The core clinical feature across all forms of spinal muscular atrophy is progressive, symmetric muscle weakness that is more pronounced in the proximal muscles (those closer to the trunk, such as the shoulders and hips) than in the distal muscles (hands and feet). Common symptoms include:

*Motor weakness and delayed milestones: Difficulty lifting the head, rolling over, sitting without support, standing, or walking, depending on age of onset.
*Hypotonia: Reduced muscle tone, often described as a “floppy” infant in early-onset cases.
Reduced or absent deep tendon reflexes.
*Tongue fasciculations: Fine, rippling movements of the tongue, seen in some infants with early-onset disease.
*Fine tremor of the fingers, sometimes noted in older children and adults with milder forms.
Respiratory muscle weakness: Weak cough, recurrent chest infections, and in severe cases, breathing difficulty, since intercostal muscles are affected more than the diaphragm.
*Bulbar weakness: Difficulty sucking, swallowing, or feeding, which can affect growth and nutrition in infants with severe disease.
*Skeletal complications over time: Scoliosis, joint contractures, and hip dislocation, resulting from prolonged muscle imbalance and reduced mobility.

Importantly, spinal muscular atrophy does not affect intellect, sensation, or cognitive development. Children with even severe forms typically show normal alertness and social engagement, which is a distinguishing clinical feature that separates it from many other severe neurological conditions of infancy.