The epidemiology of LD is highly variable according to the type of LD, the spoken language and the tools used for the diagnosis. International epidemiological studies report a prevalence of 4-17% for dyslexia, 2-8% for dysorthography and 1-5% for dyscalculia [35
]. SLD are more frequent in males than females [37
Different languages have different writing systems and variations in prevalence depend on factors like the spelling opacity of each language. The Italian language has a shallow orthographical system, for this reason, we would expect a lower prevalence of SLD in Italy, where, however, the prevalence range is very large, between 0.88% and 10% [41
]. In Italy, SLD represent about 30% of the users of local Neuropsychiatry Services and about 50% of patients which undergo rehabilitation [47
Comorbidity are very common in Neuropsychiatric diseases, including LD, during developmental age. Understanding comorbidity is important because the presence of an additional disorder may affect the expression and severity of the clinical picture, requiring specific treatments and interventions. Patients with comorbidity compared to those without comorbidity usually exhibit more severe neurocognitive impairment, negative academic experience and social outcomes and lower treatment response.
Dyslexia is the most extensively investigated learning disorder in the national and international studies regarding its features and also its comorbidity.
Language disorders may precede or be associated with dyslexia. International studies have estimated that 30-40% of children with Specific Language Disorder receive a diagnosis of reading disorder later on [48
] and a percentage between 55% [51
] and 77% [52
] of dyslexics meets the diagnostic criteria for Specific Language Disorder. In Italy, comorbidity with Specific Language Disorder has been found from 15% to 20% of dyslexic children [53
]. These data support the hypothesis that Language Disorder and Dyslexia may have common genetic or etiologic factors and may be different manifestations of the same cognitive impairment [55
In addition, children with LD often present motor, sensory, perceptual abnormalities [59
]. Huc Chabrolle et al. [15
] in a review found that the impairment of motor development is a feature of nearly 50% of patients with dyslexia and that dyslexia is common among dyspraxic patients. Motor coordination disorder was reported in a percentage from 10.3% to 26% of dyslexics [53
]. These data support the “cerebellar theory” of dyslexia [64
] according to which, the cerebellum, that is responsible for motor control and automate overlearned tasks (i.e. reading), in LD may exert an insufficient motor control influencing articulation, phonological representation and ability to form appropriate connections between graphemes and phonemes.
Comorbidity with other disorders also is known in LD. It is reported that approximately 60% of patients with dyslexia also meet the criteria for at least one neuropsychiatric disorder [65
]. Comorbidity with ADHD is present from 10% to 50% of LD children, while comorbidity with dyslexia is present from 25 to 40% of ADHD patients [65
]. Comorbidity with anxiety and mood disorders has been reported in some studies but in others no difference was detected in the symptoms of anxiety and depressed mood among children with and without LD [53
We did not find comparative data between SLD and LD NOS in literature.
In our sample we detected a comorbidity with neuropsychopathological disorders in both analyzed subgroups with some differences. A more significant presence of language and motor coordination disorders was found in the LD NOS compared to SLD subgroup. This could be linked to a higher degree of functional impairment in LD NOS patients, which presented, in the majority of cases, intellectual disability that might interfere with the normal evolution of neurolinguistic and motor development.
A meaningful presence of ADHD, anxiety and depressed mood was detected in the SLD subgroup. Some old and less replicated studies have suggested that reading disorder might be the primary deficit which causes secondary symptoms of ADHD [76
]. Recent data have shown that there are common cognitive deficits between the two disorders [80
] according to a possible similar genetic etiology, as demonstrated by families studies in twins [81
]. Our results might support these latter theories, as demonstrated by the higher frequency of ADHD in SLD patients. With regards to anxiety and depressed mood, a bi-directional relationship between anxiety, depression and academic achievement has recently been hypotized [83
]. Anxiety and depressed mood could negatively impact learning process, alternatively children with LD may develop anxiety and mood problems, because they often reported adverse academic experiences. In our study we can assume that the symptoms of anxiety and depression are more frequent in the SLD subgroup, due to the greater introspective capacities of these children which are more aware of their difficulties compared to LD NOS patients, most of which have cognitive impairments.
In May 2013, it was published the DSM-5 [84
] that provides the diagnostic category of LSD as a single, overall diagnosis. New criteria give detailed specifiers for the areas of reading, mathematics and written expression and specifiers for grade of severity (mild, moderate, severe). The classification system DSM-5 does not provide diagnostic category LD unspecified/NOS but admit that SLD can co-occurs with neurodevelopmental (e.g. ADHD, communication disorders, developmental coordination disorder, autistic spectrum disorder) or other mental disorders (e.g. anxiety disorders, depressive and bipolar disorders). Further investigations, in according to new classification criteria, are need to better define comorbidities and LSD prognostic profiles to implement appropriate intervention strategies.