Introduction - Dorsal root ganglia (DRG) can be the selective target of either acquired or genetic diseases, and can be associated with immune-mediated and neoplastic diseases, viral infections, vitamin intoxication or deficit and neurotoxic drugs. Hereditary sensory neuron diseases (SNDs) can be transmitted as autosomal dominant tract (HSAN-1), to be recessive (HSAN-2 and Tangier), or X-linked (Fabry's disease). They also cause ataxia in more complex syndromes: Friedreich disease, SCA, vitamin E deficiency and mitochondrial diseases. Diagnosis - Afferent and efferent axonal projections give DRG neurons the typical T shape. To ascertain the sufferance of the sensitive cell bodies, apart from DRG direct histological analysis, we have to demonstrate the contemporary degeneration of the peripheral and central leg of sensory axons. Peripheral deficit can be verified by routine neurophysiological examination or nerve biopsy. Somatosensory evoked potentials can also reveal abnormal conduction throughout the posterior columns. However, their practical value may be limited by attenuated sensory nerve action potentials (SNAPs) amplitude. Afferent projections of large-size neurons ascend directly throughout the posterior columns which transduce proprioceptive and tactile sensation. New tools in ascertaining central pathways sufferance in SND patients are now given by T 2-weighted Nuclear Magnetic Resonance (MRI-T2) sequences demonstrating high signal intensity in the posterior columns of patients with neuronopathy of different aetiologies. Clinical and neurophysiological features - As happens in motor neuron diseases, SNDs are non-length dependent; their signs and symptoms being determined by the group of cells involved and not by axonal length. In the absence of motor symptoms and demyelination, ataxia and a patchy distribution similar to that seen in multiplex mononeuropathies but involving both proximal and distal body region, can be markers of involvement of sensory perikarion. Ataxia, quite rare even in the latest phases of axonal polyneuropathies, can be present at the onset of SND. Routine electrophysiological studies contribute to the diagnosis of sensory neuronopathies, showing non-length dependent abnormalities of sensory nerves. Widespread loss or marked decrease in amplitude of SNAPs are typically found even in patients with asymmetrical or patchy presentation. Sometimes, sensory nerves can be more affected in the upper rather than lower limbs. Right-to-left difference can be observed in a limited percentage of patients. Nuclear Magnetic Resonance - MRI typically demonstrates signal hyperintensity in the spinal cord that is more easily detected at the cervical level due to the larger size of the cord and the greater impairment of distal axons. However, it extends longitudinally throughout the dorsal and lumbar levels. Small myelinated and unmyelinated fibres make synapses and decussate before ascending the spinal cord. This prevents the degeneration of spinothalamic tracts and precludes the possibility to demonstrate signal intensity changes on MRI scan. Skin biopsy - Morphometry of small diameter fibres in sural nerve biopsy reflects the number of both autonomic and somatic fibres, which cannot be distinguished. This is possible by examination of intraepidermal nerve fibres (IENF), which are somatic unmyelinated axons arising from small-size DRG neurons. Patients with sensory neuronopathy may show a non-length dependent loss of IENF in the limbs, which differentiate them from the length-dependent skin denervation typical of axonal polyneuropathy. A skin biopsy showing a non length-dependent pattern of denervation can be the only tool for the diagnosis of a ganglionopathy limited to small diameter ganglion cells.
|Translated title of the contribution||Sensory Neuron Diseases|
|Number of pages||18|
|Journal||Rivista Italiana di Neurobiologia|
|Publication status||Published - 2006|
ASJC Scopus subject areas
- Clinical Neurology