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Upper Motor Neuron Weakness - Patterns in Neurologic Exam
Reading time: 10 minutes
UMN = Upper Motor Neuron
LMN = Lower Motor Neuron
Introduction
Following on from our previous blog on tone we will be discussing the different patterns of muscle weakness in this one. Remember that neurologic exam is like a treasure hunt, decide what to do next, based on what you’ve found earlier!
Once we have established that the increased tone represents spasticity, our next job is to assess the muscle power. Recall that spasticity implies an upper motor neuron lesion referring to the pyramidal tract. Previously, I would often get lost in grading the muscle power with such accuracy that I wouldn’t know how to interpret it apart from saying which muscles are weak.
What would be more helpful, is to look at the pattern of muscle weakness. Recall that muscles are arranged in pairs like flexor and extensor groups etc. When you examine, identify which muscle groups are predominantly affected – look at the pattern (see figure 1).
Figure 1
3 patterns of UMN weakness
There are 3 common types of pyramidal pattern of muscle weakness that you need to identify in the context of spasticity (UMN lesions):
- Unilateral weakness – commonly seen in hemiparesis or hemiplegia
- Symmetric/ asymmetric
- Monoplegia
1. Unilateral pyramidal pattern of muscle weakness
The best way to remember this pattern, is to think of the gait in someone who’s had a stroke with hemiparesis i.e. unilateral (same sided arm and leg) pyramidal pattern of weakness.
How do they walk? Recall that they walk with the affected side held in a particular posture. The upper limb is held in a flexed position with the shoulder adducted while the opposite occurs in the ipsilateral leg. The leg is held in extension with stronger abduction. This is because the extensors are weak in the upper limb while the flexors are weak in the lower limb. Hence, they trace a semi-circle with their foot while taking a step, because they are unable to flex the knee! (See figure 2). Hence the name of this gait – circumduction.
Figure 2
The pattern in right sided hemiparesis would look something like this in your notes:
Remember that hemiplegia implies significant weakness e.g. 0/5, while hemiparesis suggests milder weakness which is still significant for the patient! Unilateral pyramidal pattern of weakness points to the brain or spinal cord as the site of lesion.
2. Symmetric/ asymmetric
Pyramidal pattern of weakness occurring bilaterally (symmetric or asymmetric) often points to the spinal cord as the site of lesion e.g. cord compression, spastic paraparesis etc. Remember that in cord compression, LMN signs occur at the level of compression while UMN signs occur below it.
3. Monoplegia
Occasionally you might find the pyramidal pattern of weakness affecting a single limb. For example, sudden onset unilateral lower limb UMN weakness is typical of a frontal lobe stroke and can be associated with urinary incontinence.
Recall that UMN pattern of weakness means that the site of lesion can be anywhere along the upper motor neuron pathway that extends from the cerebral cortex to the anterior horn cell in the spinal cord. Placing the lesion on this pathway will be explained in follow-up blogs.
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