Diagnosis
Differential diagnosis can be challenging due to the number of overlapping symptoms, shared between several motor neuron diseases. Frequently, the diagnosis is based on clinical findings (i.e. LMN vs. UMN signs and symptoms, patterns of weakness), family history of MND, and a variation of tests, many of which are used to rule out disease mimics, which can manifest with identical symptoms. Classification
Motor neuron disease describes a collection of clinical disorders, characterized by progressive muscle weakness and the degeneration of the motor neuron on electrophysiological testing. The term "motor neuron disease" has varying meanings in different countries. Similarly, the literature inconsistently classifies which degenerative motor neuron disorders can be included under the umbrella term "motor neuron disease". The four main types of MND are marked (*) in the table below.All types of MND can be differentiated by two defining characteristics:
Is the disease sporadic or inherited? Is there involvement of the upper motor neurons (UMN), the lower motor neurons (LMN), or both?Sporadic or acquired MNDs occur in patients with no family history of degenerative motor neuron disease. Inherited or genetic MNDs adhere to one of the following inheritance patterns: autosomal dominant, autosomal recessive, or X-linked. Some disorders, like ALS, can occur sporadically (85%) or can have a genetic cause (15%) with the same clinical symptoms and progression of disease.UMNs are motor neurons that project from the cortex down to the brainstem or spinal cord. LMNs originate in the anterior horns of the spinal cord and synapse on peripheral muscles.
ALS, PBP, PLS)

Lower and upper motor neuron findings
Motor neuron diseases are on a spectrum in terms of upper and lower motor neuron involvement. Some have just lower or upper motor neuron findings, while others have a mix of both. Lower motor neuron (LMN) findings include muscle atrophy and fasciculations, and upper motor neuron (UMN) findings include hyperreflexia, spasticity, muscle spasm, and abnormal reflexes.Pure upper motor neuron diseases, or those with just UMN findings, include PLS.Pure lower motor neuron diseases, or those with just LMN findings, include PMA.Motor neuron diseases with both UMN and LMN findings include both familial and sporadic ALS. Causes
Most cases are sporadic and their causes are usually not known. It is thought that environmental, toxic, viral, or genetic factors may be involved. DNA damage
TARDBP (TAR DNA-binding protein 43), also referred to as TDP-43, is a critical component of the non-homologous end joining (NHEJ) enzymatic pathway that repairs DNA double-strand breaks in pluripotent stem cell-derived motor neurons. TDP-43 is rapidly recruited to double-strand breaks where it acts as a scaffold for the recruitment of the XRCC4-DNA ligase protein complex that then acts to repair double-strand breaks. About 95% of ALS patients have abnormalities in the nucleus-cytoplasmic localization in spinal motor neurons of TDP43. In TDP-43 depleted human neural stem cell-derived motor neurons, as well as in sporadic ALS patients’ spinal cord specimens there is significant double-strand break accumulation and reduced levels of NHEJ. Associated risk factors
In adults, men are more commonly affected than women.