Comorbidity in developmental dyslexia

Question 1

What is comorbidity?

Answer 1

The simultaneous presence of 2 chronic diseases or conditions in a patient. They are associated in time but not related.

Question 2

What is co-occurrence?

Answer 2

Occurring together or simultaneously. Associated by not necessarily related.

Question 3

Give an example of co-morbidity and one of co-occurrence.

Answer 3

Co-morbidity: heart disease and diabetes (have similar cause so occur together but not actually related).
Co-occurrence: diabetes and asthma (they can occur together but are not related at all, many people have asthma and not diabetes)

Question 4

Moll et al. (2014)

Answer 4

Dyslexia co-occurs with other developmental disorders at rates that are higher then chance

Question 5

Moll et al. (2017)

Answer 5

Single learning disorders are as common as comorbid LDs in spelling, reading and arithmetic. Relative commonality of pairs of disorders shows differences in aetiology of comorbidity.

Question 6

Kaplan et al (1998)

Answer 6

Dyslexia co-occurs with dyspraxia (DCD): 50% with dyslexia meet DCD criteria

Question 7

Gilger et al. (1992)

Answer 7

Dyslexia co-occurs with ADHD: 25-40% of children with ADHD have dyslexia.

Question 8

Kaplan et al. (2006)

Answer 8

ADHD- DCD- Dyslexia: 36% of children with DCD met criteria for ADHD and Dyslexia.

Question 9

Catts et al. (2005)

Answer 9

Dyslexia co-occurs with specific language impairments: 31-35% of children with SLI develop later literacy difficulties

Question 10

Bishop & Pennington (2009)

Answer 10

Reading difficulty more likely In multi deficit models genes will be shared and independent factors contributing to cognitive skills and have probabilistic influences (not on/off) and so act as a quantitative trait loci rather than a major gene.
More Genome Wide Association Studies to reduce linkages found by chance.in children with both Speech sound disorder and Language impairment.

Question 11

Landerl & Moll (2010)

Answer 11

11-56% of children with Developmental dyscalculia (DD) have dyslexia.
(17-70% of children with dyslexia have DD)

Question 12

Kadesjo & Gillberg (2001)

Answer 12

Half the population of 7 y/o in a swedish town clinically examined and parents and teacher interviewed. Diagnosed with ADHD, DCD, ODD, Aspergers, Tic disorders and reading/writing disorders.
Found that pure disorders are the exception not the rule.

Question 13

Willcutt et al. (2007)

Answer 13

Twins with reading difficulty and ADHD. Both individually highly heritable. Genetic influences primarily responsible for association between RD and inattention. Association between RD and Hyperactivity/impulsivity did not have significant genetic influence.

Question 14

What is the alternate form explanation for co-morbidity in dyslexia?

Answer 14

The co-morbid version of dyslexia and another disorder is another type of dyslexia.

Question 15

What is the random multiformity explanation for co-morbidity in dyslexia?

Answer 15

Where having one disorder increases risk to have another disorder.

Question 16

What is the extreme multiformity explanation for co-morbidity in dyslexia?

Answer 16

Extreme cases of a disorder lead to increased risk of the second co-morbid disorder.

Question 17

What is the three independent disorders explanation for co-morbidity in dyslexia?

Answer 17

Co-morbid version of e.g. dyslexia with ADHD is another form of dyslexia

Question 18

What is the correlated liabilities explanation for co-morbidity in dyslexia?

Answer 18

The correlation between risk factors for a disorder means that disorders who’s risk factors correlate will occur together more often.

Question 19

What is the direct causal model explanation for co-morbidity in dyslexia?

Answer 19

The 2 co-morbid disorders have a cause in common.

Question 20

What do estimates for co-morbidity being greater than chance indicate and what potential limitations may also cause this?

Answer 20

A shared causal risk factor such as genetic variants, causal mechanisms and environmental factors. May be due to biased sampling or rater bias.

Question 21

Bishop & Pennington (2009) biased sampling

Answer 21

Biased sampling as many participants are clinic referred and the chance of being referred increases with co-morbidity. Also there is parent and teacher bias from the interviews.

Question 22

Semrud-Clikeman et al. (1992)

Answer 22

Results occur at a rate better then chance in clinic referred samples: 38% ADHD in LD

Question 23

Wilcutt & Pennington (2000)

Answer 23

Results occur at a rate better than chance in non-referred community samples: boys with RD 31% ADD, without RD 9%, and for girls with RD 22% and without 5%.

Question 24

What is rater bias?

Answer 24

You are more likely to rate ADHD symptoms highly if you already have reading difficulties.

Question 25

Wilcutt (2008)

Answer 25

Adult, parent and child rating all showed greater ADHD ratings in children with reading difficulties.

Question 26

What is some evidence for rater bias in dyslexia research?

Answer 26

Pre-reading skills are associated with ADHD ratings prior to reading instruction, before overt difficulties (Wilcutt & Betjemann et al., 2007)

Question 27

What is phenocopy?

Answer 27

an individual showing features characteristic of a genotype other than its own, but produced environmentally rather than genetically.
The experience of one disorder leads to symptoms of another.

Question 28

Rice, Smith & Gayan (2009)

Answer 28

The candidate genes for reading disability, KIAA0319, had a strong effect on language phenotypes. The findings are consistent with a multiple gene model of the comorbidity between language impairments and reading disability.

Question 29

McGrath et al (2011)

Answer 29

614 twin study. Naming speed, processing speed, WM (VWM for D) and executive functioning (PA for D) overlap between dyslexia and ADHD.
Processing speed was the only cognitive variable with significant unique relationships to RD and ADHD dimensions, particularly inattention. Moreover, the significant correlation between reading and inattention was reduced to non‐significance when processing speed was included in the model, suggesting that processing speed primarily accounted for the phenotypic correlation (or comorbidity) between reading and inattention. Supports the power of multiple deficit approaches for complex developmental disorders. The overlap between ADHD and RD is due to a shared cogntive deficit in processing speed.

Question 30

What reasons are there for dyslexia overlap with language impairment (LI)?

Answer 30

Phonological association with speech and language development, dyslexia affects learning phonological representations may impede semantic learning.

Question 31

What reasons are there for dyslexia overlap with speech sound disorder (SSD)?

Answer 31

SSD affects articulatory gestures and this may impact on phonological awareness.

Question 32

What are the strengths of single deficit models of dyslexia?

Answer 32

Useful to try to focus in on the most primary symptom that can be explained, has a simple initial focus for idea generation.

Question 33

What are the limitations of single deficit models of dyslexia?

Answer 33

Ignore symptoms not correlated with the symptom of interest, relies on pure cases which varies with theory used to define pure case, age, cog processing, and compensation).
Behaviourally defined disorders don’t have single causes at different levels of analysis, co-morbidity is due to shared genetic and cognitive risk factors and liability is continuous (disorders are based on thresholds).

Question 34

What do multiple deficit models look at in dyslexia? (citation)

Answer 34

(Pennington, 2006)
Behaviours, co-morbidity, interactive development, neural systems, cognitive processes, risk and protective factors, GxE interactions and correlations.

Question 35

What are the implications of co-morbidity?

Answer 35

Ignorance of overlap ignores nuances of aetiology, there aren’t many pure cases of disorder when taken into account.
Patients should be assessed across domains (e.g. literacy and arithmetic) as co-morbidity is likely, definitions may not capture the learning domains affected, absence/presence of co-morbidity affects intervention as less likely to respond well if co-morbid.