The relation between sensory loss and communicative and cognitive development in children with congenital deafblindness

Abstract

People with congenital deafblindness (CDB) are a heterogenic group, and CDB is defined in a variety of ways in the scientific literature. In this study, we aim to examine whether some of the heterogeneity may be more easily resolved from the perspective of ability than from the perspective of impairment. In order to do this, we take as a starting point for our investigations the communication systems that are used and the different sense modalities they require. Information about almost the entire known population of children with CDB in Denmark (age = 3–18 years, N = 71) was collected using a questionnaire form, covering degree of visual and hearing impairments, intellectual disability, level of expressive communication and use of communication systems. No correlation was found between severity of CDB based on degree of sensory impairment and level of intellectual and communicative disability within the population. However, whether or not the child with CDB was able to make use of residual senses to access a linguistic culture (spoken or signed) correlated significantly with both cognitive and communicative ability. In addition, the two groups had inverse correlations between number of systems used for communication and communicative ability. The actual systems used for communication may be useful for categorizing people with CDB into severity subgroups for scientific study and for intervention planning. In addition, the acquisition of a tactile language for the subgroup of people with CDB who do not utilize a visual or auditory linguistic culture should be given special attention in research and practice.

Keywords

Congenital deafblindness developmental psychopathology dual sensory impairment intellectual disability language development tactile modality

Introduction

The scientific study of congenital deafblindness (CDB) is a relatively small discipline. Not many researchers take on the challenge of describing and explaining the complexities of the population. One unresolved issue is that CDB is defined in a variety of ways in the scientific literature. One of the discrepancies in terminology is between medical/functional and ability/functioning definitions, roughly corresponding to the legal definitions of deafness and blindness in United States (medical/functional) and the Nordic definition of deafblindness (ability/functioning) (Ask Larsen & Damen, 2014a; Danermark & Möller, 2008; Rönnberg et al., 2002). The former takes the degree of the two separate sensory impairments as the defining factors, and the latter stresses the total functional disability in relation to communication, access to information and mobility (Ask Larsen & Damen, 2014a; Dammeyer, 2012). In line with these two approaches, the population with CDB are often further subdivided into categories according to the combination and severity of the individual sensory losses, as when applying the Deafblind Severity Index (DbSI) promoted by Dalby et al. (2009), or according to onset of functional deafblindness relative to development of communication (e.g. Dammeyer, 2012). CDB is usually distinguished from acquired deafblindness. This is done on the basis of the onset of deafblindness relative to either chronological age (e.g. before 2 years of age; cf. Dalby et al., 2009) or developmental age – that is, before language acquisition (Dammeyer, 2011; Rødbroe & Janssen, 2006). Finally, some take the medical aetiology, such as congenital syndromes like CHARGE syndrome (Thelin & Fussner, 2005; Vervloed et al., 2006) or Congenital Rubella Syndrome (Armstrong, 2010; Dammeyer, 2010a), to be the defining factor or inclusion criteria in studies of deafblindness. According to Ask Larsen and Damen (2014a), at least five criteria influence how CDB is defined: (1) medical aetiology of deafblindness, (2) level of sensory impairment, (3) sensory functioning, (4) onset age and (5) onset relative to communicative development. Some of which (1, 2, 3) concern the definition of deafblindness in general, while others (1, 4, 5) concern the distinction between congenital and acquired deafblindness.

Regarding the definition of deafblindness in general, discrepancies between definitions based on sensory impairment, sensory functioning or medical aetiology are evident (Ask Larsen & Damen, 2014a). Some people cannot use their residual senses for communication, information and mobility even though they are not legally blind or deaf. Conversely, people with only minor residual vision or hearing may experience a high function. Also, far from all, people with one of the medical conditions (e.g. CHARGE syndrome or Congenital Rubella Syndrome) associated to deafblindness actually become deafblind.

Discrepancies between onset based on age or relative to language acquisition and the use of medical aetiology as the inclusion criteria create different distinctions between congenital and acquired deafblindness in the literature. The onset of deafblindness may occur well beyond 2 years of age but still before language acquisition, as language acquisition may be delayed or even absent in people with CDB. Others may have already begun language acquisition before the onset of deafblindness at the age of 2. Likewise, even though the medical aetiology may be congenital, deafblindness as a disability may occur much later, as the sensory impairments increase. Accordingly, studies will include or exclude people differently in their CDB samples.

In addition, the CDB definition used for inclusion is not always reported. The diverse and at times unclear definitions of CDB and, consequently, diverse or even obscure inclusion criteria in the literature make it difficult to compare qualitative descriptions as well as quantitative results. According to Saunders and Echt (2007), a clear definition of dual sensory loss in general is lacking.

Another issue in deafblindness research is difficulties in assessing psychological and social parameters in a valid manner (Ask Larsen & Damen, 2014b). Even though there do exist assessment tools developed for people with dual sensory loss, ‘there are no universally accepted instruments for assessing young children who are deaf-blind’ (Rowland et al., 2010). Several researchers report that one cannot rely on visual or auditory test items or even on co-operation from the person with CDB (Carvill, 2001; Dammeyer, 2011; Holte et al., 2006), and interpretation of results from standardized norms is dubious, at best (McInnes & Treffry, 1982; Rocheleau & Mack, 1930; Rowland, 2009).

The third issue to be addressed here is that the CDB population, even when considering the discrepancies regarding definition and inclusion criteria addressed above, is a heterogeneous group with many different and complex aetiologies (Dammeyer, 2010c). This heterogeneity makes it difficult to make inferences from population studies to the individual person with CDB.

These three issues, contradictory definitions, difficulties of assessment and heterogeneity of the population, make it difficult to conduct systematic research related to deafblindness and to compare studies.

The few surveys that have been conducted on the population uniformly report delay in communicative development in relation to CDB (Dalby et al., 2009; Dammeyer, 2010b; Dammeyer & Ask Larsen, 2016; Narayan & Bruce, 2006). Likewise, a high prevalence in the CDB population of mental and behavioural disorders is reported in the literature (Dammeyer, 2011). Also, an association between limited communication abilities and reduced cognitive function is reported (Dammeyer, 2010b). Interestingly, no correlation between degree of sensory impairments and cognitive or communicative delay within the population was found in these studies and was not found to be reported in any other studies. This leaves us with the question of how dual sensory impairment is related to intellectual disability, when no correlation between sensory impairments and degree of intellectual disability is apparent within the group. This calls for research that explores in more detail what the risk factors of CDB for communicative and cognitive ability are and on how these risk factors are related.

No study has yet found any relation between the degree of sensory losses and communicative and cognitive ability within the CDB group (Dalby et al., 2009; Dammeyer, 2010b, 2011; Dammeyer & Ask Larsen, 2016; Narayan & Bruce, 2006). Even though CDB in general is clearly disabling in these areas (Dammeyer, 2014), the within-group differences are not explained by the severity of sensory losses. In a group of children with CDB, the current study aims at investigating the unclear relation between severity of CDB and cognitive and communicative ability.

As an attempt to understand this unclear relation, we will examine in this study the systems that are actually used for communication in order to determine whether some of the heterogeneity may be more easily understood from a biopsychosocial perspective on ability (World Health Organization [WHO], 2001), that considers participation, activity and environmental factors, rather than from a solely medical perspective of impairment, which only considers body structure and function (Bøttcher & Dammeyer, 2016). This study examines the hypothesis that systems in use for communication (spoken language, visual sign language [VSL], tactile sign language [TSL], etc.) instead of the degree of sensory impairment may be useful as an indication of the impact of deafblindness.

The distribution of use of communication systems based on different sense modalities may be interpreted as reflecting the total complexity of the disability (here CDB), including impairments, participation, activity and personal and environmental factors, as defined by the World Health Organization (WHO, 2001) in the International Classification of Functioning, Disability and Health (ICF). Figure 1 shows some of the complexities in the relation between sensory impairments and cognitive and communicative ability. The relation is mediated by the aspects of social interaction such as partner competency, educational approach and participation, as well as additional medical conditions and impairments. Cognitive and communicative ability, as well as communication system use, may be conceptualized as the level of functioning of this complex interplay of factors.

Figure 1.

Communication system use as indicator of communicative and cognitive development.

Looking at level of functioning this way, we may understand some of the heterogeneity (e.g. severity of CDB and support requirements) of subgroups within the group that is not explained by measurements of the sensory impairments alone. Ability (measured by participation in communication systems based on different sensory modalities) and not impairment (i.e. structure of the senses) becomes the indicator of the severity of CDB.

Method

Participants

This study was based on the data from a survey on the Danish population of children with CDB. Recruitment was made through the Centre for Deafblindness and Hearing Impairment (CDH), Aalborg, Denmark, offering national services for all children and adults with CDB. The sample in this study was directly taken from the Danish support system for CDB with no possibility to control the original inclusion assessment procedure. On one hand, this increases the risk of uncontrolled confounding factors on comparison between subjects in the sample but, on the other hand, it makes the sample very true to the actual population included in practice, and thus the sample fits the aim of this study well.

All children in Denmark up till 18 years of age (n = 82) identified with CDB by CDH were invited to participate in this study. Informed consent forms were sent to the parents or the legal guardians of all 82 children. The parents were asked whether they needed the form translated or read aloud in their own language. Except the parents of two children, all accepted to participate and gave permission to use data from their children’s files. The parents of another five children did not return the informed consent forms. Children below 3 years of age (n = 4) were excluded from this study to avoid including children who have not developed language because of low age. In sum, N = 71 children were included in the study, which is almost the entire known population of children with CDB in Denmark (age = 3–18 years) (Dammeyer, 2010c). The consultants informed the deafblind child about his or her participation in the project in a way that was appropriate to the individual child. Ethical review and approval was obtained at CDH.

Procedure

Of the 71 children included in the study, questionnaires were filled out by the consultants from the CDH. The consultants were all experts in CDB support and they had seen the children regularly over the previous 6 months as a minimum. The consultants were selected to give the information instead of the parents because they were experts in assessing behaviour, communication and language development in children with CDB. They had all expertise knowledge of and assess to a range of CDB communication methods. The consultants were required to search their files on the participants for the information they needed for filling in the forms. If information was missing, they were asked to gather the information either by conference with staff and family closer to the participant or by direct observation. Hearing impairment was assessed by an otolaryngologist and vision impairment by an ophthalmologist, to ensure data validity.

Measures

The survey included questions on the children’s sensory impairment, cognitive ability, communicative ability and which communication systems were used. In addition, demographic data such as age and gender were collected. As no existing or validated questionnaires for assessing children with CDB exist, the questions were constructed for this study.

Information about sensory impairment

Hearing impairment was reported in five categories based on decibel hearing loss (dBHL): (1) deaf (>80 dBHL), (2) severe hearing impaired (61–80 dBHL), (3) moderate (41–60 dBHL) and (4) no hearing impairment (<41 dBHL). An additional group (5) included those with cochlear implant (CI). None reported no hearing impairment.

Vision impairment was similarly reported in four categories: (1) blind (worse than 1/60 or visual field <6%), (2) severe visual impairment (worse than 6/60 and equal to or better than 1/60 or visual field 6%–10%), (3) moderate visual impairment (worse than 6/18 and equal to or better than 6/60 or visual field 11–90%) and (4) no visual impairment (equal to or better than 6/18 or visual field >90%). None reported no vision impairment.

Measures of cognitive ability

Level of intellectual disability was obtained from the case files of the participants by the consultants. In all cases, clinical assessment was conducted by psychologists with professional experience in administering formal tests to children with dual sensory loss. Leiter-R or another non-verbal instrument appropriate for use with children with hearing impairments and/or a non-visual instrument appropriate for use with children with visual impairments was employed in cases where the child was able to participate. If the child was not able to participate in formal testing, expert judgement of cognitive ability was made with support from parents and teacher report (Vineland II). Because of the dual sensory loss, it is difficult to use formal testing to assess cognitive ability in many cases (Ask Larsen & Damen, 2014b). Of the data available, it was meaningful to group cognitive ability into three groups: no intellectual disability (IQ > 69), mild (IQ = 50–69), moderate (IQ = 35–49) or profound (IQ < 35) intellectual disability.

Measures of communicative ability

Language level was rated in two different ways. First, a separate question regarded whether the child was considered to communicate on a prelinguistic level. Second, the consultants were asked to rate the expressive communicative level in any communication system used of the children on an ordinal scale including the following categories: (1) communicates by social interaction, (2) using gestures, (3) single words/signs utterances, (4) two words/signs utterances, (5) utterances of three or more words/signs, (6) produce actual sentences and (7) use language without any delay relative to age.

Measures of communication systems

The consultants were asked to report information about the child’s use of communication systems. They were asked whether the child used VSL, spoken language (Speech), TSL, self-made/nonconventional/idiosyncratic signs (Home Signs), alternative and augmentative communications (AAC) systems (i.e. pictures, etc.) and preverbal communication. It was allowed to mark more than one communication system.

Analyses

In order to examine relation between sensory impairments and cognitive and communicative ability, we conducted an explorative analysis of the correlations between the reported communication systems used, the measures of communicative and cognitive ability and the measurements of sensory impairments.

CDB severity

In order to operationalize the DbSI (Dalby et al., 2009), the hearing impairment measure was categorized into two categories: (a) deaf (>80 dBHL) to severe hearing impaired (61–80 dBHL) and (b) moderate hearing impairment (41–60 dBHL) and those with CI. Similarly, vision impairment was categorized into two categories: (c) blind (worse than 1/60 or visual field <6) to severe visual impairment (worse than 6/60 and equal to or better than 1/60 or visual field 6–10) and (d) moderate visual impairment (worse than 6/18 and equal to or better than 6/60 or visual field 11–90). The resulting categories of sensory impairment were finally combined into the four categories of the DbSI, most commonly used in the literature (Dammeyer, 2014): (1) ‘deaf and blind’ (a + c), (2) ‘blind with functional hearing’ (c + b), (3) ‘deaf and functional vision’ (a + d) and (4) ‘functional vision and hearing’ (b + d).

Communication system complexity

The measures on communication system use were analysed in terms of number and combination of systems in order to describe and measure the complexity of the communication of different subgroups and of the group as a whole. A complexity variable was construed on the basis of this analysis, starting with none of the reported systems, through the single systems (beginning with the non-linguistic), and ending up in all the possible combinations of the single systems. The range of this variable can be seen in Figure 4.

Indirect measure of sensory ability: VSL or speech use yes/no

In addition, a binary variable of system use (VSL or Speech use yes/no) was construed on the basis of the given information with regard to system use, dividing the group in those utilizing VSL or Speech at any level from those not utilizing VSL or Speech.

Statistical analyses

Pair-wise correlation coefficients and significance (Kendal’s tau-b, two-tailed) were calculated for all combinations of the variables communication ability and cognitive ability and the variables of sensory impairment, communication system in use and the three constructed variables (CDB group, complexity of communication system use, and the binary variable VSL or Speech use yes/no). Data were analysed in SPSS 20.0 using a significance level of both .01 and .05 because of multiple comparisons.

Results

Sensory impairment

Expressive communicative and cognitive ability was found to correlate significantly (r_τ = .775, p < .001). Slightly significant correlations between visual impairment and the expressive communicative ability (r_τ = .219, p = .030) and cognitive ability (r_τ = .218, p = .037) were found.

No significant correlations between hearing impairment and expressive communicative and cognitive ability were found. Accordingly, we found a slightly significant correlation between the CDB groups, based on sensory impairments, and expressive communication (r_τ = .277, p = .019) and cognitive ability (r_τ = .243, p = .015) (see also Figure 2).

Figure 2.

Distribution of CDB severity group, level of communicative ability and the binary variable VSL or Speech use yes/no.

Communication system use

We found a high correlation between the binary variable (VSL or Speech use yes/no) and both expressive communicative (r_τ = .748, p < .001) and cognitive ability (r_τ = .645, p < .001). As seen in Figure 3, the children with CDB who use neither VSL nor Speech often scored low on both communicative and cognitive ability.

Figure 3.

Distribution of level of intellectual disability, communicative ability and the binary variable VSL or Speech use yes/no.

When splitting the sample according to the binary variable VSL or Speech use yes/no, we found a highly significant correlation (r_τ = .807, p < .001) between the use of more systems of communication and better expressive communication in the group that did not use any of the two linguistic systems VSL or Speech and, conversely, a highly significant correlation (r_τ = −.411, p = .001) between fewer systems of communication and better communicative ability in the group that used one or both of the two linguistic systems. This finding is also seen in Figure 4 showing the distribution of communication systems used and the combination of them with respect to expressive communicative ability. The group that utilizes VSL or Speech used more systems in total than the groups that did not use VSL or Speech (see Table 1).

Figure 4.

Distribution of used communication systems (1 = ACC system(s), 2 = idiosyncratic signs/home signs, 3 = tactile sign language, 4 = visual sign language, 5 = Speech) and level of communicative ability and the binary variable VSL or Speech use yes/no.

Table 1.

Correlations between complexity of system use and level of expressive communication.

Speech or VSL use	r	p	N
No	.807**	<.001	32
Yes	–.411**	.001	39

VSL: visual sign language.

Correlation is significant at the .01 level (Kendall’s tau-b, two-tailed).

Impairment and communication system use

Regarding the correlations between use of communication systems and sensory impairment, we found a high correlation between visual impairment and the use of VSL (r_τ = .366, p = .001) and a lesser, but significant, correlation between visual impairment and the combined binary variable VSL or Speech use yes/no (r_τ = .281, p = .014) with 71.8% of all VSL or Speech users being only moderately visually impaired.

No significant correlations between hearing impairment and communication system use were found, although the correlation between hearing impairment and spoken language was almost significant (r_τ = .213, p = .053) (see Table 2).

Table 2.

Correlations between sensory impairments and VSL and Speech use.

		Visual impairment	Hearing impairment
VSL or Speech use yes/no	r	.281*	.039
	p	.014	.722
	N	71	71
VSL use yes/no	r	.366**	–.118
	p	.001	.285
	N	71	71
Speech use yes/no	r	.128	.213
	p	.262	.053
	N	71	71

VSL: visual sign language.

Correlation is significant at the .05 level (Kendall’s tau-b, two-tailed). **Correlation is significant at the .01 level (Kendall’s tau-b, two-tailed).

Discussion

The findings in this study indicate that the population of people with CDB is best conceptualized as consisting of two distinct groups: one that makes use of visual and spoken linguistic structures, and one that does not. This may have implications for both research on CDB and CDB practice.

Implications for CDB research

Level of sensory impairment did not fully reflect the variability of communicative and cognitive ability in the population. This calls for an approach where the ability and functioning in relation to the different sensory modalities are given centre stage in the description of the population with CDB. The results also suggests that this can be done by examining which communication systems are actually used. In order to understand the implications of the sensory impairments, it may be helpful to examine in more detail why these and not other communication systems based on other sensory modalities are used by the individual person with CDB. We argue that it is only when it is taken into consideration whether or not the person with CDB is able to make use of his residual senses to access a linguistic culture (spoken or signed), that the differences in the group may be explained in more detail. The apparent heterogeneity of the group is less pervasive when we distinguish between those who use their visual and auditory senses for accessing linguistic culture and those who must rely on the bodily-tactile modality for communication.

A question that arises from the present study is whether or not people with dual sensory losses who are able to develop VSL or spoken language in an age-adequate manner should be considered a subgroup of the population of people with CDB. The inclusion criteria used in this survey, which are directly reflecting the practice of the official support system for people with CDB in Denmark, show, that practice to some degree reflects the confusion that we find in the scientific literature regarding how to define CDB. A closer attention from the research community to the theoretical and methodological issues of definition and inclusion, would give practice a better foundation for the identification and assessment of CDB.

Implications for CDB practice

Our findings indicate that the two groups of people with CDB (VSL or Speech use yes/no) may have different needs in terms of communicative approach. The group of people with no use of visual or spoken language may profit from a strong focus on the tactile modality with all the use of gestural, augmentative and alternative communication forms possible, while those benefitting from either or both of the two linguistic systems, VSL or Speech, may use a multimodal approach in the initial stages of language acquisition. Those in the latter group, who do not develop complex language, may benefit from the multimodal approach as a more permanent means of communication.

If access to a linguistic culture is determining for communicative and cognitive development, access to a TSL is crucial for those who cannot benefit from either VSL or Speech. In this study, most of those using TSL (n = 16) were also using VSL (n = 9), Speech (n = 1) or both VSL and Speech (n = 5). Only one child used TSL and not the other two linguistic modalities. This child scored remarkably higher on communicative ability than the rest of the group not using VSL or Speech (‘Utterances of three or more words/signs’ compared with ‘Social interaction’ to ‘Single-word/sign utterances’). The child was, in the same vein, reported to have no intellectual disability.

One major issue to resolve is how to accommodate TSL to language development in the bodily-tactile modality for those who cannot benefit from VSL or Speech. The possibility of TSL as a first language for those who rely on the bodily-tactile modality throughout communicative development is only a possibility for the few, only few cases are reported in the literature (Dammeyer et al., 2015; Rødbroe & Janssen, 2006). This could be taken as an argument for research and practice to give more attention to how to support bodily-tactile communication development, understood as TSL acquisition. It is also an argument for research and practice to focus on how to support the preverbal social interaction and communication of children with sensory loss (Damen et al., 2015).

Limitations

Several limitations of the findings in this study must be taken into consideration when interpreting the findings. The first issue concerns the definition of CDB and inclusion criteria. The sample in this study is directly taken from the Danish support system for CDB with no possibility to control the original inclusion assessment procedure. The Danish support system uses the Nordic definition of deafblindness, which is a functioning-based definition (Ask Larsen & Damen, 2014a). Comparison with other samples will have to take this into consideration.

Second, and related to this, is the well-known problem with statistical calculations based on relatively small sample sizes, as the one in this survey. As CDB is a very rare condition with a prevalence reported to be only 1:10,000 or 1:31,000 (Dammeyer, 2010c), this is a general precondition for research on CDB. This issue makes the discussion of how to define and delimit the group even more important for future research.

Third, the measurement of communicative and cognitive ability used here likewise face the problems outlined in the introduction with regard to measurement validity in relation to CDB. The strong correlation between communicative and cognitive ability measures do, however, call for further research on whether this correlation should be interpreted as a causal relation from cognitive ability to communicative ability or, conversely, as a causal relation from communicative ability through poorly accommodated assessment procedures to cognitive measurement, or rather as part of a more complex system of interrelations including programme approach and partner competency regarding the bodily-tactile modality.

Fourth, we would like to draw attention to the preliminary and explorative character of the reliance of actual communication system use as a measure of sensory ability applied in this study. In order for this to have more than face validity, an in depth study would be necessary, examining the dynamic relations between the ability of the person with CDB to make use of a communicative system as a function of cognitive ability and the availability, accessibility and usefulness of such a communicative system as functions of educational approach, sensory impairment and relevance. No causal interpretations should be made on the basis of this study with regard to why VSL or spoken language is used or not.

The discussion of which bodily-tactile communication system is the most optimal for access to a linguistic culture is in this study not considering the use of Tadoma or other systems as a bodily-tactile means for access to spoken language (cf. Reed et al., 1982, 1984). This is due to the fact that Tadoma is not used systematically in the Danish support system. Comparison with a sample from, for example, the United States where Tadoma has a strong tradition would be interesting.

Finally, future studies should also investigate the significance of early intensive support from caretakers fluent in TSL and apply longitudinal data of the children’s development. Another topic of interest for future research is outcome of CI use, which was not addressed in this study. One previous study has reported a positive communicative outcome from CI use but not always in relation to spoken language development (Dammeyer, 2008).

Conclusion

This study is partly affected by the same difficulties regarding population definition, inclusion practice and measurement validity that were described in the introduction of this article, but it has, nonetheless, pointed out some possibilities for a re-interpretation of severity of CDB in terms of ability instead of impairment.

The complex dynamic relationships between environmental social and physical factors and the specific (sensory, physical and cognitive) impairments of the person with CDB are associated with a certain communicative strategy. This strategy cannot be predicted from the sensory impairments alone, even though visual impairment seems to have some effect on whether or not VSL or speech is accessible to the person with CDB. Instead, we may examine the systems that are actually used in the social interaction, and take this as a measure of the de facto ability of the person with CDB. We may hypothesize that for the children with CDB who make use of residual senses to access VSL or spoken language (at any level), the impact on communicative and cognitive development will be (reported as) less severe than for those children with CDB who do not make use of these linguistic resources. However, it should be noted that, not in all cases and for several reasons the systems used do not reflect the ability of the person with CDB.

This study analysed a simple way to subdivide the population of people with CDB into two distinct groups with very different characteristics and approach requirements: one group with access to a auditory or visual linguistic culture, and another group without such access. Based on the findings, we suggest that the actual communication systems of use are used as indication of de facto sensory ability as a supplement to measures of sensory impairment, in order to take the complex interplay of biopsychosocial factors on ability into consideration. We recommend future research and further development of practice that focus on how to support communication and language development in the bodily-tactile modality in terms of TSL acquisition.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship and/or publication of this article.

ORCID iD

Jesper Dammeyer

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