From the Research
To test for flaccidity in a lower motor neuron lesion, a comprehensive neurological examination focusing on muscle tone, strength, reflexes, and atrophy is essential, as outlined in the most recent study 1. This approach begins with assessing muscle tone through passive movement of the affected limbs, noting abnormal laxity or decreased resistance characteristic of flaccidity. The examination should also include testing muscle strength using the Medical Research Council (MRC) scale from 0-5, with lower motor neuron lesions typically showing weakness in specific muscle groups corresponding to the affected nerve roots or peripheral nerves. Checking deep tendon reflexes, which will be diminished or absent in lower motor neuron lesions, is also crucial. Additionally, looking for muscle atrophy, fasciculations (spontaneous muscle twitching), and reduced muscle bulk, which develop over time with denervation, provides valuable diagnostic information. Electromyography (EMG) and nerve conduction studies can confirm lower motor neuron involvement by demonstrating denervation potentials, fibrillations, and reduced motor unit recruitment, as discussed in 2. These findings collectively help distinguish lower motor neuron lesions from upper motor neuron problems, which instead present with spasticity, hyperreflexia, and preserved muscle bulk. The flaccidity occurs because lower motor neuron damage interrupts the final common pathway between the spinal cord and muscles, eliminating both voluntary control and baseline muscle tone, a concept supported by the understanding of lower motor neuron syndromes 3, 4. Key aspects of the clinical history and examination, as highlighted in 1, are critical for localizing the lesion and generating a differential diagnosis, ultimately guiding appropriate ancillary testing and treatment. Given the recent advances in genetic testing and targeted therapies for neuromuscular disorders, as mentioned in 1, timely and accurate diagnosis is more important than ever for achieving better clinical outcomes.