Hematopoietic cell transplantation (HCT) is a potentially curative treatment for children and adults with malignant and non-malignant diseases. Despite increasing survival rates, long-term morbidity following HCT is substantial. Neurocognitive dysfunction is a serious cause of morbidity, yet little is known about neurocognitive dysfunction following HCT. To address this gap, collaborative efforts of the Center for International Blood and Marrow Transplant Research and the European Society for Blood and Marrow Transplantation undertook an expert review of neurocognitive dysfunction following HCT. In this review, we define what constitutes neurocognitive dysfunction, characterize its risk factors and sequelae, describe tools and methods to assess neurocognitive function in HCT recipients, and discuss possible interventions for HCT patients with this condition. This review aims to help clinicians understand the scope of this health-related problem, highlight its impact on well-being of survivors, and to help determine factors that may improve identification of patients at risk for declines in cognitive functioning after HCT. In particular, we review strategies for preventing and treating neurocognitive dysfunction in HCT patients. Lastly, we highlight the need for well-designed studies to develop and test interventions aimed at preventing and improving neurocognitive dysfunction and its sequelae following HCT.
According to the Worldwide Network for Blood and Marrow Transplantation (WBMT)  and the World Health Organization (WHO), over one million hematopoietic cell transplants (HCT) have been performed worldwide and approximately 50,000 HCT procedures are performed annually [2, 3]. By 2030, an estimated half-million HCT recipients in the United States (US) will be long-term survivors . These survivors are at risk for late effects that may adversely affect their quality of life (QOL) and increase morbidity and mortality [5, 6]. Neurocognitive dysfunction, including symptoms such as memory impairment, impaired concentration, and difficulty in performing multiple tasks simultaneously, has been recognized as a common complication in cancer patients [7, 8]. Neurocognitive dysfunction can significantly impact the early and late post-HCT course, and it has emerged as a major cause for post-transplant morbidity and mortality .
In adult HCT survivors, an incidence of neurocognitive dysfunction of up to 60% has been documented at 22–82 months post-HCT [10,11,12]. Neurocognitive dysfunction is associated with risk factors such as pre-transplant chemotherapy, use of total body irradiation (TBI) in conditioning, immunosuppressive therapies, length of hospital stay, and graft-vs.-host disease (GVHD) [10, 12,13,14,15,16]. For children undergoing HCT, special considerations include the presence of non-malignant disorders that impact neurocognitive function even without transplant (e.g., sickle cell anemia) and prior intense chemotherapy or radiation for malignant diseases during developmentally vulnerable periods, leading to language and speech delays .
Current gaps exist in our characterization of neurocognitive dysfunction following HCT and include: (1) an operational definition, (2) neurocognitive issues in adults and children, (3) risk factors, (4) assessment, and (5) interventions. To address this, the Late Effects and Quality of Life Working Committee of the Center for International Blood and Marrow Transplant Research (CIBMTR) and the Complications and Quality of Life Working Party of the European Society for Blood and Marrow Transplantation (EBMT) provide an expert review to characterize the state-of-the-science of neurocognitive dysfunction following HCT, and to build upon this data with general recommendations for clinical practice and future areas of research.
Neurocognitive function domains
Neurocognitive function refers to the activities of the brain that generate the complex behaviors of day-to-day life. While a large number of brain structures may be involved in generating these behaviors, unique neurocognitive functions can be described most comprehensively by evaluating eight domains (Table 1) . Notably, neurocognitive evaluation in children may also include an assessment of academic achievement and global intelligence.
Neurocognitive dysfunction in HCT
Neurocognitive dysfunction describes a negative change in neurocognitive function that is independent of normal aging and may affect activities of daily living, including social interactions, complex behaviors, and occupational or academic functioning; this change may have a profound effect on quality of life . Neurocognitive dysfunction may be assessed in relation to a subject’s prior abilities, if known, or in relation to a normative population.
Characterization of neurocognitive dysfunction challenges
A variety of issues hamper the ability to characterize and understand neurocognitive dysfunction following HCT. First, it is unclear whether self-appraisals of neurocognitive dysfunction correlate with objective neurocognitive test results, and most studies do not include an analysis of the patients’ perspectives. In the few studies that have performed this analysis, correlations between the patient’s perspective and the test results varied [10, 19,20,21]. Second, the heterogeneity in study designs, testing methods, and cutoffs makes it challenging to identify the neurocognitive domains most affected by HCT. Furthermore, definitions of neurocognitive dysfunction vary between studies, and analysis and interpretation of longitudinal data can be hampered by the practice effect of repeating tests over time and the high attrition rate due to adverse medical outcomes [19, 22]. Neurocognitive testing also depends on the patient’s ability to communicate in English or the local language of the health-care providers, thereby excluding minorities that may be less proficient in these languages. Finally, cultural differences and contextual understanding of neurocognitive function may impact neurocognitive testing, bias results, and lessen the validity of findings .
Neurocognitive issues in adults
A recent survey performed in a heterogeneous group of more than 400 survivors and caregivers by a patient advocacy group (www.bmtinfonet.org) showed that finding information about neurocognitive dysfunction was the top concern for patients and second most important concern for caregivers (personal communication). Moreover, Bevans and colleagues  studied 171 adult survivors of allogeneic HCT and found that difficulty with concentration was one of the most prevalent physical symptoms reported by 3-year survivors. Historically, HCT has not often been an option for individuals over 55 years of age; however, with advances in treatment options such as reduced intensity regimens and supportive care measures, patients 65 years of age and older are now candidates for HCT. There is a scarcity of evidence regarding neurocognitive dysfunction and older HCT recipients. In the few studies that have reported findings in this population, results suggest regardless of age, HCT survivors have more neurocognitive dysfunction than healthy individuals . Further, age was not associated with outcomes such as graft-vs.-host disease, non-relapse mortality or overall survival .
Despite the demand for information about neurocognitive dysfunction, assessment is complicated because many patients have neurocognitive dysfunction prior to transplant (see Table 2). Indeed, when neurocognitive function was evaluated prior to HCT, up to 58% of adults had some level of neurocognitive dysfunction. In a multi-institutional study, Scherwath and colleagues  followed 102 adult allogeneic HCT recipients and found that prior to HCT 4–24% of the patients demonstrated scores consistent with neurocognitive dysfunction across various domains, including verbal fluency, fine motor function, and verbal memory .
In addition to this confounding factor, only a limited number of researchers have examined the course of neurocognitive dysfunction following HCT. Thus far, studies have revealed that among adults, neurocognitive function declines in the first few months following HCT in a subset of patients, and then partially recovers over time (Table 2). For example, in one study, Syrjala and colleagues  prospectively assessed neurocognitive function among 92 allogeneic HCT survivors at a single center. Their results showed that by the end of the first year following HCT the neurocognitive functioning of most survivors recovered to pre-transplant levels in the majority of domains, excluding grip strength and motor dexterity . Importantly, pre-transplant impairment on each test was identified in 15 to 32% patients .
In another study, Scherwath and colleagues  found that at 1-year post-HCT 41% of patients demonstrated neurocognitive dysfunction on at least one of the domains assessed compared with 47% of patients who experienced neurocognitive dysfunction at baseline. Also, 56% of survivors demonstrated decline at both Day + 100 and 1-year post-HCT and 17% of survivors developed cognitive decline starting at 1-year. Finally, in a recent systematic review conducted by Phillips and colleagues , researchers failed to identify a statistically significant change in neurocognitive function following HCT. Although this review included 11 studies and 404 patients, the authors highlighted important methodological limitations including heterogeneous samples, no control groups, small sample sizes, and a high prevalence of neurocognitive dysfunction prior to HCT . These studies also failed to differentiate neurocognitive dysfunction from “chemo brain” or “chemo fog,” which is experienced by patients undergoing treatment for cancer [28, 29].
In cases where neurocognitive functioning does not recover, evidence suggests that neurocognitive dysfunction may persist in the long-term and negatively affect the quality of life of survivors. Indeed, Syrjala and colleagues documented that 41.5% of survivors compared with 19.7% of controls continued to demonstrate at least mild neurocognitive dysfunction at 5 years post-HCT [16, 26]. Many patients with neurocognitive dysfunction have a poor self-image and are often unable to resume pre-transplant activities, such as attending work or school. In fact, nearly half of patients remain on disability or sickness benefits following HCT due to multiple factors, including neurocognitive dysfunction . Not surprisingly, higher incidences of anxiety, fatigue, depression, emotional distress, and poor physical and social functioning, have also been reported among HCT survivors with neurocognitive dysfunction [10, 21]. These side effects may lead to difficulty with medication management, including dosing errors and non-adherence, in the early period following HCT .
The aforementioned data support the notion that neurocognitive dysfunction is a prevalent complication following HCT in adults. Moreover, it is of upmost importance among adult HCT survivors. The demonstration of neurocognitive dysfunction prior to HCT among adults suggest that it may be a result of the disease itself as well as previous treatments.
Despite limited data, results also suggest that neurocognitive dysfunction may occur across the continuum of HCT survivor care and may also be associated with decrements in physical, emotional, and social health. Unfortunately, these decrements in well-being may also have important ramifications with respect to treatment compliance and subsequent increased risk for morbidity and mortality following HCT.
Neurocognitive issues in children
Neurocognitive dysfunction and associated decrements in intelligence quotient (IQ) have been noted in children when comparing pre and post-HCT scores [31,32,33]. For example, Shah and colleagues  found domain-specific alterations, including lower verbal and performance IQ scores at 5 years post-transplant; however, other researchers found no significant changes in these areas of neurocognitive function [34,35,36,37,38,39,40]. Although Simms and colleagues  found that parent ratings of their child’s academic ability were lower than those of a normative sample, other investigators [35, 37, 41] found academic achievement of children post-HCT to be within normal limits. Barrera and colleagues  noted deficits in academic achievement, and found that family (e.g., cohesion) and clinical factors (e.g., diagnosis) were predictors of neurocognitive function. Evidence suggests that other domains may also be impacted by neurocognitive dysfunction, including adaptive skills such as activities of daily living (e.g., dressing one’s self) diminished social competence, self-esteem, and emotional well-being in the first year following HCT [20, 22, 42].
Notably, studies have shown that younger age at diagnosis and treatment are associated with the most significant declines in neurocognitive function [33, 35, 36, 43]. Although IQ and academic achievement may remain within normal ranges for younger children post-HCT [34, 41], they may experience deficits in executive functioning skills, such as sustained attention, inhibition, response speed, and visual-motor integration skills . Research has indicated that younger autologous HCT recipients experience neurocognitive dysfunction, including impairment in visual memory and visual-motor skills . In addition, deficits in fine motor skills appear to be more pronounced in HCT recipients who received cranial irradiation at a younger age than those who received cranial irradiation at older ages [15, 31, 35].
To date, prospective longitudinal data in this area of research are limited. Longitudinal evaluation of neurocognitive functioning is important because it may elucidate differences over time as well as among specific domains. For example, Shah and colleagues  found that some patients develop domain-specific declines that eventually improve (e.g., visual-motor skills), whereas other patients develop domain-specific declines that are progressive and chronic (e.g., verbal skills). Significantly, patients in this study were unable to acquire new skills at a rate comparable to age-matched healthy peers, although this may have been due to changes in the sample across time as well as the unreliability of small sample sizes. The necessity for longitudinal evaluation in children is also evident when focusing on academic achievement. As an example, lower academic achievement has been noted, particularly as time since transplant increases [39, 45].
To date, literature reporting neurocognitive function of children post-HCT is inconclusive, conflicting, and often focused on specific domains such as IQ and academic functioning. Notably, studies of neurocognitive dysfunction have suggested that age at the time of diagnosis and HCT is a potentially important moderating variable such that younger age may be deleterious. Despite a need for additional longitudinal data, results also suggest that neurocognitive dysfunction may occur across the continuum of HCT survivor care for children as well.
Reported risk factors associated with neurocognitive impairment after HCT are presented below.
Transplant conditioning includes the administration of chemotherapeutic agents, TBI or both prior to stem cell infusion. Chemotherapeutic agents that cross the blood–brain barrier and TBI have a direct cytotoxic effect upon the brain. Table 3 displays the most common agents used in transplant conditioning regimens and their side effects. A TBI dose of 12 Gy is the mainstay treatment of myeloablative conditioning regimens for acute lymphoblastic leukemia [46, 47] and the neurotoxic effects of this treatment have been studied in adults and children. Neuro-toxic effects with the use of reduced intensity conditioning regimens, have been documented . For example, fludarabine, a common component of reduced intensity conditioning regimens, may be associated with neuro-toxic effects in both adults and children. It may be important, therefore, to tailor individual conditioning regimens balancing potential neurotoxic effects of the administered agents in the context of desired overall and disease-free survival.
While researchers have demonstrated that TBI and chemotherapy are neurotoxic, the specific effects of TBI and chemotherapy on the patients’ neurocognitive functioning in the peri-transplant period are unknown. Different techniques of administering TBI between centers make data analyses complex, and as a result, conclusions are elusive. For example, Harder and colleagues  found mild to moderate late neurocognitive dysfunction in 60% of the patients who had received high-dose chemotherapy with TBI up to 12 Gy compared to healthy population norms. Others report no systematic effects of conditioning intensity on neurocognitive function; [14, 48] and a recent meta-analysis found no significant associations between TBI and neurocognitive dysfunction .
The potential adverse effect of myeloablative doses of TBI on neurocognitive function has been reported in young children with leukemia [14, 16, 49]. Addition of cranial or cranio-spinal irradiation, which may be added to TBI, may further impact neurocognitive function . Other data in children reveal that the effects of TBI and cranial irradiation on neurocognitive function are relatively modest and variable [34,35,36,37,38,39]. Notteghem and colleagues  evaluated 76 children with extracranial solid tumors following autologous HCT using chemotherapy-only conditioning. They found that the percentage of children falling into the below average range for IQ was greater than that of children in the general population and over a third of participants had severe reading or writing difficulties. Research has also shown executive function and visual-spatial skills to be below age level in children who received busulfan .
GVHD and immunosuppressive therapies
Allogeneic HCT recipients who develop GVHD may need immunosuppressive therapy for extended periods of time. These include calcineurin inhibitors such as cyclosporine and tacrolimus, which are known to have neurotoxic effects including tremor, posterior reversible encephalopathy syndrome (PRES) and thrombotic microangiopathy (TMA). Studies have shown that subgroups of children who received unrelated allogeneic HCT and developed GVHD demonstrated increased risk of neurocognitive dysfunction [32, 37]. Despite potential association between GVHD and neurocognitive dysfunction, at present we are limited to conjecture regarding the possible effects.
Immune defects post-HCT as well as immunosuppressive therapy used during allogeneic HCT increases the risk for viral infections, including cytomegalovirus (CMV), Epstein-Barr virus (EBV), and human herpesvirus 6 (HHV6). These infections may specifically affect non-verbal memory functions, attention and speed of cognitive performance [50,51,52,53,54,55]. Mild neurocognitive dysfunction associated with viral infections may not be identified by clinical or cognitive screening [50,51,52,53, 56, 57].
Unlike patients with hematological malignancies, patients with non-malignant disease may have neurocognitive dysfunction that is often related to their primary disease. For example, patients with adrenoleukodystrophy have disease-specific neurological dysfunction prior to HCT. These patients may have lesions in their central nervous system (CNS) that can affect both their physiological and psychological functioning. Similarly, patients with sickle cell disease often experience cerebral ischemic events prior to HCT that can affect their overall neurocognitive functioning. Finally, patients with severe combined immunodeficiency due to adenosine deaminase deficiency may have neurocognitive dysfunction prior to HCT that is a result of their disease [54, 55].
Other risk factors
Risk factors for neurocognitive dysfunction following HCT include female gender, younger age, higher body mass index (BMI), absence of social partner, allogeneic HCT, extensive chronic GVHD, higher intensity pre-HCT cancer treatment, and use of narcotics, corticosteroids, tricyclic antidepressants and sedatives [14, 58, 59]. In some studies, pre-HCT functioning [41, 44] and socioeconomic status are strong predictors of neurocognitive function following HCT . However, other researchers have failed to find similar associations . Behavioral problems such as sleep deprivation, fatigue, and depression may adversely affect neurocognitive function [60, 61]. Finally, researchers have noted a negative relationship between pre-HCT anxiety and post-HCT neurocognitive function . Collectively, the evidence indicates there are many factors that could impact neurocognitive dysfunction and need to be examined for possible interventions targeting modifiable factors.
Both subjective and objective measures have been used to assess neurocognitive function in HCT. However, no standard recommendations exist for the timing or types of measures to assess neurocognitive function in either adults or children. Table 4A, B summarize tests for specific neurocognitive domains, applicable age ranges, average administration times, and general descriptions for each assessment tool. These tests are common in the published literature and address the domains that are most affected by neurocognitive dysfunction. All commonly used neurocognitive tests are standardized measures that are psychometrically validated and widely available in multiple languages [62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79].
Researchers and clinicians currently use the following instruments to assess the neurocognitive function of adults before and after HCT: the Mini Mental State Examination (MMSE), the Cognitive Abilities Screening Instrument (CASI), the Cognitive Assessment Screening Test (CAST), the Cambridge Neuropsychological Test Automated Battery (CANTAB), and the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS) . However, the use of these screening tools is controversial. The National Comprehensive Cancer Network (NCCN) does not recommend these screening tools for use in cancer patients, including HCT patients , likely because these screening tools were developed for patients with dementia and may not be sensitive enough to address the subtle neurocognitive dysfunction found in HCT patients. Given the drawbacks of these assessments, it may be more applicable for researchers and clinicians to assess patients based on identified risk factors; thus, future research should focus on the development of a standardized risk factor profile for patients who may be at risk of poor neurocognitive functioning post-HCT.
Researchers and clinicians may consider assessing neurocognitive function of children prior to HCT, 1 year following HCT, and then at the beginning of each new stage of education. It should be noted that some children can be challenging to assess because they may not be old enough to perform specific assessments. As a result, deficits in neurocognitive function may only appear in the long-term along with increasing age and tasks that require higher executive functioning. In addition, to date, researchers have not developed assessment tools that can reliably predict future neurocognitive deficits in more complex domains (e.g., math, reading and executive function) in children. Clinicians should consider the impact of other factors, such as protective isolation, missed schooling, and socialization with peers, when assessing the neurocognitive function of children post-HCT. These factors are difficult to measure, but they may have a significant impact on the neurocognitive function and development of children over time.
Self-report measures and interview
Because the sole use of objective measures does not provide clinicians with a complete picture of the patient’s level of daily functioning, it is important to include self-report measures as well as a clinical interview in the assessment process. Self-report measures capture the patients’, parents’, or teachers’ assessment of neurocognitive function and serve as an additional tool to screen for neurocognitive dysfunction. Similarly, the clinical interview collects information, including previous education, occupation, medical and psychiatric history, and cognitive history [48, 81] in order to guide intervention for patients with neurocognitive dysfunction .
One self-report measure, The Childhood Cancer Survivor Study-Neurocognitive Questionnaire (CCSS-NCQ), addresses specific self-reported concerns about neurocognitive function in long-term survivors of childhood cancer, and it can be used with patients’ post-HCT. The CCSS-NCQ, which was developed in conjunction with the Behavior Rating Inventory of Executive Function—Adult Version (BRIEF-A), uses similar items and includes novel items specific to outcomes in survivors of childhood cancer . Versions for younger children are also available—the Brief-Pre (for pre-school children), the Brief-P (for school-age children), and the Brief-SR (for older children). In order to ensure the most accurate findings, a qualified neuropsychologist, who is aware of the relationship between mental health and subsequent neurocognitive assessment, should administer the assessment tools, interpret the results, and provide a report to clinicians [84,85,86,87,88,89,90,91,92].
In addition to the use of subjective and objective measures, neurologic specific biomarkers of central nervous system injury, neuroinflammation, and neuroimaging, should be examined as potential tools to evaluate neurocognitive dysfunction following HCT. Biomarker discovery is a promising area of inquiry that may facilitate a deeper understanding of the impact of HCT on the central nervous system. From a clinical perspective, biomarkers may help define risk and identify protective factors for neurocognitive dysfunction as well as help monitor patient response to treatment. Biomarkers may also help elucidate the potential relationship between distressing symptoms, such as sleep deprivation, anxiety/depression, and infection, and neurocognitive dysfunction, leading to better care and quality of life for patients after HCT.
Biomarkers of CNS injury and neuroinflammation
Biomarkers of neurologic injury have been historically studied in stroke patients and patients with brain metastasis [93,94,95,96,97]. Previous studies have identified associated biomarkers of neurocognitive function such as O6-methylguanine–DNA methyltransferase , neuron-specific enolase (NSE) , S100B  and neurotransmitters such as glutamate and gamma-amniobutyric acid (GABA). However, to date, these biomarkers have not been studied in patients with CNS damage caused by chemotherapy or radiation [101, 102]. Chemotherapy and radiation utilized in HCT conditioning may result in the stimulation of inflammatory pathways and associated elaboration of various cytokines, adhesion molecules and chemokines from leukocytes, fibroblasts, and endothelial cells. Pre-clinical models have shown that chemotherapy and radiation regulates expression of tumor necrosis factor-alpha, intracellular adhesion molecule-1, and interleukin (IL)-1 . These inflammatory markers have been detected in the blood of patients who received radiation . Similarly, serum levels of inflammatory cytokines have been measured in stroke patients [105, 106] and correlated with neurocognitive dysfunction among newly diagnosed breast cancer patients . Markers of oxidative stress have been associated with neurocognitive dysfunction among childhood leukemia patients, but similar studies have not been conducted among HCT recipients . Among HCT survivors, Bhatia and colleagues have characterized various single nucleotide polymorphisms in combination with neurocognitive assessment tools . The results of these studies underscore the need for additional longitudinal studies in HCT patients evaluating select blood-based biomarkers in combination with imaging modalities and neuropsychological assessment tools.
Magnetic resonance (MR)-based imaging and positron emission tomography techniques, including structural and functional MR imaging, diffusion tensor imaging, and MR spectroscopy, may play an important role as biomarkers for neurocognitive dysfunction following HCT. In multiple previous studies, researchers have used these techniques to detect neurocognitive dysfunction following the diagnosis and treatment of cancer. For example, Cao and colleagues  evaluated dynamic contrast-enhanced MR imaging as a biomarker to predict radiation-induced neurocognitive dysfunction. MR changes including reduced neuroanatomic volumes have also been associated with neurocognitive dysfunction among survivors of childhood leukemia; however, similar studies have not been conducted among HCT survivors .
Building on this work among HCT recipients, Correa and colleagues  utilized neuroimaging techniques and neuropsychological testing to study 28 adult HCT recipients conditioned with TBI and high-dose chemotherapy or high-dose chemotherapy alone. They noted gray matter loss and a concomitant increase in ventricular volume in patients 1-year following HCT, and no corresponding changes in healthy participants in the control group. Despite the noted changes in neuroimaging, statistically significant differences in rates of neurocognitive dysfunction were not found.
Physical and psychological symptoms associated with cancer and cancer treatment may also be associated with neurocognitive dysfunction. In this area of research, most studies have focused on fatigue and depressive symptoms [10, 19, 29, 30]. For example, one longitudinal study examined cancer-related symptoms associated with neurocognitive dysfunction and found significant relationships over time among several domains of neurocognitive function and symptoms such as fatigue, depression, and perceived stress . Another study examined patients with multiple myeloma who completed autologous HCT and found similar associations between neurocognitive function and symptoms (e.g., depression) .
In 2002, Harder and colleagues  focused on neurocognitive dysfunction of patients receiving HCT within the past 22–82 months and found that neurocognitive dysfunction was present in 60% of participants and that fatigue was a strong predictor of neurocognitive dysfunction; however, a correlation with depression was not reported in this study . Similarly, Booth-Jones and colleagues noted significant relationships between fatigue and depression and neurocognitive dysfunction in a cohort of patients at least six months following HCT . However, it should be noted that two studies found no significant relationship between fatigue or depression and neurocognitive dysfunction [19, 29], and that two other studies found anxiety to be significantly associated with neurocognitive dysfunction [30, 113].
Awareness of neurocognitive dysfunction in HCT recipients is important for timely introduction of psychosocial support and other interventions, but there is a significant void in high-quality data to assess interventions in this area. Several approaches aimed at prevention or reduction of neurocognitive dysfunction have been studied in patients receiving systemic chemotherapy and/or radiation therapy, but to date, no prospective studies have been conducted and relevant interventions still need to be evaluated in HCT patients. Four potential strategies to mitigate the risks or improve outcomes of neurocognitive dysfunction after HCT are listed below and in Table 5.
Strategy 1: Interventions to minimize therapy related neurocognitive toxicity
In order to reduce neurocognitive dysfunction, clinicians may consider reducing the use of neurotoxic therapies such as prophylactic cranial radiation, TBI, or neurotoxic agents [114, 115] or the substitution of busulfan for TBI-based conditioning during treatment . Similarly, in cases where the patient does not need radiation to control disease (e.g., non-malignant diseases), clinicians may choose to reduce or eliminate neurotoxic agents given concerns for long-term sequelae.
Strategy 2: Management of acute CNS toxicities after allogeneic HCT
TBI has been associated with CNS complications within the first 100 days in adults and those patients with known seizure history may experience increased seizures . PRES occurring in the first 100 days after allogeneic HCT is associated with neurocognitive dysfunction  and requires careful management strategies . Identification of PRES and tight control of hypertension as well as a careful search for and removal of the etiologic agent remains a mainstay of management. For example, sirolimus, cyclosporine or tacrolimus have been associated with PRES and may be withdrawn if they are felt to be contributing to the development of PRES . TMA and genetic susceptibility to TMA  can also be associated with neurocognitive dysfunction and also require prompt identification and management .
Strategy 3: Non-pharmacologic interventions
For adults, re-education or job training may be beneficial. For children, approaches include cognitive remediation strategies and educational interventions [121, 122]. Establishment of school re-entry programs that involve teachers early, tutoring in the immediate period following HCT, enlisting the school system to provide an individualized educational plan, and accommodations based upon a patient’s individual deficits should be considered [45, 122]. Poor recruitment and adherence to these educational programs remains a challenge and requires improvement in accessibility and convenience for children and their families .
Cognitive rehabilitation for childhood cancer survivors in the form of intensive therapist-delivered training such as the cognitive remediation program has shown encouraging initial results . The application of computer-based techniques to support optimal neurocognitive function may also be considered in children and adults. The systematic use of computer-based cognitive training is associated with significant improvements in working memory attention problems and processing speed in childhood cancer survivors with attention and working memory deficits [124, 125].
Integrative therapies may also be useful to improve neurocognitive function (e.g., strategies to improve diet, exercise and stress management) following HCT. For example, nutraceuticals such as vitamin therapy and other supplements may improve neurocognitive function and need to be examined before any conclusions can be made regarding their efficacy in HCT patients. Campbell and colleagues  found aerobic exercise improved neurocognitive function in cancer patients. Current investigation is ongoing to examine the potential benefit of exercise on neurocognitive dysfunction (NCT02533947) in adults. Lastly, health behaviors such as abstinence from tobacco use, and consuming alcohol in moderation, may support healthy neurocognitive functioning following HCT.
Strategy 4: Pharmacologic interventions
These approaches include therapies with a variety of pharmacologic agents such as stimulants; however, data in HCT recipients is lacking. Therapy with methylphenidate is associated with short- and long-term improvements in attention, concentration, executive function, and memory in childhood cancer survivors with neurocognitive dysfunction [122, 124, 127]. However, rebound symptoms (psychosis, depression and attention problems) may arise with long-term use . With perceived effects in social skills and behavior, further study focusing on the impact of methylphenidate on academic functioning is warranted.
The acetylcholinesterase inhibitor, donepezil, was studied in adult patients with primary brain tumors and showed improved attention, concentration, language function, verbal and figure memory, and mood . Breast cancer patients taking modafinil have shown improvement in memory and attention . Administration of recombinant human growth hormone may be associated with improved cognition; sustained attention and cognitive-perceptual performance in young adult survivors of childhood cancer .
Recommendations for research and clinical practice
Several significant gaps in our knowledge support our proposed recommendations for future research and the general recommendation for clinical practice shown in Table 6. Current practice recommendations are difficult to suggest due to the lack of adequately powered randomized controlled trials; however, the literature suggests a burden of neurocognitive dysfunction in HCT recipients and their caregivers. There is no evidence supporting standard drug or other intervention prophylaxis in all or even in currently definable subgroups of patients. There is also limited data to justify choice of conditioning based on predicted neurocognitive effects, and therefore conditioning treatments should be guided by primary disease. However, clinicians need to balance the need for high intensity conditioning regimens and disease control with short- and long-term sequelae of these therapies.
Clinicians may inform and counsel their patients of the signs of neurocognitive dysfunction prior to HCT, such as difficulty concentrating or remembering important dates, and conduct appropriate assessments at each follow-up visit to enable early intervention. Supportive treatment may be considered based on dominating symptoms. Moreover, referral for a neuropsychiatric consult may be also considered. Awareness of the risk factors and likelihood of neurocognitive dysfunction after HCT is important for counseling patients pre-transplant but also to help earlier identification of emerging toxicities to guide referrals to appropriate specialist and help management.
This review examined extant literature in key areas to characterize the state-of-the-science regarding neurocognitive dysfunction in patients who have completed HCT. Several significant gaps in our knowledge support our proposed recommendations for future research and the general suggestions for clinical practice. Future studies focusing on specific populations including various pediatric populations and older adult population are needed to delineate neurocognitive dysfunction following HCT as well as define potential risk and protective factors for patients who suffer from the condition and represent unmet needs. In addition, researchers should focus on the development and validation of a sensitive screening tool for neurocognitive dysfunction that can be used by clinicians who treat patients after HCT. Moreover, the combination of a wider application of neurocognitive assessments with newly developed biomarkers may prove to be a powerful combination of tools utilized to define at-risk HCT recipients. These data can then be utilized to develop and evaluate precision interventions focused on prevention and amelioration of neurocognitive dysfunction. With properly designed studies, appropriate interventions and practice guidelines can be developed. Emerging knowledge on evaluation and intervention may lead to better neurocognitive outcomes.
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Conflict of interest
The authors declare that they have no conflict of interest.
David Buchbinder and Debra Lynch Kelly contributed equally to this work.
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