Flaccidity

Flaccidity refers to a marked decrease in muscle tone, resulting in limbs that are soft and readily moved with little resistance to passive stretching. In neurology, it is often described as hypotonia with reduced resistance to passive movement and can accompany or be distinguished from weakness. Flaccidity differs from spasticity or rigidity, which involve increased tone.

It may be transient or persistent in a range of disorders. In acute spinal cord injury there is flaccidity during spinal shock, followed by spasticity as recovery progresses. Persistent flaccidity suggests underlying lower motor neuron or muscular disease.

Common causes can be grouped into neurogenic, myopathic, and other categories. Neurogenic flaccidity arises from lower motor neuron lesions or peripheral nerve injury (for example, poliomyelitis, spinal muscular atrophy, Guillain-Barré syndrome, radiculopathy). Myopathic flaccidity results from muscular dystrophies or inflammatory myopathies, where the muscle itself is intrinsically weak. Central causes include congenital or acquired hypotonia in infants or after brain injury, though tone abnormalities from central lesions may evolve over time.

Clinical features include decreased resistance to passive movement, limp and floppy limbs, and reduced or absent deep tendon reflexes. Muscle may show atrophy; fasciculations can occur with lower motor neuron involvement. Distinguishing flaccidity from weakness is important, as strength testing may be preserved despite low tone in some conditions.

Diagnosis involves careful clinical assessment and targeted investigations. Electrodiagnostic studies (EMG and nerve conduction) help classify neurogenic versus myopathic causes. Imaging (MRI), CSF analysis (notably in Guillain-Barré syndrome), metabolic screens, and genetic testing may be employed depending on the suspected etiology.

Management focuses on treating the underlying cause and maintaining function. This includes supportive care, physical and occupational therapy, respiratory support when needed, and progressive rehabilitation plans. Prognosis varies widely with etiology and severity.