Abstract

1. Background

The improvement in the overall survival of childhood acute lymphoblastic leukemia (ALL) currently exceeding 80% is considered as a real success in modern pediatric hematology [1]. There are many potential reasons for this contemporary breakthrough. It has been attributed to the introduction of new chemotherapeutic agents, enhanced supportive care and risk-adapted therapy [1]. As survival rates have significantly increased, more emphasis has been paid on the long-term side effects of the ALL treatment, also in scientific research.

As shown repeatedly, ALL therapeutic protocols cause changes in the central nervous system. Undoubtedly, white matter disturbances induced by demyelination and vascular abnormalities play a role in cognitive impairment caused by radiotherapy [2]. In turn, there is a significant gap in the knowledge of central nervous system-related chemotherapy toxicity. Direct destructive effects on cerebral endothelial cells, brain white matter, blood flow and glucose metabolism as well as modification of immunological mechanisms could be involved in the development of central nervous system damage [3–5].

The above described changes may contribute to the development of cognitive impairment in ALL survivors. Several meta-analyses concerning neuropsychological outcomes after treatment for childhood ALL have been developed so far. All of them unanimously emphasize the heterogeneity of the studied populations and the lack of a uniform cognitive impairment analysis scheme in ALL patients. In a literature review of neuropsychological consequences of ALL chemotherapy approximately two thirds of analyzed studies found declines in different aspects of cognitive functioning [6]. Cousens et al. [7] analyzed 31 studies reporting cognitive function in ALL children after cranial irradiation. Decrements were found in intellectual function, amounting to 10 intelligence quotient points. Campbell et al. [8] performed a complex meta-analysis of 28 studies, those with cranial irradiation, as well as studies in which treatment solely consisted of chemotherapy. As has been shown, ALL survivors represented significant deficits in intellectual and neurocognitive functioning which resulted in worse academic achievements. Moreover, ALL patients treated with cranial irradiation performed weaker on intellectual functioning than those who received only intrathecal chemotherapy. In turn, Peterson et al. [9] analyzed neuropsychological results of chemotherapy-only treatment for childhood ALL. Based on 13 articles published until 2004, some evidence for mild fine motor, executive function and verbal memory weaknesses exist in these patients. In addition, the direct relationship between higher levels of methotrexate and executive dysfunction has been recently reported [10]. This was also confirmed by the diverse activity of particular brain regions visualized with structural and functional magnetic resonance imaging [10].

Currently, the American Academy of Pediatrics indicates neuropsychological follow-up as an important element of long-term care for cancer survivors [11]. However, this recommended approach is not without drawbacks. There is still insufficient evidence to guide the specific timing of comprehensive neuropsychological assessment for ALL children. Moreover, neuropsychometric evaluation can be expensive, time-consuming and provide limited insight into the neurobiological basis of cognitive dysfunction. As a consequence, there is a need for simple functional methods to screen which patients need extensive neuropsychological testing and possible rehabilitation to optimize their learning capabilities and academic achievements. Conventional EEG recordings have not turned out to be useful in predicting late effects of oncological treatment [12] but the endogenous event-related potentials, which detect the neuronal electrical activity associated with cognitive processing, seem to be more promising and encouraging as earlier studies suggested [13, 14]. These diagnostic methods are known to give objective information about both attention-dependent and independent central auditory processing with quite simple and inexpensive test arrangements.

The aim of our study was to assess the value of screening of subtle neurocognitive dysfunction with P300 event-related potentials in childhood ALL survivors as well as to compare the observed changes in irradiated and non-irradiated groups of patients.

2. Methods

A group of 136 patients, 66 males (48.5%), aged 4.9 to 27.9 (average 13.5 ± 5.3) years who have completed ALL therapy, were included in the study. The study group was divided in 3 subgroups according to treatment protocols introduced gradually by Polish Leukemia/Lymphoma Study Group (Fig. 1). Applied modifications of treatment protocols were presented in Tables 1 and 2. ALL therapy was conducted according to modified New York (NY) (30 patients, 17 males, 56.7%) and subsequent revisions of modified Berlin-Frankfurt-Münster (BFM) (106 patients, 49 males, 46.2%) regimens. Patients treated with BFM protocols were divided into two further groups. 32 children (14 males, 43.8%) were treated with previous modified BFM (pBFM) protocols (BFM 81, 83, 86 and 87) in which, as in the NY program, prophylactic and/or therapeutic central nervous system radiotherapy in addition to chemotherapy was used. In turn, 74 children (35 males, 47.3%) were treated with the BFM95 protocol without radiotherapy. Two of these children also received a second-line chemotherapy due to recurrence of the disease. Central nervous system involvement was found in 7 children, including single patients treated with NY and BFM95 and 5 patients treated with pBFM. None of the analyzed patients underwent allogeneic hematopoietic stem cell transplantation.

Cumulative doses of vincristine in NY programs amounted 26 to 89 mg/m² (60.8 mg/m² on average) and 30 mg/m² in BFM programs. In two children with recurrent disease the cumulative dose of vincristine was 35 mg/m². The radiotherapy dose in pBFM group was 13-36.4 Gy (mean 18.4 Gy), while in the group treated with NY programs − 18.2–24 Gy (mean 18.3 Gy).

The control group consisted of 58 patients, 34 males (58.6%), aged 6–17 years (mean 12.2 ± 3.3 years), who were hospitalized after a single syncope episode (n = 29) and healthy subjects (n = 29) who volunteered for consultation and consented to the examination (Fig. 1).

3. Results

Mean age of children at the time of starting treatment was 5.1 ± 3.2 years. In turn, mean age at the time of screening for cognitive disorders was 13.5 ± 5.3 years. The time that elapsed from the completion of treatment to performed screening ranged from 1.5 to 21.8 years.

Mean age of starting treatment in NY group was 6.5 ± 4.5 years and mean control age − 14.0 ± 5.6 (Table 3). Children treated with pBFM developed ALL at the earliest (4.4 ± 3.1 years) and were controlled at the latest (18.3 ± 4.0 years). In this group, the average time from onset of the disease to control was therefore the longest. In turn, the difference between the average age of ALL onset (4.9 ± 2.5 years) and the average age of control (11.2 ± 4.0 years) was the shortest in the BFM95 group. However, the statistical analysis did not show any significant differences in the mean age of ALL onset. In turn, the mean age of cognitive control was significantly different (P < 0.001). Intergroup differences were shown between particular treatment regimens as well as in their direct comparisons with the control group (Table 3).

In groups with or without radiotherapy, the average age of starting treatment was similar, while patients with radiotherapy were significantly older at the time of control examination (mean age: 16.3 vs 11.2 years, P < 0.001) (Table 4).

4. Discussion

According to our best knowledge, the presented data constitute the largest report about the implementation of event-related potentials in childhood ALL population. As we showed in our study, abnormalities in screening assessment of P300 potential were detected in more than a quarter of ALL survivors. Moreover, due to the inclusion of ALL patients treated with different protocols, a significant effect of the type of treatment on the nature of neuropsychological disorders in endogenous evoked potentials was observed. The analyzed protocols contribute to the prolongation of latency and reaction time of P300 potential. In turn, the use of regimens quite similar in terms of used chemotherapy regimens but containing radiotherapy reduces its amplitude. These abnormalities can be used to provide a more accurate characterization of subtle neurocognitive dysfunction in childhood ALL survivors.

The endogenous potentials analyzed in current study are the result of changes in the electrical voltage associated with information processing. They do not depend directly on the type of stimulus but on thinking processes and are classified as long-latency potentials constituting an electrophysiological indicator of cognitive processes [16]. The P300 potential is determined by the positive wave with the highest amplitude recorded in the central-parietal midline leads in response to the processing of the auditory or visual stimulus. Stimulation with two distinct acoustic stimuli (oddball paradigm) is commonly used. The P300 wave arises after 300–800 ms. This wave occurs when the stimulus recognition is associated with a high level of subjective uncertainty and is a measure of the degree of attention devoted to a particular cognitive task. It is assumed that it is generated in the hippocampus and in the temporal and parietal lobes of the cerebral cortex [16].

Among the greatest advantages of neurophysiological techniques are their high sensitivity, non-invasiveness and the ability to repeat them at relatively low costs. They are an objective although non-specific neurological diagnostic tool. Endogenous potentials are widely used in clinical practice, in particular in the diagnosis of oligosymptomatic disease processes, mainly dementia syndromes [17]. Their serial execution also allows to track the dynamics of the disease process and monitor the treatment, therefore they are helpful in determining the prognosis.

However, the usefulness of event-related potentials in the diagnosis and monitoring of adverse effects of childhood ALL treatment has not been sufficiently understood yet. As already mentioned, event-related potentials reflect the synchronized post-synaptic potentials generated by the depolarization of neurons, primarily the large pyramidal cells of the cerebral cortex. Its latency is a measure of the time needed for the cognitive processes preceding the cognitive assessment of the task situation, while the wave amplitude defines the involvement of cognitive structures. The changes observed by us indicate slower and more effortful target detection. Prolonged latency and a reduction in the amplitude of the P300 potential have already been shown in ALL survivors. However, all previous studies have been conducted on small groups of patients. P300 latency has been found to peak later and to have a smaller amplitude in childhood cancer survivors [13, 18]. However, a study by Lahteenmaki et al. [13] was performed on a heterogeneous group of only 19 cancer survivors, in which there were 11 patients with ALL. In turn, Uberall et al. included only 13 long-time ALL survivors in their study. Our results are also consistent with the results by Sato et al. [19] who showed a significant increase in P300 latency in 33 patients treated with chemotherapy and radiotherapy compared to patients treated with chemotherapy alone and to the control group. Moore et al. also made similar observations on an equally large group of childhood cancer survivors [20]. Järvelä et al. demonstrated the usefulness of P300 potentials in monitoring of central nervous system toxicity of ALL therapy in 27 patients. They showed a relationship between progressive deterioration of mental performance and prolongation of the peak latency as well as poorer enhancement of P300 amplitude after treatment [21]. The previous observations presented above have also been confirmed by recently published preliminary results by Brace et al. [14]. Decreased amplitude of particular P300 components were observed in the analyzed small group of 8 ALL survivors treated exclusively with chemotherapy protocols.

Our study may also have potential therapeutic implications in the future. N-methyl-D-aspartate (NMDA) channels have a central role in the generation of event-related potentials [22]. Differences in particular parameters of P300 potentials between ALL survivors and controls are consistent with altered neurotransmission through NMDA receptors [23]. Recent preclinical study has revealed that memantine, non-competitive NMDA receptor antagonist, reduces the incidence of cognitive deficits in rats treated with intrathecal methotrexate [23]. Memantine has also shown promising effects in randomized trial among adults treated with cranial radiation for brain tumors [24]. Potentially, a group of patients with subtle neurocognitive dysfunction identified on the basis of screening with P300 event-related potentials can therefore experience the benefits of prophylactic use of NMDA antagonists. Such behavior may protect this selected group of patients from the development of symptomatic cognitive impairment. However, large randomized trials using NMDA antagonists in patients with childhood ALL are necessary to confirm this hypothesis.

Our study has several limitations. First, our study compared different protocols previously used in clinical practice. However, it was our deliberate intention. Thanks to this it is possible to study the impact of radiotherapy withdrawn from many protocols currently used in ALL on P300 potentials. Second, neuroimaging and neuropsychological correlations with neurophysiological results were not performed. However, the purpose of our study was only to evaluate the value of cognitive disorders screening. The performed neurophysiological studies informed about maintaining the functional integrity of the nervous system. However, none of the patients exceeded the 5% margin of uncounted discriminating stimuli which indicates the correct concentration of attention. It should also be emphasized that all patients carried out their school duty in an undisturbed manner or they worked and were fully independent after completing their education. Third, genetic methods, which are increasingly used in the diagnosis of cognitive disorders in the pediatric population, have not been used [25, 26].

5. Conclusions

Endogenous P300 event-related potentials may be useful in screening assessment of late cognitive impairment in ALL survivors. The type of treatment protocol significantly modulates the individual parameters of the registered P300 potentials. Understanding with the analysis of event-related potentials how ALL survivors brain responses are affected post-treatment will elucidate the type of the cognitive deficits and provide insights into new potential targets for intervention or prevention strategies.

6. Abbreviations

ALL: acute lymphoblastic leukemia, NY: New York protocol, BFM: Berlin-Frankfurt-Münster protocol, EEG: electroencephalography, IFCN: International Federation of Clinical Neurophysiology, NMDA: N-methyl-D-aspartate

7. Declarations

7.1 Ethics approval and consent to participate

The study protocol was complied with the Declaration of Helsinki and was approved by the local Ethics Committee (Consent No. KBET/131/B/207). All parents, adolescent patients and adult patients signed written informed consent before inclusion in the study.

7.2 Consent for publication

Not applicable

7.3 Availability of data and materials

The datasets used and analysed during the current study are available from the corresponding author on reasonable request.

7.4 Competing interests

The authors declare that they have no competing interests.

7.5 Funding

Not applicable

7.6 Authors' contributions

SK, KS and SS contributed to the study concept and design. SK and SS performed diagnostic tests and collected relevant clinical data. KS conducted statistical analysis and wrote sections of the manuscript. SK, KS and SS critically revised the article. All authors were responsible for the integrity and accuracy of the data and approved the submitted version.

7.7 Acknowledgements

Not applicable

8. References

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9. Tables

Table 1. The main modifications of Berlin-Frankfurt-Münster regimens.

Abbreviations: ARA-c: cytarabine, BFM: Berlin-Frankfurt-Münster protocol, HC: hydrocortisone, MTX: methotrexate.

Table 2. The main modifications of New York regimen.

Abbreviations: ARA-c: cytarabine, HC: hydrocortisone, MTX: methotrexate, NY: New York protocol.

Table 3. Comparison of P300 potential parameters in individual protocols and control group.

Abbreviations: BFM: Berlin-Frankfurt-Münster protocol, NY: New York protocol. Significant intergroup differences between study groups have been marked with asterisks (*). 

* P=0.039 NY vs control group, P<0.001 pBFM vs control group, P=0.026 BFM95 vs control group, P=0.001 NY vs pBFM, P=0.010 NY vs BFM95, P<0.001 pBFM vs BFM95

** P<0.001 NY vs pBFM, P=0.021 NY vs BFM95

*** P=0.001 control group vs NY, P<0.001 vs pBFM, P<0.001 vs BFM95

**** P=0.039 NY vs control group

Table 4. Comparison of P300 potential parameters in irradiated and non-irradiated groups of patients.

Abbreviations: BFM: Berlin-Frankfurt-Münster protocol, NY: New York protocol.