reduction therapy, gene therapy, and pharmacological chaperone therapy, have also been introduced, but their efficacy has not been confirmed yet [4, 7].
There is also evidence for the use of recombinant human growth hormone (hGH) as a supportive treatment for growth impairment in this group of patients. Still, the findings of the studies regarding its efficacy are inconclusive [8].
Considering the association between treatment options and growth in patients with MPS and the importance of appropriate management of these disorders, this study aims to review the available data on growth status and related treatment factors in patients with MPS type I and MPS type II systematically.

Methods
Data sources
This study was designed as a systematic review and conducted based on the 2020 PRISMA (the preferred reporting items for systematic reviews and meta-analyses) guidelines. The main outcome of this study was to determine the growth status (mainly height z-score) of patients with MPSI and MPSII from reviewed studies and its association with different treatment options.
A systematic literature search was performed through PubMed, Scopus, and the Web of Science based on the keywords and the following search strategy: (((“Mucopolysaccharidosis I” [Title/Abstract]) OR (“mucopolysaccharidosis II” [Title/Abstract]) OR (hunter syndrome [Title/Abstract]) OR (sulfoiduronate sulfatase deficiency [Title/Abstract]) OR (iduronate 2-sulfatase deficiency [Title/Abstract]) OR (lipochondrodystrophy [Title/Abstract]) OR (hurler syndrome [Title/Abstract]) OR (mucopolysaccharidosis type Ih [Title/Abstract]) OR (pfaundler-hurler syndrome [Title/Abstract]) OR (scheie syndrome [Title/Abstract]) OR (alpha l iduronidase deficiency [Title/Abstract]) OR (Hurler-Scheie syndrome [Title/Abstract]) AND ((growth [Title/Abstract])OR (weight [Title/Abstract]) OR (height [Title/Abstract]) OR (growth [MeSH Terms])) AND (stem cell transplantation, hematopoietic, [Title/Abstract]) OR (enzyme therapies [Title/Abstract]) OR (genetic therapies [Title/Abstract]) ))). Two independent reviewers (Seyed Ebrahim Tabatabayipoor, Homyra Raispour) did the literature search through the studies published until March 2023. 

Study selection
Inclusion and exclusion criteria 
All studies in English or Persian that have reported the growth status of patients with MPSI and MPSII were explored. Studies before 1990 were excluded. Letters, editorial articles, non-relevant ones, duplicates, reviews, conference abstracts, and studies in other languages were excluded. Using EndNote citation manager software, version X9 all duplicates were removed. The remaining articles were assessed using the title and the abstract. Non-related studies and reviews were excluded. The full text of the remaining articles was read, and non-related ones were excluded. Study selection was done by two independent reviewers (Seyed Ebrahim Tabatabayipoor, Silva Hovsepian). References of the final articles were also reviewed to find the references that cannot be found through the search. 

Quality assessment 
The STROBE (strengthening the reporting of observational studies in epidemiology) statement, a checklist of items that should be included in reports of observational studies, was used for quality assessment. Two reviewers independently did quality assessment (Seyed Ebrahim Tabatabayipoor, and Silva Hovsepian), and expert opinion was considered (Noushin Rostampour and Silva Hovsepian)) in the case of controversies. 

Data extraction 
The author’s name, year of publication, country, type of MPS, growth status, treatment options, and any associations regarding growth status and diseases and treatment variables in the article were extracted by two independent reviewers (Seyed Ebrahim Tabatabayipoor, Silva Hovsepian). 

 Results
From the initially retrieved 743 references, 100 were removed due to being duplicates, 31 articles were evaluated by reading the full text, and 20 were included in the systematic review [9-28] (Figure 1). The list of these studies and the extracted data regarding the growth of patients with MPSI and MPSII are summarized in Table 1.













Growth status was evaluated in 11 studies on patients with MPSI and 12 on patients with MPSII.

MPSI and growth
The samples in studies that evaluated growth in MPSI patients ranged from 5 to 670 participants [9-12, 15, 17, 21, 24-28]. Two studies with a larger sample size (670 and 295) included patients from the USA, the UK, Canada, Argentina, and France [24, 27]. The first study that evaluated growth in MPSI patients included 5 patients [9]. Treatment options in the studied population were ERT (3 studies) [9, 21, 27] and HSCT (6 studies) [10, 11, 12, 15, 17, 26]. Some studies compared ERT and HSCT treatments with those without treatment [27, 26]. Three studies evaluated the outcomes of the supplementary use of hGH treatment on the growth of patients with MPSI [11, 15, 17].

Height, growth, and growth velocity
The most significant impairment in growth was seen for height. The height SDS (standard deviation score) was <-2 in most studies. Almost all studies indicated delayed growth with lower SDS for patients with MPSI. Based on the evaluated research findings, patients with MPSI had normal growth during the first years of life (0-24 months), and the delay was initiated later in their lives. The reported time of growth delay was different in evaluated studies and between males and females. The age when the delay was initiated was lower in those not receiving treatment, or the age of treatment was later. Growth velocity also had a similar pattern.

Sex differences
The mean z-score of height in boys was lower than in girls [12]. One study reported that though growth impairment continued till 18 years of age in both sexes, the decreasing rate appeared earlier in boys than girls [24]. 
One study indicated that the height z-score decline over time was less significant in females than males [27].

Treatments
In most studies, the treatment choice was HSCT. In two studies [10, 26], the findings indicated significant improvement in the growth of the patients under treatment with HSCT. One study compared ERT with no treatment and reported significant growth improvement compared to non-treated patients [27].
Two studies showed that having a history of total body irradiation (TBI) during treatment could significantly slow growth development [10, 11]. In one study, there was no significant association between the use of TBI, number of transplants, genotype, donor source, post-transplant enzyme levels, and height SDS [12]. 
In one study, the height z-score for normal, impaired, and low enzyme levels were -2.6 (1.8), -4.2 (2.1), and -4.4 (3.2), respectively [10].
One study demonstrated no difference in various treatment options, including HSCT, ERT+HSCT, and TBI+HSCT [17].

Age at treatment initiation
In two studies for ERT [27] and HSCT [10], findings indicated that treatment initiation at younger age was associated with better growth in both sexes.

Type of donor
Short stature was higher in patients with bone marrow (BM) donor recipients than those with a cord blood donor (88% vs 46%) [10]. In another study, there was no such association [12].

Growth hormone treatment 
From 3 studies that evaluated the efficacy of hGH therapy for growth impairment, 2 studies reported improved growth velocity and increased height z-score in patients treated with hGH [11, 15]. They reported 1.7-2.1 cm/y higher growth velocity in the group that received hGH. In another study, the results failed to show a significant difference in growth velocity and height SDS of patients with and without hGH treatment. They indicated that in a patient with growth hormone deficiency (GHD) who received hGH, the growth velocity was significantly higher than those who received hGH but had no GHD [17]. All of the studies did not report serious adverse effects due to hGH therapy in MPSI patients [11, 15, 17].

MPSII and growth
The studied population in studies that evaluated growth in MPSII patients ranged from 6 to 676 participants [13-23, 25, 28]. The two studies with a larger sample size (679 and 609) included patients from the UK, the USA, Italy, Brazil, Germany, and Switzerland [22, 23]. Treatment options in the studied population were ERT (8 studies) [13, 14, 16-18, 19-21] and HSCT (1 study) [19]. Two studies evaluated the outcomes of the additional use of hGH treatment on the growth of patients with MPSII [14, 15].

Height, growth, and growth velocity
The most significant impairment in growth was seen for height. The height SDS was <-2 in most studies. Almost all studies indicated delayed growth with lower SDS for patients with MPSII. Based on the evaluated research findings, patients with MPSII had normal growth during the first years of life (2-5 years, median 3 years of life), and the delay was initiated later in their lives. The reported time of delay in growth was different in evaluated studies based on the severity of MPSII and in males and females. The age when the delay was initiated was lower in those not receiving treatment, or the age of therapy was later. The growth delay initiation was also lower in the severe form than in a mild form of MPSII. Growth velocity also followed a similar pattern.
In the reviewed studies, 3 studies compared MPSII growth based on the treatment initiation age [13, 16, 18]. Their classification was different (<10 and >10 years old, 8-11 and 12-15 years old, and <6, 6-10, 10-20 years old). Comparing the findings of the studies, it is suggested that the best growth improvement was seen with the age of treatment initiation of <6 years old.

Treatments
In most studies, the treatment was ERT, and most reported growth improvement in patients under treatment with ERT. However, it seems that the age of treatment initiation is the most important factor [16, 18].
In 2 studies [10, 25], the findings indicated significant improvement in the growth of the patients under treatment with HSCT. One study’s comparison of ERT with no treatment group indicated significant growth improvement compared to non-treated patients [26].
In a study that compared the effectiveness of different treatment options on patients’ growth, the findings indicated that HSCT could be a recommended option at an early stage in MPSII patients, considering its effectiveness on the brain or heart involvement [19].

Growth hormone treatment 
From 3 studies that evaluated the efficacy of hGH therapy for growth impairment, 2 studies reported improved growth velocity and increased height z-score in patients treated with hGH [15, 17]. One study did not show a significant difference in growth velocity and height SDS of patients with and without hGH treatment [17]. One study indicated that those treated with hGH had greater growth velocity [15].

Cognitive involvement
In 3 studies, the association between cognitive involvement and the patient’s growth has been evaluated [16, 18, 22], but the findings were not similar. In one study, patients without cognitive involvement had more pronounced height deficits than those with cognitive involvement [18]. In two studies, there was no significant difference in linear growth of the patients with and without cognitive involvement [16, 22]. One study indicated that patients with cognitive involvement had increased weight, body mass index, and head circumference [22].

Type of mutation
In 2 studies, the association between height z-score and mutations has been investigated. One study did not find any association between the type of mutation and height z-score [18]. One study demonstrated that in patients with deletions or larger nonsense mutations, the growth impairment, especially height, was more prominent than in those with other types of mutations [16].

Discussion
This study systematically reviewed studies that evaluated growth patterns, especially height SDS, in patients with MPSI and MPSII. Our findings indicated that treatment would improve growth in the MPS patients. Some variables regarding the treatment are important, such as the age of treatment initiation, combination therapy, and hGH therapy. Some factors related to the characteristics of the patients, including genotype (type of mutation) and disease severity, are also key factors in this field. Patients with MPSI and MPSII had normal growth and height during the first years of life, but after 2-5 years, their growth rate decreases progressively.
In healthy children, growth tracking is used to evaluate their health status. Still, in children with a rare disease such as MPS, height monitoring is considered a critical variable for assessing disease severity and progression, as well as the efficacy of treatment. 
Based on reviewed papers, we concluded that the growth impairment was initiated later in patients with MPSII than in MPSI. 
During the first years of life, the growth parameters for both types of MPS are similar to those of the healthy population. Further, in MPSII, the patients have better growth patterns than the healthy population. However, the disease’s severity or the treatment initiation age could have an important role in this field.
In this study, we evaluated the growth pattern of MPSI and MPSII because the accumulated GAGs in both types are dermatan sulfate (DS) and heparan sulfate (HS). It is suggested that the differences in height and growth patterns of the patients with MPSI and MPSII could be explained by the difference in the levels of DS accumulation. Evidence indicated that in MPSI patients, the accumulation of DS is higher, and in MPSII, the accumulation of HS is higher. In MPSI patients, DS accumulation is higher at birth than in MPSII. It is suggested that this accumulation affects the growth of patients with MPSI. In patients with MPSII, the accumulation of DS is more gradual, which could result in the later onset of growth impairment [29, 30].
The age of treatment initiation is one of the most important and well-documented factors. Most of the studies [10, 13, 16, 18, 27] show a significant association between the age of treatment initiation and the height of SDS, and it is suggested that it is one of the main factors in this field. In studies among MPSII patients, different age groups have been evaluated. Treatment initiation before 6 years of life seems more appropriate for improving height SDS [6].
Few studies reported the differences between male and female patients in MPSI, and the reports indicated that the height of SDS in boys was lower than in girls [12]. Some of them indicated that the rate of height SDS decline over time was less significant in female patients, and the time of height decline onset is earlier in males than females [27, 24]. The exact mechanism for the reported sex differences is not clear yet. Recently, Polgreen et al. attributed this difference to the later initiation of ERT in females than males [27]. They reported that though the age at diagnosis was similar in both sexes, the ERT was initiated later in females than males, especially in some geographic regions.
In MPSI patients, the association between donor type and height SDS has been investigated in 2 studies [10, 12]. The results were not similar. One study did not find an association [12], and another one reported that short stature was higher in patients who had bone marrow (BM) donor recipients than those with a cord blood donor (88% vs 46%) [10].
Previous research indicated that using cord blood (CB) is considered an appropriate donor source due to its better chimerism, leading to better therapeutic effects [31]. In addition, recent evidence indicates that the CB cell has a capacity for differentiation, regeneration, and cellular repair. Many studies have supported the hypothesis that due to the characteristics of CB cells, including being a rich source of progenitor and non-hematopoietic cells and capacity for transdifferentiation, they could induce an earlier clinical response [32, 33].
In reviewed studies among patients with MPSII, there was controversy regarding the type of mutation and height z-score. Cho et al. in Korea did not report such an association [18]. Whereas, Jones et al., in a study from different countries with larger sample sizes, classified the severity of MPSII based on the type of mutation and found a significant association between the type of mutations representative for disease severity (large deletion, recombination, deletion or nonsense mutation) and height z-score. They demonstrated that height deficit in patients with mutations related to severe forms of the disease was more significant than those with less severe mutations, including insertion, splicing, missense mutation, or insertion/deletion [16]. Thus, it is suggested that the growth impairment would be more pronounced in patients with severe mutations and consequently severe phenotype.
In reviewed studies, we did not find a study that evaluated the association between cognitive involvement and growth in patients with MPSI. In MPSII patients, the findings were controversial. Two studies did not report any significant association [16, 22], and one reported better growth in those with cognitive involvement [18].
Parini et al., in a multi-national study among a large sample size of patients from different countries, found no association between cognitive involvement and linear growth. Based on their suggestion in MPSII patients, DS is related to growth, and HS is mainly related to intellectual development [22]. Some studies demonstrated no association between CNS involvement in MPS and growth development [14, 16]. Given that the HS accumulation is higher than DS in MPSII patients, it could explain the findings. Another explanation is that MPSII patients with cognitive involvement exhibit a severe form of the disease and mostly die at a younger age and may miss the adult height. However, the ethnic and genetic background of the patients also could have a role in this issue [5].
In some studies, among patients with MPSI and MPSII, the efficacy of hHG therapy on the growth tendency and growth velocity has been evaluated. Polgreen et al. conducted some studies [11, 15, 17], but the findings were not similar. However, more studies in this field are needed, with larger sample sizes and wider genetic backgrounds. One of the critical findings that could be used in clinical practice was that the efficacy of hGH was more significant in patients with growth hormone deficiency than those without it. So, the effectiveness of hGH in this group of patients is supported. Another important finding was that the rate of adverse effects related to hGH was not high, and the reported adverse effects were not serious.
In the reviewed studies, we did not find studies regarding the effectiveness of gene therapy on the height of MPSI and MPSII patients, and available studies were mainly conducted in animal studies [34-36].
The main limitation of this review was the heterogeneity of the studies that evaluated growth, especially height, among MPSI and MPSII patients. In some fields, such as the association of type of donor, the effectiveness of hGH therapy, and cognitive involvement, few studies are available, and the findings are controversial.

Conclusion
The findings of this review indicated that growth impairment is common in patients with MPSI and MPSII. Treatment improved the growth development in this group of patients, but it would not normalize. Some patients’ characteristics, such as disease severity and type of mutations, impact the treatment efficacy and height gain. From treatment-related factors, the most important factor is the age of treatment initiation. Regarding reminder factors such as type of donor, hGH administration, and combination therapies, current findings are inconclusive, and more studies are needed.

Ethical Considerations
Compliance with ethical guidelines
There were no ethical considerations to be considered in this research.

Funding
This study was funded by the Vice Chancellor of Research, Isfahan University of Medical Sciences (No.: 340243).

Authors contributions
All authors equally contribute to preparing all parts of the research.

Conflicts of interest
The authors declared no competing interests. The authors confirmed no concerns of financial involvement with organizations, entities, or individuals with an interest in the subject matter or materials discussed in the manuscript and no conflict of interest.

Acknowledgements
The authors acknowledge resources and support from the Vice Chancellor of Research, Isfahan University of Medical Sciences. 

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