football and american football are possibly associated with als, and that there was not enough evidence to say whether or not physically demanding occupations are associated with als. a 2016 review found the evidence inconclusive and noted that differences in study design make it difficult to compare studies, as they do not use the same measures of physical activity or the same diagnostic criteria for als. however, research published in 2021 suggested that there was a positive causal relationship between als and intense physical exercise in those with a risk genotype. sports both football and american football have been identified as risk factors for als in several studies, although this association is based on small numbers of als cases. a 2012 retrospective cohort study of 3,439 former nfl players found that their risk of dying from neurodegenerative causes was three times higher than the general us population, and their risk of dying from als or alzheimer's disease was four times higher. however, this increased risk was calculated on the basis of two deaths from alzheimer's disease and six deaths from als out of 334 deaths total in this cohort, meaning that this study does not definitively prove that playing american football is a risk factor for als. some nfl players thought to have died from als may have actually had chronic traumatic encephalopathy (cte), a neurodegenerative disorder associated with multiple head injuries that can present with symptoms that are very similar to als. football was identified as a possible risk factor for als in a retrospective cohort study of 24,000 italian footballers who played between 1960 and 1996. there were 375 deaths in this group, including eight from als. based on this information and the incidence of als, it was calculated that the football players were 11 times more likely to die from als than the general italian population. however, this calculation has been criticized for relying on an inappropriately low number of expected cases of als in the cohort. when the lifetime risk of developing als was used to predict the number of expected cases, football players were no more likely to die of als than the general population. smoking smoking is possibly associated with als. a 2009 review concluded that smoking was an established risk factor for als. a 2010 systematic review and meta-analysis concluded that there was not a strong association between smoking and als, but that smoking might be associated with a higher risk of als in women. a 2011 meta-analysis concluded that smoking increases the risk of als versus never smoking. among smokers, the younger they started smoking, the more likely they were to get als; however, neither the number of years smoked nor the number of cigarettes smoked per day affected their risk of developing als. pathophysiology neuropathology the defining feature of als is the death of both upper motor neurons (located in the motor cortex of the brain) and lower motor neurons (located in the brainstem and spinal cord). in als with frontotemporal dementia, neurons throughout the frontal and temporal lobes of the brain die as well. the pathological hallmark of als is the presence of inclusion bodies (abnormal aggregations of protein) known as bunina bodies in the cytoplasm of motor neurons. in about 97% of people with als, the main component of the inclusion bodies is tdp-43 protein; however, in those with sod1 or fus mutations, the main component of the inclusion bodies is sod1 protein or fus protein, respectively. the gross pathology of als, which are features of the disease that can be seen with the naked eye, including skeletal muscle atrophy, motor cortex atrophy, sclerosis of the corticospinal and corticobulbar tracts, thinning of the hypoglossal nerves (which control the tongue), and thinning of the anterior roots of the spinal cord. aside from the death of motor neurons, two other characteristics common to most als variants are focal initial pathology, meaning that symptoms start in a single spinal cord region, and progressive continuous spread, meaning that symptoms spread to additional regions over time. prion-like propagation of misfolded proteins from cell to cell may explain why als starts in one area and spreads to others. the glymphatic system may also be involved in the pathogenesis of als. biochemistry this figure shows ten proposed disease mechanisms for als and the genes associated with them. it is still not fully understood why neurons die in als, but this neurodegeneration is thought to involve many different cellular and molecular processes. the genes known to be involved in als can be grouped into three general categories based on their normal function: protein degradation, the cytoskeleton, and rna processing. mutant sod1 protein forms intracellular aggregations that inhibit protein degradation. cytoplasmic aggregations of wild-type (normal) sod1 protein are common in sporadic als. it is thought that misfolded mutant sod1 can cause misfolding and aggregation of wild-type sod1 in neighboring neurons in a prion-like manner. other protein degradation genes that can cause als when mutated include vcp, optn, tbk1, and sqstm1. three genes implicated in als that are important for maintaining the cytoskeleton and for axonal transport include dctn1, pfn1, and tuba4a. there are a number of als genes that encode for rna-binding proteins. the first to be discovered was tdp-43 protein, a nuclear protein that aggregates in the cytoplasm of motor neurons in almost all cases of als; however, mutations in tardbp, the gene that codes for tdp-43, are a rare cause of als. fus codes for fus, another rna-binding protein with a similar function to tdp-43, which can cause als when mutated. it is thought that mutations in tardbp and fus increase the binding affinity of the low-complexity domain, causing their respective proteins to aggregate in the cytoplasm. once these mutant rna-binding proteins are misfolded and aggregated, they may be able to misfold normal proteins both within and between cells in a prion-like manner. this also leads to decreased levels of rna-binding protein in the nucleus, which may mean that their target rna transcripts do not undergo normal processing. other rna metabolism genes associated with als include ang, setx, and matr3. c9orf72 is the most commonly mutated gene in als and causes motor neuron death through a number of mechanisms. the pathogenic mutation is a hexanucleotide repeat expansion (a series of six nucleotides repeated over and over); people with up to 30 repeats are considered normal, while people with hundreds or thousands of repeats can have familial als, frontotemporal dementia, or sometimes sporadic als. the three mechanisms of disease associated with these c9orf72 repeats are deposition of rna transcripts in the nucleus, translation of the rna into toxic dipeptide repeat proteins in the cytoplasm, and decreased levels of the normal c9orf72 protein. mitochondrial bioenergetic dysfunction leading to dysfunctional motor neuron axonal homeostasis (reduced axonal length and fast axonal transport of mitochondrial cargo) has been shown to occur in c9orf72-als using human induced pluripotent stem cell (ipsc) technologies coupled with crispr/cas9 gene-editing, and human post-mortem spinal cord tissue examination. excitotoxicity, or nerve cell death caused by high levels of intracellular calcium due to excessive stimulation by the excitatory neurotransmitter glutamate, is a mechanism thought to be common to all forms of als. motor neurons are more sensitive to excitotoxicity than other types of neurons because they have a lower calcium-buffering capacity and a type of glutamate receptor (the ampa receptor) that is more permeable to calcium. in als, there are decreased levels of excitatory amino acid transporter 2 (eaat2), which is the main transporter that removes glutamate from the synapse; this leads to increased synaptic glutamate levels and excitotoxicity. riluzole, a drug that modestly prolongs survival in als, inhibits glutamate release from pre-synaptic neurons; however, it is unclear if this mechanism is responsible for its therapeutic effect. diagnosis an mri of the brain (axial flair) looking at a person as if from above that shows increased t2 signal as a small white region within the posterior part of the internal capsule around the center of the image, consistent with the diagnosis of als. no single test can provide a definite diagnosis of als. instead, the diagnosis of als is primarily based on the symptoms and signs the physician observes in the person and a series of tests to rule out other diseases. physicians obtain the person's full medical history and usually conduct a neurologic examination at regular intervals to assess whether signs and symptoms such as muscle weakness, atrophy of muscles, hyperreflexia, babinski's sign, and spasticity are worsening. a number of biomarkers are being studied for the condition, but so far are not in general medical use. diagnostic criteria an mri of the brain looking at a person from side-on that shows increased t2 signal as a white region in the posterior part of the internal capsule that can be tracked to the motor cortex, consistent with the diagnosis of als. the diagnosis of als is based on the el escorial revised criteria and the awaji criteria. the original el escorial criteria had four levels of diagnostic certainty, based on how many of the four spinal cord regions were involved: bulbar, cervical, thoracic, and lumbar. definite als was defined as upper motor neuron (umn) and lower motor neuron (lmn) signs in three spinal cord regions, probable als as umn and lmn signs in two regions, possible als as umn and lmn signs in only one region, and suspected als as lmn signs only. the el escorial revised criteria, also known as the airlie house criteria, dropped the "suspected als" category and added a "laboratory-supported probable als" category. the awaji criteria give abnormal emg tests the same weight as clinical signs of lmn dysfunction in making the diagnosis of als, thus making the "laboratory-supported probable als" category unnecessary. the only three categories in the awaji criteria are definite als, probable als, and possible als. the el escorial revised criteria are specific for als, which means that someone who meets the criteria is very likely to have als; however, they are not especially sensitive for als, which means that someone who does not meet the criteria can still have als. their sensitivity is particularly poor in the early stages of als. the awaji criteria have better sensitivity than the el escorial revised criteria, especially for bulbar-onset als. a 2012 meta-analysis found that the el escorial revised criteria had a sensitivity of 62.2%, while the awaji criteria had a sensitivity of 81.1%; both sets of criteria had a specificity of about 98%. the el escorial criteria were designed to standardize patient groups for clinical trials but are not as useful in clinical practice; possible als as described by the el escorial criteria is almost always clinically als. differential diagnosis because symptoms of als can be similar to those of a wide variety of other, more treatable diseases or disorders, appropriate tests must be conducted to exclude the possibility of other conditions. one of these tests is electromyography (emg), a special recording technique that detects electrical activity in muscles. certain emg findings can support the diagnosis of als. another common test measures nerve conduction velocity (ncv). specific abnormalities in the ncv results may suggest, for example, that the person has a form of peripheral neuropathy (damage to peripheral nerves) or myopathy (muscle disease) rather than als. while a magnetic resonance imaging (mri) is often normal in people with early-stage als, it can reveal evidence of other problems that may be causing the symptoms, such as a spinal cord tumor, multiple sclerosis, a herniated disc in the neck, syringomyelia, or cervical spondylosis. based on the person's symptoms and findings from the examination and from these tests, the physician may order tests on blood and urine samples to eliminate the possibility of other diseases, as well as routine laboratory tests. in some cases, for example, if a physician suspects the person may have a myopathy rather than als, a muscle biopsy may be performed. a number of infectious diseases can sometimes cause als-like symptoms, including human immunodeficiency virus (hiv), human t-lymphotropic virus (htlv), lyme disease, and syphilis. neurological disorders such as multiple sclerosis, post-polio syndrome, multifocal motor neuropathy, cidp, spinal muscular atrophy, and spinal and bulbar muscular atrophy can also mimic certain aspects of the disease and should be considered. als must be differentiated from the "als mimic syndromes", which are unrelated