A 62-year-old man presents with acute onset of dysmetria, intention tremor, and dysdiadochokinesia affecting the right limbs. MRI shows an acute infarct in the right superior cerebellar peduncle. All of the following cerebellar functions are impaired in this patient EXCEPT:

Explanation

## Superior Cerebellar Peduncle Infarct — Functional Consequences ### Anatomy and Connectivity of the SCP **Key Point:** The superior cerebellar peduncle (SCP) is the **output** pathway of the cerebellum. It carries: - Purkinje cell axons (inhibitory) to deep cerebellar nuclei - Deep cerebellar nucleus axons → ventral thalamus (VL/VA) → motor cortex, brainstem - Proprioceptive feedback from spinal cord to cerebellum (spinocerebellar tracts) An SCP infarct disrupts **cerebellar output and feedback**, not motor command initiation. ### Clinical Findings in SCP Infarct | Finding | Mechanism | Cerebellar Function Affected? | |---------|-----------|-------------------------------| | **Dysmetria** (overshoot/undershoot) | Loss of error correction; cannot compare intended vs. actual trajectory | ✓ Yes — error detection | | **Intention tremor** | Mismatch between planned and actual movement; oscillation during correction attempts | ✓ Yes — feedback control | | **Dysdiadochokinesia** (irregular alternating movements) | Cannot coordinate agonist-antagonist timing; loss of smooth alternation | ✓ Yes — muscle timing | | **Preserved strength, no paralysis** | Motor command pathway (cortex → spinal cord) is intact | ✓ Motor initiation preserved | | **Preserved reflexes** | Reflex arc (sensory → motor neuron) is intact | ✓ Spinal circuits intact | ### Why Motor Command Initiation Is NOT Impaired **High-Yield:** The patient CAN initiate movement (no paralysis, strength intact). What is lost is the **refinement** of that movement. The motor cortex and brainstem still generate the command; the cerebellum cannot execute its error-correction role. **Clinical Pearl:** A patient with cerebellar infarct will attempt to reach for an object and will make the movement, but it will be jerky and inaccurate (dysmetric). They are not paralyzed — their brain is still sending the "move" command. The problem is that the cerebellum cannot fine-tune it in real time. **Mnemonic: SCP = Smoothness, Coordination, Precision** — all lost in SCP infarct. But **S**trength and **S**tarting (initiation) are preserved.