Brain plasticity following corpus callosum agenesis or loss: a review of the Probst bundles

The corpus callosum is the largest axonal tract in the human brain, connecting the left and right cortical hemipheres. This structure is affected in myriad human neurodevelopmental disorders, and can be entirely absent as a result of congenital or surgical causes. The age when callosal loss occurs, for example via surgical section in cases of refractory epilepsy, correlates with resulting brain morphology and neuropsychological outcomes, whereby an earlier loss generally produces relatively improved interhemispheric connectivity compared to a loss in adulthood (known as the "Sperry's paradox"). However, the mechanisms behind these age-dependent differences remain unclear. Perhaps the best documented and most striking of the plastic changes that occur due to developmental, but not adult, callosal loss is the formation of large, bilateral, longitudinal ectopic tracts termed Probst bundles. Despite over 100 years of research into these ectopic tracts, which are the largest and best described stereotypical ectopic brain tracts in humans, much remains unclear about them. Here, we review the anatomy of the Probst bundles, along with evidence for their faciliatory or detrimental function, the required conditions for their formation, patterns of etiology, and mechanisms of development. We provide hypotheses for many of the remaining mysteries of the Probst bundles, including their possible relationship to preserved interhemispheric communication following corpus callosum absence. Future research into naturally occurring plastic tracts such as Probst bundles will help to inform the general rules governing axon plasticity and disorders of brain miswiring.

Surgical outcomes of open and laser interstitial thermal therapy approaches for corpus callosotomy in pediatric epilepsy

OBJECTIVE

Corpus callosotomy (CC) is a palliative surgical intervention for patients with medically refractory epilepsy that has evolved in recent years to include a less-invasive alternative with the use of laser interstitial thermal therapy (LITT). LITT works by heating a stereotactically placed laser fiber to ablative temperatures under real-time magnetic resonance imaging (MRI) thermometry. This study aims to (1) describe the surgical outcomes of CC in a large cohort of children with medically refractory epilepsy, (2) compare anterior and complete CC, and (3) review LITT as a surgical alternative to open craniotomy for CC.

METHODS

This retrospective cohort study included 103 patients <21 years of age with at least 1 year follow-up at a single institution between 2003 and 2021. Surgical outcomes and the comparative effectiveness of anterior vs complete and open versus LITT surgical approaches were assessed.

RESULTS

CC was the most common surgical disconnection (65%, n = 67) followed by anterior two-thirds (35%, n = 36), with a portion proceeding to posterior completion (28%, n = 10). The overall surgical complication rate was 6% (n = 6/103). Open craniotomy was the most common approach (87%, n = 90), with LITT used increasingly in recent years (13%, n = 13). Compared to open, LITT had shorter hospital stay (3 days [interquartile range (IQR) 2-5] vs 5 days [IQR 3-7]; p < .05). Modified Engel class I, II, III, and IV outcomes at last follow-up were 19.8% (n = 17/86), 19.8% (n = 17/86), 40.2% (n = 35/86), and 19.8% (n = 17/86). Of the 70 patients with preoperative drop seizures, 75% resolved postoperatively (n = 52/69).

SIGNIFICANCE

No significant differences in seizure outcome between patients who underwent only anterior CC and complete CC were observed. LITT is a less-invasive surgical alternative to open craniotomy for CC, associated with similar seizure outcomes, lower blood loss, shorter hospital stays, and lower complication rates, but with longer operative times, when compared with the open craniotomy approach.

Effective posterior extension of callosotomy by gamma knife surgery

Drop attacks are the most responsive seizure type to open callosotomy, however, surgical complications can worsen the prognosis. Various less invasive techniques have been explored in an effort to minimize the risk. We present a patient who suffered from life-threatening traumatizing drop attacks in whom previous open anterior callosotomy and vagal nerve stimulation were unsatisfactory. Following posterior extension of the callosotomy by non-invasive gamma knife surgery, the rate of drop attacks declined from 30 a day to once a day, or every few days over a four-month period, without complications. Open callosotomy is an invasive and high risk treatment option for patients with drop attacks. The procedure has a potential for complications and neurological consequences that can worsen the functional capacity of a patient who already suffers with disability. Recently, in an attempt to decrease the invasiveness associated with this technique, additional technical refinements and less invasive procedures have been explored in a few studies. Here, we report a case of refractory epilepsy with life-threatening traumatizing DA, in which the patient was treated by radiosurgical posterior callosotomy after unsatisfactory open anterior callosotomy and vagal nerve stimulation.

Effects of Small Temperature Differences Detected in Callosal Circuits of the Anterior Cingulate Cortex

We measured the sensitivity of cortical circuit activity to small differences in local cortical environments by studying how temperature affects the trajectory of epileptiform events (EEs). EEs evoked via blockade of GABA-A receptors were recorded extracellularly from mouse coronal brain slices containing both hemispheres of anterior cingulate cortex synaptically connected by corpus callosum axons. Preferentially illuminating one hemisphere with the microscope condenser produced temperature differences of 0.1 °C between the hemispheres. The relatively warmer hemisphere typically initiated the EEs that then propagated to the contralateral side, demonstrating temperature directed propagation. Severing the callosum following one hour of EEs showed that the warmer hemisphere possessed a higher rate of EE generation. Further experiments implied that intact callosal circuits were required for the increased EE generation in the warmer hemisphere. We propose a hypothesis whereby callosal circuits can amplify differences in respective hemispheric activity, promoting this directionality in seizure propagation.

On the role of the corpus callosum in interhemispheric functional connectivity in humans

The relation between structural and functional connectivity has profound implications for our understanding of cerebral physiology and cognitive neuroscience. Yet, this relation remains incompletely understood. Cases in which the corpus callosum is sectioned for medical reasons provide a unique opportunity to study this question. We report functional connectivity assessed before and after surgical section of the corpus callosum, including multiyear follow-up in a limited subsample. Our results demonstrate a causal role for the corpus callosum in maintaining functional connectivity between the hemispheres. Additionally, comparison of results obtained in complete vs. partial callosotomy demonstrate that polysynaptic connections also play a role in maintaining interhemispheric functional connectivity.

Resting state functional connectivity is defined in terms of temporal correlations between physiologic signals, most commonly studied using functional magnetic resonance imaging. Major features of functional connectivity correspond to structural (axonal) connectivity. However, this relation is not one-to-one. Interhemispheric functional connectivity in relation to the corpus callosum presents a case in point. Specifically, several reports have documented nearly intact interhemispheric functional connectivity in individuals in whom the corpus callosum (the major commissure between the hemispheres) never develops. To investigate this question, we assessed functional connectivity before and after surgical section of the corpus callosum in 22 patients with medically refractory epilepsy. Section of the corpus callosum markedly reduced interhemispheric functional connectivity. This effect was more profound in multimodal associative areas in the frontal and parietal lobe than primary regions of sensorimotor and visual function. Moreover, no evidence of recovery was observed in a limited sample in which multiyear, longitudinal follow-up was obtained. Comparison of partial vs. complete callosotomy revealed several effects implying the existence of polysynaptic functional connectivity between remote brain regions. Thus, our results demonstrate that callosal as well as extracallosal anatomical connections play a role in the maintenance of interhemispheric functional connectivity.

Bimanual simultaneous movements and hemispheric dominance: Timing of events reveals hard-wired circuitry for action, speech, and imagination

The evidence that speech is the marker of hemisphere of action is overwhelming. Thus, contrary to the commonly accepted belief, the evidence indicates that both sides of the body are under the same command (major hemisphere) and that the nondominant side of the body is a callosum-width farther from the major hemisphere. Substantial controversy exists, however, as to the best method for determining the laterality of motor control in an individual case. According to the new understanding, ie, the one-way callosal traffic circuitry underpinning laterality of motor control, the larger excursions of effectors located opposite (contralateral to) the command center while performing bimanual simultaneous drawing tasks provides the best noninvasive and inexpensive approach for demonstrating the laterality of the major hemisphere of a person (who is able to perform such tasks). Here, it is documented pictorially that bimanual simultaneous drawing of geometrical designs or straight lines, as well as moving the arms simultaneously from side to side (or up and down) while noting the difference of speed of the two arms (represented by the distance between the two index fingers), both provide a reliable indication of the laterality of a person’s major hemisphere. In all these maneuvers the nondominant side of the body (even the diaphragms) lags behind the dominant side by an interval equal to the interhemispheric transfer time (IHTT). This lagging behind of the nondominant side of the body in bimanual simultaneous movements is the footprint of directionality of callosal traffic underpinning the laterality of motor control evidenced by worsening of the delay of the nondominant side following callosotomy (uncoupling). Here, the historical precedence of a novel understanding in motor control together with its neurological implications in daily life as well as in laterality of seizure onset are briefly addressed, pointing out the deleterious effects of Sir Isaac Newton’s influence in neurological research on interhemispheric connectivity by suggesting symmetrical representation of visual sense of space in the human brain.