Motor skills that had been learned may be lost, but generally the functioning of the brain and ability to think are not affected.In MDDS associated with mutations in SUCLA2 or SUCLG1 that primarily affect the brain and muscle, hypotonia generally arises in infants before they are 6 months old, their muscles begin wasting away, and there is delay in psychomotor learning (learning basic skills like walking, talking, and intentional, coordinated movement). The spine often begins to curve (scoliosis or kyphosis), and the child often has abnormal movements (dystonia, athetosis or chorea), difficulty feeding, acid reflux, hearing loss, stunted growth, and difficulty breathing that can lead to frequent lung infections. Sometime epilepsy develops.In MDDS associated with mutations in RRM2B that primarily affect the brain and muscle, there is again hypotonia in the first months, symptoms of lactic acidosis like nausea, vomiting, and rapid deep breathing, failure to thrive including the head remaining small, delay or regression in moving, and hearing loss. Many body systems are affected. The Charlie Gard case was associated with this sub form of the disease.In MDDS associated with mutations in DGUOK that primarily affect the brain and the liver, there are two forms. There is an early-onset form in which symptoms arise from problems in many organs in the first week of life, especially symptoms of lactic acidosis as well as low blood sugar. Within weeks of birth they can develop liver failure and the associated jaundice and abdominal swelling, and many neurological problems including developmental delays and regression, and uncontrolled eye movement.
MDDS is associated with mutations in the genes TK2, SUCLA2, RRM2B, DGUOK, POLG, TYMP, SUCLG1,  and TWNK.Myopathic MDS is strongly correlated to a variety of mutations in the gene TK2, seeing a reduction of TK2 activity to less than 32% in people with MDS found with the mutation. Because TK2 plays a key role in the mitochondrial salvage pathways of several deoxyribonucleoside triphosphates (dNTPs), a lowered activity would lead to less cycling of nucleotides. This lack of nucleotide recycling is detrimental since the mitochondria cannot synthesize entirely new deoxynucleotides, and the inner membrane of the mitochondria prevents the negatively charged nucleotides of the cytosol from entering.The SUCLA2 gene codes for the beta-subunit of SCS-A. This enzyme catalyzes the synthesis of succinate and coenzyme A into succinyl-CoA, but is also associated with the complex formed by nucleoside diphosphate kinase (NDPK) in the last step of the dNTP salvage pathway.The RRM2B gene, which is expressed in the cell nucleus, codes for one of two versions of the R2 subunit of ribonucleotide reductase, which generates nucleotide precursors required for DNA replication by reducing ribonucleoside diphosphates to deoxyribonucleoside diphosphates. The version of R2 encoded by RRM2B is induced by TP53, and is required for normal DNA repair and mtDNA synthesis in non-proliferating cells. The other form of R2 is expressed only in dividing cells.The DGUOK gene encodes for mitochondrial deoxyguanosine kinase (dGK), which catalyzes the phosphorylation of deoxyribonucleosides into nucleotides.