Enlarged SI and SII somatosensory evoked responses in the CLN5 form of neuronal ceroid lipofuscinosis

Objectives: To examine in detail the activation of the primary (SI) and secondary (SII) somatosensory cortex in CLN5, the Finnish variant of late infantile neuronal ceroid lipofuscinoses (NCL). Methods: Somatory evoked magnetic fields were recorded with a 122-channel planar gradiometer in response to median nerve stimulation in 5 CLN5 patients (aged 8.8–16.7 years) and in 10 healthy age-matched controls. Results: The first two responses from contralateral SI, N20m and P35m, were 6–20 times stronger in the patients than in the controls. The morphology of the subsequent deflections from SI was abnormal in the patients: a prominent N45m was detected, while the normally present P60m deflection was missing. In 4 patients the contra- and in two patients the ipsilateral SII responses were also enlarged. Furthermore, the SII activation was detected at shorter latency in patients than in controls. Conclusions: At SI, CLN5 is associated with a selective enhancement of the early cortical responses. We propose that the enlargement of N20m most likely reflects increased synchronous input from thalamus, whereas the altered morphology of the following responses may reflect defective interneuronal inhibition at the cortex. The enlargement of SII responses shows that the imbalance between excitation and inhibition in CLN5 extends outside the primary somatosensory areas