Clinical Correlation: Hereditary Sensory And Autonomic Neuropathies

Clinical correlation: hereditary sensory and autonomic neuropathies (HSAN)                Rare neurological disorders caused by specific genetic mutations, producing complex phenotypes that are expressed in infancy. HSAN III (Riley-Day syndrome) is the most common, largely limited to Jews of Eastern European descent. Single point mutation on chromosome 9q. Characteristic features include hypertension with anxiety, greatly elevated temperature and pain thresholds, and a relative indifference to pain. Prognosis. o By the time they are 20, 6% of patients will require walking aids. o By the time they are 30, increased to 18%. o By the time they are 40, increased to 30%. o By the time they are 50, increased to 59%. Although motor strength is normal, patients have absent tendon jerks and Hreflexes (reflex from electrical stimulation), leading to decrease proprioceptive ability. Could be explained by very low spinal excitability or lack of functional muscle spindles. There are no histological data on muscle spindles, but it is known that the volume of the dorsal root ganglia was reduced by ~30% in patients with HSAN III. HSAN IV patients have normal spindle activity and do not exhibit ataxia, and have preserved tendon and H-reflexes. Passive joint angle matching at the knee joint was measured using digital inclinometers on the side of each knee. The reference knee was passively rotated by one experimenter into one of six angles, each commencing from 0o (vertical): 7o, 14o or 21o (flexion or extension). Each block of angles was presented in quasi-random fashion 8 times. Indicator knee was then passively moved (by another experimenter). o Either in the same or opposite direction to which the reference knee was placed. o Rate of ~2o/s. o Until subject reported that its perceived angle matched that of the reference knee. Results showed that proprioceptive acuity was reduced for HSAN III patients, but was normal for the HSAN IV and cerebellar patients. o Range of error between reference and indicator knee was wide in HSAN III, but 3 of 18 were within the normal range. Ataxia was assessed using the 5-item Brief Ataxia Rating Score (BARS). o Gait. o Finger-to-nose pointing. o Knee-to-tibia test. o Oculomotor. o Dysarthria (abnormal speech).   There was a wide difference across the HSAN III patients, and had a mean total BARS value comparable to patients with cerebellar ataxia. o Matching error was positively correlated to the total ataxia score. o Slope and correlation coefficients were inversely correlated. Although muscle spindles are absent in HSAN III, tactile afferents appear to be normal. o Using this information, scientists placed bands near the knee to increase the stretching of skin during passive movement. o It was noted that proprioceptive acuity greatly improved compared to controls. Sense of effort  During voluntary contractions, an efference copy (corollary discharge), is generated that allows comparison of the actual sensory feedback generated during the muscular contraction with the expected sensory feedback.  If a muscle is weakened, either by muscular fatigue or pathology, a greater motor command is required to generate the required force, and hence the expected sensory feedback.  The required increase in motor command is perceived as an increase in effort. This perception is referred to as the sense of effort. Golgi     tendon organs Encapsulated sensory endings located in the musculotendinous junctions. Simple stretch receptors, similar to Ruffini endings in the skin and joints. Their axons are rapidly-conducting Ib afferents. They are silent when the muscle is relaxed, and do not respond when the muscle is stretched passively.  They are activated during muscle contraction and can sense the forces generated by individual motor units.  Overall, help calibrate our sense of effort. Conclusion  Muscle spindles are the primary source of proprioceptive input, but cutaneous afferents can also contribute.  The contribution provided by joint receptors is low, unless the other two sources are not available.  Efference copy also contributes to proprioception; the expected sensory input associated with a particular motor task is compared with the actual sensory input.  Our sense of effort increases when the muscle is weakened and hence the required motor command increased.  Proprioceptive inputs provide the reference signals for our body map, which is constantly being updated.