Abnormal subcortical somatosensory evoked potentials indicate high cervical myelopathy in achondroplasia

Children with achondroplasia may have high cervical myelopathy due to steno sis of the cranio-cervical junction resulting in neurological disability an d an increased rate of sudden death. To detect myelopathy we recorded somat osensory evoked potentials (SEPs) after median nerve stimulation in 30 pati ents with achondroplasia aged 13 months to 18 years (mean 6 years). in addi tion to the conventional technique of recording the cortical N20 and the ce ntral conduction time (CCT), we employed a noncephalic reference electrode recording the subcortical waveforms N13b and P13, generated near the cranio -cervical junction. The findings were related to the clinical status and MR I results. Eighteen patients had MRI evidence of spinal cord compression wi th indentation or narrowing of the upper cervical cord, and 13 showed signs of myelomalacia. Seven patients had neurological abnormalities. The sensit ivities of the SEPs were 0.89 for cervical cord compression, 0.92 for myelo malacia and 1.0 for the clinically symptomatic patients. There were no fals e-positive results. The subcortical SEPs were more sensitive than the conve ntional recordings. However, the conventional SEPs were highly specific in the most severely affected patients; here the specificity was 1.0 for patie nts with myelomalacia and 0.96 for symptomatic patients. Postoperative SEPs improved after occipital decompression in two children. Conclusion The analysis,of somatosensory evoked potentials, in particular o f subcortical tracings, is useful in the detection of early cervical myelop athy in children with achondroplasia. Early neurosurgical decompression-may prevent irreversible damage.