From the Guidelines
Upper motor neuron (UMN) signs are characterized by increased muscle tone, hyperreflexia, and weakness affecting groups of muscles, whereas lower motor neuron (LMN) signs are marked by decreased muscle tone, hyporeflexia, and weakness in individual muscles. The distinction between UMN and LMN signs is crucial for localizing neurological lesions and guiding diagnostic workup and treatment approaches. UMN signs result from damage to neurons in the brain or spinal cord, as seen in conditions like stroke, which can lead to spasticity, a component of the upper motor neuron syndrome, as noted in the guidelines for adult stroke rehabilitation and recovery 1. In contrast, LMN signs occur from damage to motor neurons in the anterior horn of the spinal cord or peripheral nerves, as observed in conditions such as amyotrophic lateral sclerosis (ALS), which can present with both UMN and LMN signs, including hypertonicity, hyperreflexia, muscle fasciculations, weakness, and atrophy 1.
Key Differences
- UMN signs:
- Increased muscle tone (spasticity)
- Hyperreflexia
- Clonus
- Babinski sign (upgoing toes)
- Weakness affecting groups of muscles
- LMN signs:
- Decreased muscle tone (flaccidity)
- Hyporeflexia or areflexia
- Fasciculations
- Fibrillations
- Muscle atrophy
- Weakness in individual muscles
Clinical Implications
The differentiation between UMN and LMN signs is essential for diagnosing and managing neurological disorders. For instance, a stroke affecting the motor cortex would produce UMN signs on the opposite side of the body, while conditions like ALS or peripheral nerve injuries would demonstrate LMN signs. Electromyography and nerve conduction velocity are key tests in diagnosing ALS, with imaging relied upon mainly to exclude other conditions with similar clinical presentations rather than confirm or facilitate the diagnosis of ALS 1.
Treatment Approaches
Understanding the difference between UMN and LMN signs guides treatment approaches, including the use of botulinum toxin injections for upper limb spasticity combined with therapy in stroke patients, although its cost-effectiveness compared to therapy alone is debated 1. The management of spasticity, a common component of UMN syndrome, is crucial as it correlates with activity limitations and reduces quality of life, emphasizing the need for targeted therapeutic interventions 1.
From the Research
Upper Motor Neuron Signs
- The Babinski sign is a well-known sign of upper motor neuron dysfunction 2
- Spasticity is usually observed along with paralysis, hyperreflexia, Babinski sign and abnormal associated movements associated with dysfunction of central motor tracts 3
- Upper motor neuron lesions can result in increased muscle tone, which can be pathophysiologically described as spasticity 3
- The distribution of the increment in the muscle tone can vary, and other conditions such as rigidity, dystonia, and muscle stiffness should be differentiated 3
Lower Motor Neuron Signs
- Lower motor neuron paralysis can result in muscle weakness, atrophy, and decreased reflexes 4
- The integrity of the lower motor neuron (LMN) is important to assess in patients with cervical level spinal cord injury (SCI) 4
- LMN status can be easily evaluated using surface electrical stimulation, which can contribute to the development of a lifetime upper extremity care plan for maximizing function and quality of life 4
- Differentiation between upper and lower motor neuron paralysis is crucial for emerging treatments designed to restore upper extremity function 4
Key Differences
- Upper motor neuron signs are typically characterized by increased muscle tone, spasticity, and hyperreflexia, while lower motor neuron signs are characterized by muscle weakness, atrophy, and decreased reflexes 3, 4
- The Babinski sign is a specific indicator of upper motor neuron dysfunction, while decreased speed of foot tapping may be a more useful sign for identifying upper motor neuron weakness 2
- Neurophysiological examinations, such as surface electromyography and magnetic stimulation, can provide supportive objective data for diagnosing upper motor neuron lesions 3