These are:
Parkinsonism - slow, stiff movement, writing becomes small and spidery
Cerebellar dysfunction - difficulty coordinating movement and balance
Autonomic nervous system dysfunction - impaired automatic body functions, including one, some, or all of the following:postural or orthostatic hypotension, resulting in dizziness or fainting upon standing up
urinary incontinence or urinary retention
impotence
constipation
vocal cord paralysis
dry mouth and skin
trouble regulating body temperature due to sweating deficiency in all parts of the body
loud snoring, abnormal breathing or inspiratory stridor during sleep
other sleep disorders including sleep apnea, REM behavior disorder
double vision
muscle twitches
Cognitive impairment

Genetics
One study found a correlation between the deletion of genes in a specific genetic region and the development of MSA in a group of Japanese patients. The region in question includes the SHC2 gene which, in mice and rats, appears to have some function in the nervous system. The authors of this study hypothesized that there may be a link between the deletion of the SHC2 and the development of MSA.A follow-up study was unable to replicate this finding in American MSA patients. The authors of the U.S. study concluded that "Our results indicate that SHC2 gene deletions underlie few, if any, cases of well-characterized MSA in the US population. This is in contrast to the Japanese experience reported by Sasaki et al., likely reflecting heterogeneity of the disease in different genetic backgrounds. "Another study investigated the frequency of RFC1 intronic repeat expansions, a phenomenon implicated in CANVAS; a disease with a diagnostic overlap with MSA.
Histopathological investigation on six cases of pathologically confirmed MSA, using antibodies directed at a variety of α-synuclein epitopes, revealed substantial variation in α-synuclein protein deposition across both cases and brain regions within cases, providing evidence for strains of aggregated conformers that may differentially promote pathological prion-like spread.In 2020, researchers at The University of Texas Health Science Center at Houston concluded that protein misfolding cyclic amplification could be used to distinguish between two progressive neurodegenerative diseases, Parkinson’s disease and multiple system atrophy, being the first process to give an objective diagnosis of Multiple System Atrophy instead of just a differential diagnosis. Classification
MSA is one of several neurodegenerative diseases known as synucleinopathies: they have in common an abnormal accumulation of alpha-synuclein protein in various parts of the brain. Other synucleinopathies include Parkinsons disease, the Lewy body dementias, and other more rare conditions. Old terminology
Historically, many terms were used to refer to this disorder, based on the predominant systems presented. These terms were  discontinued by consensus in 1996 and replaced with MSA and its subtypes, but awareness of these older terms and their definitions is helpful to understanding the relevant literature prior to 1996. These include striatonigral degeneration (SND), olivopontocerebellar atrophy (OPCA), and Shy–Drager syndrome. A table describing the characteristics and modern names of these conditions follows:

Current terminology
The current terminology and diagnostic criteria for the disease were established at a 2007 conference of experts and set forth in a position paper.