Diseases are a common origin for brain damage, not injury-related but rather result of a prolonged condition, developing in a mid/long-term basis. They are presented in a separate group to differentiate them from incidental, acute insult (previous section). They include pathologies like epilepsy and dementia, neurodegenerative illnesses and psychiatric/motor disorders.
3.2.2. Neurodegenerative diseases
Neurodegenerative diseases are becoming increasingly common worldwide, especially in developed countries where life expectancy is higher and thus dementia is more likely to appear [
167]. They entail a progressive, inevitable deterioration and ultimate death of cells within the nervous system (both central and peripheral, though mostly in the brain [
168]) causing neurological dysfunction, ranging from memory loss - e.g. Alzheimer’s - to motor impairment - sclerosis, Parkinson’s -. They can be classified according to etiology into amyloidoses (Alzheimer’s, Creutzfeldt-Jakob’s), tauopathies (Pick’s disease, CTE), alfa-synucleinopathies (Lewy bodies, multiple system atrophy) and TDP-43 proteinopathies (some types of sclerosis) [
169].
Although very diverse in nature, they all share some common features. Firstly, instead of a static neuronal loss - typical for metabolic/toxic NTBI -, necrosis progressively affects certain cells due to their vulnerability and it spreads according to the brain’s structural and functional connectivity [
169] (neural pathways). Secondly, they are related to a chronic immune malfunction. Lastly, they generally cause inflammation. [
170]
Dementia-type Dementia is the gradual loss of mental skills such as memory, thinking, learning and judgement. It affects around 50 million people globally - mainly over 65 years [
171] with symptoms such as behavioural changes and/or deficits in communication, orientation and memory. While cell deterioration is part of the normal ageing process, dementia implies There have no definitive cure - the only attainable goal being the slowing of the decay - and ultimately make the patient dependent on others. The main risk factors are lower education, hypertension, hearing impairment, smoking, obesity, depression, sedentary lifestyle, diabetes, isolation, TBI, alcohol and air pollution [
172]. Most common illnesses causing dementia are Alzheimer’s and Lewy’s bodies, although its origins can also be vascular (strokes), STDs or trauma (CTE). Dementia is not to be mistaken for delirium, which is sudden, transitory and mainly distorts attention mechanisms.
Alzheimer’s Disease is the most prominent dementia-inducing disease (half of all cases [
173]), affecting around 5% of the European population [
174] - the most deeply affected continent due to its advanced life expectancy. A-
plaques and neurofibrillary tangles (NFTs) are generated by accumulation of amyloid
-peptides and hyperphosphorilated
-proteins (and/or demyelinization [
175,
176]), causing inflammation and degeneration of the brain tissue and so excessive, non-programmed cell death (necrosis) and volume changes (gyri shrink, sulci grow - overall volume loss up to 50%) [
80]. It can also be the result of mutation (genes APP, PS1, PS2), though very rarely (less than 1 in 20 cases).
AD deteriorates memory, thinking processes and alters emotions by provoking diffuse neuronal loss, synaptic degeneration (correlated with NFT distribution) and reactive gliosis (abnormal glial and astrocyte growth) [
80] altering the complex cellular micro-environment in the brain [
177]. It starts as a localized neural loss (locus ceruleus, entorhinal cortex) but eventually becomes diffuse (amygdala, hippocampus, frontal/parietal cortices). As neurons die all over the brain, paths between them get longer while local clustering remains in the unaffected areas, yielding an efficiency loss as "small-world" characteristics fade away [
178] - selective hub vulnerability [
179] - although that depends on the characterization of such graphs (sample size, brain area, measurements) [
180].
The disease’s progress can be tracked by synaptic activity, namely the post-synaptic density protein PSD-95 [
181,
182,
183] and other biomarkers [
184]. Enhancing neuroplasticity [
66] and some immunotherapeutic drugs [
184] or boosting the endogenous cannabioid system (ECBS) in the brain [
185,
186] can help slow down the advancement of AD, especially in its prodromal (i.e. early) stages. A precocious diagnosis [
184] and realistic computational approaches [
179] can prove important in treatment.
Dementia with Lewy Bodies (DLB) is the result of the accumulation of the alpha-synuclein protein in the brain, creating deposits (Lewy bodies) which disturb the brain’s chemical balance. Although not as common as AD, it represents a sizeable share of cases (around 5-7,5% of all dementias [
187]). It is one of two diseases caused by Lewy Bodies, the other being Parkinson’s (PD), discussed in the next section.
Movement disorders Under a relatively new and phenomenological medical category [
188], this section encompasses the most common neurodegenerative diseases affecting motor abilities, i.e. walking, reflexes, standing, etc. as a result of a disruption or dysfunction of coordination between the CNS (brain, spinal cord) and muscles. According to the movement’s disruptive expression, they can be further subdivided into hyperkinesias (excessive), dyskinesias (unnatural) - jerky or non-jerky -, hypokinesias (decreased reach), bradykinesias (slowness), akinesias (absence) and abnormal involuntary movements [
188]. Although this subsection focuses on prolonged disorders prominent in advanced ages (+65 years), transitory movement disorders - such as tremors, dystonia or tics - are not uncommon in younger patients and can result in early Parkinson’s misdiagnosis [
189].
Parkinson’s Disease (PD) produces hypokinesia and bradykinesia - among other non-motor symptoms - stemming from Lewy’s bodies (
-synuclein deposits), presenting high comorbidity with LBD and so greatly hampering an accurate (and timely) diagnosis [
187] - which already requires very high-fidelity MRI [
190]. This deposits provoke localized necrosis in the ventrolateral substantia nigra - namely targeting dopaminergic neurons - and so dopamine underflow to the striatum within the brainstem [
191]. About 10-15% of all cases are genetic, and they can originate from prion diseases and perhaps metabolic iron accumulation [
192] as well.
PD results in axonopathy, demyelination [
193] (particularly originating in the basal ganglia [
194]) and synaptic dysfunction and reduced white matter structural connectivity [
195] - especially along certain pathways: nigro-pallidal, frontoparietal-stratal, etc. Some studies suggest this connectivity decrease and subsequent cortico-striatal topological reorganization is present in prodromal symptoms of PD, like rapid eye movement sleep behaviour disorder (iRBD) [
195]. It also disrupts functional connectivity, although reliability and reproducibility between patients undergoing different stages, treatments and severities of PD and time evolution is problematic when comparing to healthy individuals [
196], although some praiseworthy attempts exist [
197]. Treatment usually involves L-DOPA (levodopa or l-3,4-dihydroxyphenylalanine) which crosses the BBB to increase dopamine concentrations in the brain.
Lastly, one must not mistake PD for ataxia, which is an acute lack of coordination of different muscles affecting gait, speech and eye movement among others as a result of nervous damage, mainly cerebellar and reversible in some cases [
198].
Huntington’s Disease (HD) is a rare, genetic (autosomal dominant) disorder affecting the CNS and showing symptoms like chorea (involuntary, fast and abrupt movements of facial, trunk and/or limb muscles), behavioural and psychiatric degeneration - including dementia -. It can manifest itself at any point in the patient’s life, with no noticeable clinical indices until then [
199], although most diagnoses happen between 30 and 50 years of age, inevitably leaving to full dependency and death (most commonly by pneumonia or suicide) [
200]. It is caused by a cytosine-adenine-guanine (CAG) trinucleotide repeat (more than 36 times) in the huntingtin (HTT) gene on chromosome 4p and frequently treated with dopamine receptor blockers [
200].
In terms of structural connectomics, HD greatly decreases nodal betweenness centrality (i.e., reduced relative importance of certain nodes within the network) and the clustering coefficient - meaning an impaired capacity for inter-nodal information processing [
201] that can spread throughout the brain as the mutant protein propagates, shaping propagation patterns - with origin in the striatum and explaining white and grey matter deterioration - predictable via graph theory [
202]. HD disrupts as well functional connectivity in subcortical and default mode networks. In some brain areas (within putamen, insula-putamen, visual networks), functional connections are further impeded as the CAG repeat length increases; whereas the contrary is true for others (calcarine to middle frontal gyri) [
203].
Prion diseases, also known as transmissible spongiform encelopathies (TSE) are rare neurodegenerative, deadly illnesses caused by misfolded proteins (prions)
mutating irreversibly into
and causing neuronal necrosis, vacuolation and abnormal activation of microglia and astrocytes [
204]. This can take place in humans and other animals (e.g.: sheep, deer, cows), in all organs but mainly the brain and the CNS. Their incubation process is long (up to decades), during which those proteins accumulate and create microscopic holes in the brain, transforming its tissue into a sponge (hence the name).
Prion diseases (PrD) can be divided into sporadic (spontaneous and unpredictable: Creutzfeld-Jakob disease (CJD), fatal insomnia, variable protease sensitive prionopathy), familial (genetically transmitted: familial CJD and fatal insomnia, Gerstmann – Sträussler – Scheinker disease) and acquired (through introduction of contaminated tissue into the patient - ingestion, transfusion, etc.: kuru, variant CJD). CJD is the most common prion disease, affecting 1 in a million yearly (85% sporadic, 10-15% familial [
205]), manifesting itself in young adulthood if acquired (vCJD) and senescence if sporadic (sCJD) [
206]. Although uncommon and heterogeneous in etiology and diagnosis, the study of prion diseases can potentially shed light on the role of protein misfolding in more widespread neurodegenerative diseases such as Parkinson’s and Alzheimer’s [
207] and its transmission along connected structural pathways, which can be modeled as graph diffusion [
208].
Multiple sclerosis (MS) can be also considered a neurodegenerative disease [
209,
210] in its latest stages, after initial autoimmune inflammation in the CNS [
170,
211,
212]. "Sclerosis" means "abnormal hardening of body tissue". Its ultimate etiology is unclear, but it involves multiple genes increasing susceptibility along some environmental (ultraviolet B exposure), disease (Epstein-Barr virus) [
212] and genetic factors (highest incidence among European and North American populations [
210,
211]). It is usually diagnosed in early adulthood via MRI (several white matter scars/plaques over time, chronic CNS inflammation) and it can be intermittent (relapsing-remitting, 85% of cases) or chronic, with chances of drug-induced remission (secondary progressive MS) or not (primary progressive or progressive-relapsing MS) [
211].
MS affects mainly optic nerves, the brainstem and the spinal cord [
212]. comprise demyelination and neuronal loss through axon deterioration [
211] so, although remission can happen within hours or days, it is never complete because the neuronal reserve is progressively depleted - hence the neurodegenerative nature of the illness, despite partial remyelination [
212]. Primary progressive MS entails ataxia and progressive cognitive and visual failure [
212]. Lesions appear as the illness advances, affecting the connectome’s structure (inflammation, atrophy) and thus also its functioning: abnormal activation of frontal regions or hippocampus for memory tasks and upsetting the default mode network (DMN) in resting states - although its direct links to cognitive impairment have proven difficult to clarify [
213,
214].
Functional connectivity remains a benchmark for studies about MS nonetheless, with special attention to network efficiency indicators [
215,
216] - e.g. on working memory, but subjected to patient heterogeneity [
217] - with an apparent common ground altogether: altered connectivity in deep-gray matter areas, lower brain modularity, hemispheric skewness and task-independency [
218]. Treatment strategies have rapidly improved in a paliative sense but keep failing to put a remedy to continuous neurodegeneration [
219].
Amyotrophic lateral sclerosis (ALS) - also known as Lou Gehrig’s disease - is a rare (1 in 100000 [
220]) neurodegenerative disease targeting mainly motor neurons in the brain (upper, in the frontal lobe) and the spinal cord (lower) and the brainstem. As such, it is incurable and, on average, patients perish 2 to 5 years after diagnosis (usually after 60 years [
220]) due to the malfunction of the diaphragm (breath) and/or swallowing muscles (nutrition) - with notable exceptions of longevity such as Sir Stephen Hawking. Although it starts focally, ALS often spreads to other body parts. It etiology can be autosomal inheritance (10 %) by a hexanucleotide repeat expansion of gene C9orf72 (between 1/3 and 1/2 of familial cases, although there are at least other 25 genes involved [
221]) or sporadic (unclear). Half of all patients experience extra-motor conditions like behavioural changes, language impairment or executive dysfunction, and 1 in 10 show signs of frontotemporal dementia [
222].
It is proven that brain topology worsens quantitatively (leaf fraction, degree divergence and correlation) in terms of efficiency as ALS unfolds, disentangling the multiscale, assortative networks - bonding between similar nodes - typical for a healthy connectome [
223] - which translates into a functional impairment accordingly, predictably with more intensity within the motor cortex [
224]. Overall, patients show a decreased functional connectivity in the cortex (right orbitofrontal, left interior frontal) and the corpus callosum [
225] as well as an enhancement in the right angular, parietal cortex and the frontoparietal networks [
226]. All this changes could be interpreted as compensatory mechanisms - surrogates for lost or damaged brain regions.