Cytotoxic lesions of the corpus callosum (CLOCCs)

A.Prof Frank Gaillard et al.

Cytotoxic lesions of the corpus callosum (CLOCCs) represent a collection of disparate conditions that can cause signal change in the corpus callosum, usually involving the splenium. 

The term cytotoxic lesions of the corpus callosum (CLOCCs) has been proposed recently 12 as a more precise description of this phenomenon which has previously been known by a variety of terms including transient lesions of the splenium of the corpus callosum, mild encephalitis/encephalopathy with a reversible isolated SCC lesion (MERS), reversible splenial lesions and reversible splenial lesion syndrome (RESLES). CLOCCs not only better reflects current understanding of the underlying pathophysiology of these lesions but also does not explicitly imply that these lesions are confined to the splenium. As such it is probably a better term to use. 

Clinical presentation relates to the underlying pathology (see below) rather than to the callosal lesion itself. Unlike many other lesions of the corpus callosum, CLOCCs do not demonstrate convincing signs or symptoms of hemispheric disconnection, such as pseudo-neglect, alien hand syndromeapraxia of the left hand, agraphia, alexia, and visual apraxias 4

Although numerous underlying aetiologies have been identified, these lesions appear to result from a stereotyped cascade of cytokines and stimulated cells. An initial insult results in macrophages releasing inflammatory cytokines (IL-1 and IL-6) which in turn result in a cascade of changes including recruitment T-cells, break-down of the blood-brain barrier, production of TNF-α, and activation of astrocytes. The end result is a massive increase in glutamate in the extracellular fluid which, via interactions with a number of cell membrane receptors, results in an influx of water into both astrocytes and neurons which manifests macroscopically as cytotoxic oedema 12

It appears that the reason the splenium of the corpus callosum is preferentially affected is the presence of a high density of oligodendrocytes expressing large numbers of glutamate affected receptors 12

Cytotoxic lesions of the corpus callosum are seen in a wide variety of clinical settings, although exactly which conditions are listed in any one publication varies. Classically CLOCCs are seen in patients with seizures or metabolic disturbances, although many other aetiologies are recognised. Reported aetiologies include 1-5,12

Transient lesions of the splenium are only really appreciable on MRI where they have three distinct patterns 4,12

  1. well circumscribed, small, oval lesions in the midline within the substance of the splenium (most common)
  2. more extensive less well-defined irregular lesions extending throughout the splenium and into the adjacent hemispheres (boomerang sign
  3. more extensive extension anteriorly into the body of the corpus callosum

The smaller well-circumscribed lesions are the typical lesion seen in the setting of seizures/cessation of antiepileptic medication, whereas the larger lesion is more typical of other aetiologies.  

These lesions demonstrate the expected features of cytotoxic oedema 4,12

  • T1: hypointense
  • T2: hyperintense
  • DWI/ADC: restricted diffusion (ADC typically 300–500 x 10-6 mm2/s)
  • T1 C+ (Gd): no enhancement

Some studies have shown that patients generally recover completely on MRI studies within 1 month, mostly within 1 week following the neurological recovery 7.

The prognosis generally depends on the underlying cause, but in the setting of epilepsy or antiepileptic drug-related lesions, it is very good. 

Depending on the publication, some of the differentials to contemplate are included in the list of aetiologies of CLOCCs. In any case, when confronted with a splenial lesion consider:

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Article information

rID: 26390
Section: Gamuts
Synonyms or Alternate Spellings:
  • Transient lesions of the splenium of corpus callosum
  • MERS
  • Transient lesion of splenium
  • Transient splenial lesion
  • Mild encephalitis/encephalopathy with a reversible isolated SCC lesion (MERS)
  • Mild encephalitis/encephalopathy with a reversible isolated SCC lesion
  • Transient lesion of the splenium
  • Reversible splenial lesion syndrome (RESLES)
  • Reversible splenial lesion

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