Disconnection Syndromes

Abstract

Evolutionary pressures on a finite amount of cortical space led the cerebral hemispheres to evolve into subspecialised regions subserving specific functions. Impaired communication between these highly specialised brain regions gives rise to a variety of clinical syndromes known as the disconnection syndromes. While some of the original concepts proposed by the initial protagonists of the disconnection theory in the mid‐nineteenth century have been disproved, other conditions are now recognised as having a disconnectionist aetiology (e.g. autism and dyslexia). With improved neuroimaging, especially functional brain imaging, it is conceivable that the number of disorders attributable to disconnection syndromes will continue to increase. Disconnection syndromes may be under‐recognised, owing to both lack of physician knowledge of and difficulties elucidating these disorders, because of their subtle presentations (e.g. anterior callosal lesions).

Key Concepts

  • Disconnection syndromes should be considered when dealing with aphasia, alexia and agnosia and impaired hand function.
  • Autism and dyslexia can be thought of as congenital disconnection syndromes.
  • Presentations of disconnection syndromes may be subtle and need to be specifically examined for in order to avoid missing the diagnosis.
  • Disconnection syndromes involve interruption of communication between highly specialised brain regions.

Keywords: disconnection; alexia; agraphia; aphasia; autism; dyslexia

Figure 1. Axial T2‐weighted MRI crudely illustrating impaired communication between the pre‐motor areas (yellow‐ needed to perform coordinated bimanual tasks) as a result of anterior corpus callosum damage (green square).
Figure 2. Coronal T2‐weighted MRI sequence illustrating disconnection of the right motor cortex from the language regions as a result of anterior callosal damage (green cross).
Figure 3. Axial T2‐weighted MRI sequence illustrating the neuroanatomical localisation of Alexia without Agraphia, a syndrome which results from extensive damage involving the left occipital lobe (shown here in green) and the posterior commissure. The result is an inability of visual information from either occipital lobe (orange lines) to be transferred to the language centre in the left hemisphere (red area).
Figure 4. Coronal T2‐weighted MRI illustrating disconnection of the right sensory cortex (blue area), which receives sensory input from the left hand, from the language centre (red area) by an area of damage in the anterior corpus callosum (green cross). Such lesions give rise to the syndrome of tactile anomia.
Figure 5. Coronal MRI illustrating disconnection of the right uncus from the language centre as a result of callosal damage (yellow cross). Information transfer from the left uncus to the language centre is unimpaired as it does not involve trans‐callosal information transfer. Olfactory information received by the right uncus cannot be transmitted to the language centre for naming, leading to verbal anosmia for smells presented to the right nostril.
Figure 6. Sagittal FLAIR MRI sequence illustrating the current theory surrounding neuroanatomic localisation of the apraxias. The inferior parietal lobe contains learned patterns of visuo‐spatial organisation, which, in order to carry out a motor task must then be transferred to the frontal lobe via the superior longitudinal fasciculus (purple line) for translation into motor programmes which then activate motor neurons in the motor cortices in a coordinated manner. Damage to any of these areas or their connections can cause apraxia.
Figure 7. Coronal T2‐weighted MRI graphically representing disrupted connections in the syndrome of pure word deafness. As well as destroying the left auditory radiations to Wernicke's area (green lines), lesions such as those described by Liepmann involving the subcortical left temporal region (red cross) disrupt the path of the trans‐callosal fibres (yellow line) from the right auditory cortex (blue area) to the left temporal lobe.
Figure 8. Graphical representation of associative visual agnosia demonstrating disconnection of visual imagery (located in the yellow area) from their auditory associations (red area) by a temporal lobe lesion (blue box).
Figure 9. Axial T2‐weighted MRI illustrating disconnection of the visual areas in the occipital lobes from the facial recognition centre located in the right temporal lobe by a right temporal lobe lesion (blue rectangle). This gives rise to prosopagnosia (inability to recognise familiar faces).
Figure 10. In the syndrome of conduction aphasia, a lesion (green X) affecting the arcuate fasciculus (blue line) results in interruption of connections between Wernicke's (yellow area) and Broca's areas (purple area).
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Further Reading

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Mulroy, Eoin, Gleeson, Sarah, McCarthy, Allan, Kavanagh, Eoin, and Lynch, Tim(May 2015) Disconnection Syndromes. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0024022]