Developmental language disorder (DLD) refers to persisting difficulties with receptive and/or expressive language which affect a child’s functional communication and without a known cause. DLD is estimated to affect 7.5% of children (Norbury et al., 2016; Tomblin et al., 1997), often occurs in association with other disorders and which changes in its presentation over time. DLD is a relatively common but understudied disorder that adversely affects one’s quality of life (McGregor, 2020). However, in many contexts children with such difficulties go undiagnosed, or are diagnosed with other conditions (Meschi et al., 2012) and only 50% of children with DLD receive support (Norbury, 2016). Few studies examine DLD diagnosis and comorbidities over the lifespan or can determine whether DLD is becoming more recognized or recognized earlier over time. To fill this gap, we utilized two longitudinal register-based Finnish birth cohorts (FBC) for children born in 1987 and 1997 (FBC1987 and FBC1997) which included detailed data from administrative and health registers (Gissler et al., 1998; Paananen & Gissler, 2012). These offered an unprecedented opportunity to investigate the age of diagnosis, prevalence, comorbidities, and diagnostic changes in children with DLD.

1. What is the prevalence of DLD in FBC1987 and 1997 and does this change across childhood and/or between cohorts?

2. What is the prevalence of co-morbidities in DLD and does this differ significantly from children without DLD?

3. To what degree do DLD and its comorbid condition(s) occur over time and in which order?

The FBC 1987 and 1997 contained follow-up data from birth to 2016 and included 58,508 and 57,064 individuals respectively. 

DLD vs. Non-DLD: International Classification of Diseases (ICD) 9th and 10th versions were used in the cohorts

DLD:

• Children were classified as having DLD with codes:

  • ICD-9 code 3153A (expressive and/or receptive language disorder)

  • ICD-10 codes F80.1 (expressive language disorder) or F80.2 (receptive language disorder)

  • ICD-9 and ICD-10 Criteria: 2 SD below the mean on language measures. Language skills should be at least 1 SD below the nonverbal IQ standardized score.

• Using the CATALISE consensus diagnostic criteria (Bishop et al., 2017), individuals with a history of differentiating conditions were excluded from a DLD diagnosis: hearing impairment, intellectual disability, fluency disorders, autism, and brain injuries.

Non-DLD: All other cases who do not meet the above criteria

• The prevalence of diagnosed DLD identified (0.29% in FBC1987; 1.10% in FBC1997) was similar to previous findings in Finland (see Kunnari et al., 2019 for a review), but lower than those in Australia (Law et al., 2017), the UK (Norbury et al., 2016), and the US (Tomblin et al., 1997) obtained through population screening.

  • The low prevalence of DLD may be primarily due to the strict criteria defined in the ICD and to the severity of language disorder required to diagnose DLD in Finland.

    • The prevalence of DLD decreased significantly (from 7.5% to 1-2%) when the similar or the same strict diagnostic criteria were applied to the same populations in prior studies (Norbury et al., 2016; Tomblin et al., 1997).

  • The prevalence in 1987 was lower than in 1997 – likely due to differences in register coverage (hospital outpatient visits were not available before 1998).

• Children with DLD were at significantly increased risk of comorbid mood, behavioral and emotional and academic-related disorders. Notably children with DLD were more than 16 times more likely to have developmental disorders (including academic-related disorders) than their non-DLD peers. 

• As for diagnostic changes,

  • over the course of their lives, 57.71% of individuals with DLD had one or more comorbid conditions, while 42.29% only had DLD.

  • conditions that tended to be diagnosed within the same age band as DLD were other speech-language disorders, motor coordination disorders, multiple developmental disorders, and behavioral and emotional disorders.

  • conditions that tended to be diagnosed at a later age band than DLD were academic-related disorders and psychiatric disorders (i.e., mood disorders and neurotic disorders).

  • findings regarding the order of diagnosis may reflect when each condition was diagnosed in the Finnish healthcare system, not necessarily indicating causal relationships.

Only a small subset of children with DLD receive a formal diagnosis. Work is needed to understand whether children with low language not in receipt of a diagnosis of DLD are receiving the support they need within a Finnish context and whether a change to more inclusive criteria for DLD advocated by Bishop et al (2017) would improve access to support. Patterns of comorbidity are complex and change over time, but it is very clear that both academic and mental health outcomes must be considered in interventions and systems to support children with DLD.