This is a protocol for a Cochrane Review (prognosis). The objectives are as follows:

Primary objectives

Population

Children between four and eight years of age, diagnosed with a language disorder, or determined to have low language proficiency before age eight.

Intervention

Not applicable, this is a review of observational studies and will not include active interventions.

Comparator

The comparison group will be peers or siblings with typically developing language skills, that is, no identified language disorder or low language proficiency.

Outcome

We will include the following outcomes if measured when participants are at least 12 years old (adolescents and adults):

Proximal outcomes in language and literacy (outcomes within the same domain as the original assessments: omnibus tests of expressive, receptive (including listening comprehension), total language, vocabulary, grammar, narrative or expository discourse, clinical markers, such as nonsense word repetition, sentence repetition, or both)

World Health Organization quality of life outcomes across five domains:

physical (including general health, sleep and energy, sexual health);

psychological (including mental health, self-esteem, memory, learning, and concentration);

independence (including activities of daily living, occupational outcomes, dependence on medicinal and non-medicinal drugs and supports, independent living);

social relationships (including friendships, romantic relationships, parenthood, peer problems, and anti-social behaviour);

environment (including academic outcomes, work satisfaction, financial resources, societal participation in leisure/community activities, safety).

Timing

Studies must have traced the individuals into adolescence, or transition to adulthood or adulthood, or both, thus, when participants are 12 years and older.

Setting

Identification, or assessment and diagnosis using standard diagnostic algorithms. Precise measures and cutoffs on standardised tests will vary from study to study. For instance, Tomblin 1997 used the diagnostic algorithm of < -1.25 standard deviations on two or more language composite scores; and in Norbury 2016, language disorder was defined as scores of −1.5 standard deviations or below on two of five language composites in the absence of intellectual disability, existing medical diagnosis, or both.

Identification by speech-language pathologists, educational psychology services, or in the context of research (population studies). Identification includes measures of language, such as vocabulary, grammar, morphosyntax, narrative. It can be expressive measure(s), receptive measure(s), or both. We will exclude identification made on the basis of tests of phonology/speech production, pragmatics, reading, or working memory only.

We will include large-scale population studies in which low language proficiency may be determined by a cut-off of at least -1 standard deviation on at least one standard test of language to assess longitudinal relationships between single measures and quality of life outcomes in large community samples (Beitchman 2014; Caspi 2016; Thornton 2021).

The main objective of this review is to assess the long-term prognosis of an early language disorder or low language proficiency (LLP) for children aged four to eight years at baseline, and from age 12 years and up at follow-up, in areas of language and literacy, and broad quality of life outcomes in physical, psychological, independence, social relationships, and environment outcomes. This will include measures of mental and physical health, academic outcome, employment status, financial resources, and societal participation.

We will ask the following research questions: 1) To what extent do children with LLP age four to eight years show higher risk for persistent difficulties with language and literacy into adolescence and adulthood? 2) To what extent do children age four to eight years, with LLP, experience higher risk for poor quality of life across five domains of physical, psychological, independence, social relationships, and environment well-being in adolescence, and adulthood (WHO 2012)?

Secondary objectives

Secondary objectives are: 1) to understand how severity of early language problems affects long-term prognosis and quality of life, and 2) to identify gaps in the extant research. For instance, while the indicative sample of papers consistently report academic and employment outcomes, there is little evidence regarding physical or medical health outcomes.

Investigation of sources of heterogeneity between studies

We expect that there will be substantial heterogeneity between the included studies on the following variables:

Diagnostic criteria

Severity of language impairment

Method of ascertainment (population study versus recruitment from special schools or clinics)

Year of publication

Age of outcome measurement

Inclusiveness of non-verbal IQ (Specific Language Impairment versus Developmental Language Disorder)

Stability of schooling (e.g. special school consistently versus changing between special school and mainstream classrooms)

Literacy skills