From the Research
Crossed clonus is indicative of an upper motor neuron lesion affecting the corticospinal tract, specifically suggesting damage above the level of the decussation (crossing) of the pyramidal tracts in the medulla oblongata. This neurological finding occurs when stimulation of one limb produces a clonic response in the contralateral limb. It represents pathological hyperreflexia and is typically seen in conditions affecting the brain or upper cervical spinal cord. Common causes include stroke, multiple sclerosis, traumatic brain injury, or space-occupying lesions in the brain. The physiological basis involves interruption of descending inhibitory pathways that normally regulate muscle tone and reflexes, resulting in disinhibition of spinal reflex arcs, as explained by the principles of upper motor neuron and lower motor neuron neurophysiology and motor control 1. This disinhibition allows the abnormal spread of reflexes across the midline to the opposite side of the body.
When evaluating a patient with crossed clonus, it's essential to conduct a complete neurological examination to localize the lesion and determine its etiology, as this finding suggests significant central nervous system pathology requiring prompt medical attention. The role of motor cortex is critical for the motor activity, very dependent on the continuous sensory feedback, and this is essential for adapting the force and speed control, which contributes to motor learning 1. In contrast to lower motor neuron lesions, which may present with different clinical features, such as muscle atrophy and fasciculations, upper motor neuron lesions, like those causing crossed clonus, are characterized by spasticity, hyperreflexia, and a positive Babinski sign.
Key points to consider in the evaluation of crossed clonus include:
- The level of the lesion: damage above the level of the decussation of the pyramidal tracts in the medulla oblongata can cause crossed clonus
- The presence of other upper motor neuron signs, such as spasticity and hyperreflexia
- The absence of lower motor neuron signs, such as muscle atrophy and fasciculations
- The importance of a detailed clinical examination, including sacral reflexes, to distinguish upper motor neuron lesions from lower motor neuron lesions 2.
In summary, crossed clonus is a significant neurological finding that requires prompt evaluation and management, as it suggests damage to the upper motor neuron pathways and may be associated with various conditions affecting the brain or upper cervical spinal cord.