What are the types of dyslexia?

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Types of Dyslexia

Dyslexia is classified into two primary cognitive subtypes: phonological dyslexia (impaired nonword reading with relatively preserved exception word reading) and surface dyslexia (impaired exception word reading with relatively preserved nonword reading), though most individuals with dyslexia demonstrate deficits in both domains. 1

Primary Cognitive Subtypes

Phonological Dyslexia

  • Characterized by disproportionate difficulty reading nonwords compared to exception words, reflecting impaired phonological processing abilities 1
  • Individuals show deficits in phonological awareness and phoneme manipulation tasks 2, 1
  • Neural signatures include increased activation in the left inferior frontal gyrus (area 44), left supplementary motor area, left precentral gyrus, and right insula 3
  • Performance patterns differ significantly from younger normal readers, unlike surface dyslexia 1

Surface Dyslexia

  • Characterized by disproportionate difficulty reading exception words compared to nonwords, reflecting impaired orthographic processing 1
  • Individuals demonstrate deficits in orthographic knowledge tasks but relatively preserved phonological abilities 1
  • Performance resembles that of younger normal readers who have not yet fully developed orthographic representations 1
  • Neural signatures include increased activation in the left supramarginal gyrus (area PFcm) and angular gyrus (area PGp) 3

Additional Cognitive Subtypes

Research has identified a third subtype with combined deficits across multiple domains:

  • Mixed/multiple deficit subtype: Shows impairments in phonological awareness, auditory discrimination, and magnocellular (motion detection) tasks simultaneously 2
  • This represents the most common presentation, as most dyslexics demonstrate impairments in both exception word AND nonword reading compared to age-matched peers 1, 4

Attentional Subtype

  • Characterized by higher attentional costs during reading tasks 2
  • Shows visual attention deficits as the primary impairment 2

Shared Neural Marker

Both phonological and surface subtypes demonstrate increased activation in the right cerebellum (Lobule IV), suggesting a common neurobiological marker across dyslexia subtypes 3

Critical Clinical Considerations

Heterogeneity is the Rule

  • Most individuals with dyslexia have deficits in multiple domains rather than a single isolated impairment 4, 5
  • Individual assessment across letter processing, orthographic lexicon, phonological lexicon, and phoneme systems reveals complex patterns of deficits 4
  • Nine out of 15 adults showed pure phonological deficits, while others demonstrated combinations of phonological, visual magnocellular, and cerebellar deficits 5

Implications for Intervention

  • Prevention and remediation programs must be specifically targeted to each individual's deficit pattern rather than applying a one-size-fits-all approach 2
  • Subtyping based solely on exception word versus nonword reading is insufficient; comprehensive assessment of underlying cognitive mechanisms is essential 1, 4

References

2
Research

Cognitive subtypes of dyslexia.

Acta neurobiologiae experimentalis, 2008

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This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.

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