Functional limb weakness, a common symptom of Functional Neurological Disorder (FND), can present in various patterns. Two such patterns are ‘pyramidal’ and ‘inverse pyramidal’.
- Pyramidal Pattern: The term ‘pyramidal’ refers to the pyramidal tract, a major motor pathway in the central nervous system that is responsible for voluntary movement. A pyramidal pattern of weakness typically involves more weakness in the distal parts of the limbs than the proximal parts. This pattern is common in organic neurological diseases involving the corticospinal tract. In the upper limbs, the flexors (which bend the limb) may be more affected, and in the lower limbs, the extensors (which straighten the limb) may be more affected.
- Inverse Pyramidal Pattern: In contrast, an ‘inverse pyramidal’ pattern of weakness, more commonly seen in FND, involves more weakness in the proximal parts of the limbs than the distal parts. This pattern is unusual for organic neurological diseases but is frequently observed in functional weakness. An inverse pyramidal pattern might also show inconsistency, with the distribution of weakness changing over time or with repeated examination.
These patterns of weakness can provide valuable clues in the diagnosis of functional limb weakness. However, it’s crucial to note that FND is a positive diagnosis based on specific clinical features, not merely a diagnosis of exclusion. A thorough clinical assessment, including a detailed history and physical examination, is essential for an accurate diagnosis.