How do you differentiate between upper and lower motor neuron lesions?

From the Guidelines

Upper motor neuron (UMN) and lower motor neuron (LMN) lesions can be differentiated by examining specific clinical signs, with UMN lesions typically presenting with hypertonia, hyperreflexia, and preserved muscle bulk, and LMN lesions manifesting with hypotonia, hyporeflexia, and significant muscle atrophy. The key differences between UMN and LMN lesions lie in their clinical presentation and the underlying pathophysiology. UMN lesions, which affect neurons from the motor cortex to the anterior horn of the spinal cord, often present with characteristic signs such as spasticity, hyperreflexia, clonus, and an upgoing plantar response (Babinski sign), as noted in a study published in Pediatrics 1. In contrast, LMN lesions, which involve anterior horn cells or peripheral nerves, are characterized by hypotonia, hyporeflexia or areflexia, fasciculations, and significant muscle atrophy, with no Babinski sign.

Some of the key clinical signs that differentiate UMN from LMN lesions include:

• Hypertonia (spasticity) vs. hypotonia (flaccidity)
• Hyperreflexia vs. hyporeflexia or areflexia
• Presence of Babinski sign vs. absence
• Preserved muscle bulk with minimal atrophy vs. significant muscle atrophy
• Distribution of weakness: UMN lesions often affect extensors in the upper limbs and flexors in the lower limbs, while LMN lesions follow a pattern corresponding to specific nerve roots or peripheral nerves.

A more recent study published in Stroke in 2016 1 highlights the importance of spasticity as a component of the upper motor neuron syndrome, particularly in the context of poststroke rehabilitation. This study emphasizes the impact of spasticity on activity limitations, caregiver burden, and quality of life, underscoring the need for accurate differentiation between UMN and LMN lesions to guide appropriate management and rehabilitation strategies.

Given the clinical context and the need to prioritize morbidity, mortality, and quality of life, differentiating between UMN and LMN lesions based on clinical signs and understanding their distinct pathophysiologies is crucial for providing targeted and effective care. This approach is supported by guidelines and evidence from reputable sources, including the American Heart Association/American Stroke Association 1 and the American Academy of Pediatrics 1.

From the Research

Differentiating Between Upper and Lower Motor Neuron Lesions

To differentiate between upper and lower motor neuron lesions, several clinical features and diagnostic techniques can be utilized. The key is to identify the specific characteristics of each type of lesion.

• Clinical Features:
  • Upper motor neuron (UMN) lesions typically present with signs such as spasticity, hyperreflexia, and a positive Babinski sign 2.
  • Lower motor neuron (LMN) lesions are characterized by muscle wasting, weakness, and decreased or absent reflexes 3.
• Diagnostic Techniques:
  • Electrodiagnostic assessments, including nerve conduction studies and electromyography (EMG), can help distinguish between UMN and LMN lesions 4.
  • Clinical examination signs, such as the distribution and severity of UMN and LMN signs, can also be used to differentiate between the two types of lesions 5.
• Neurophysiological Studies:
  • Understanding the normal structure and function of the motor system is essential for interpreting neurophysiological studies of UMN or LMN lesions 6.
  • The role of the motor cortex, spinal cord, and cerebellum in motor control can provide insights into the underlying pathology of UMN and LMN lesions.
• Specific Disorders:
  • Certain disorders, such as amyotrophic lateral sclerosis (ALS), primary lateral sclerosis (PLS), and spinal muscular atrophy, can present with a combination of UMN and LMN signs 2.
  • The clinical features and diagnostic techniques used to differentiate between UMN and LMN lesions can also be applied to these specific disorders.