Sensory ataxia · Lobeworks/17

Sensory ataxia is a disorder of movement in which coordination fails because the brain no longer receives reliable information about the position and movement of the limbs (proprioception), while the muscles and the motor commands are intact; it is the clumsiness of a body steered blind.

Sensory ataxia. Sensory ataxia is a disorder of movement in which coordination fails because the brain no longer receives reliable information about the position and movement of the limbs (proprioception), while the muscles and the motor commands are intact; it is the clumsiness of a body steered blind.

Proprioception travels on the thick, fast myelinated fibres (Ia and II from the muscle spindles, Ib from the tendon organs, Aβ from the skin) and climbs the dorsal columns of the spinal cord; pain and temperature travel on thin Aδ and C fibres along another route. A disease that destroys the large fibres or the dorsal columns can therefore take away the sense of where the limbs are and leave pain and temperature. The patient can still move but no longer knows, eyes closed, which way a finger has been bent, and walks on a wide base watching the feet.

The Romberg sign tells it apart. Standing with feet together, the patient is steady with eyes open and sways or falls when they close: vision was standing in for the lost sense. A patient with cerebellar ataxia is unsteady with eyes open too, because the fault is in the coordination itself.

Ian Waterman is the famous case. At nineteen, in 1971, a viral illness destroyed the large sensory fibres from his neck down; he lost touch and position sense, kept pain and temperature, and relearned to walk and use his hands by watching every movement, as his doctor Jonathan Cole described in Pride and a Daily Marathon (1995).

Its causes are many. Tabes dorsalis in late syphilis (where Romberg described the sign), vitamin B12 deficiency, some chemotherapy drugs and immune attacks on the sensory ganglia all reach the same fibres.

The brain still knows how to move the hand; what it has lost is the feedback that says where the hand is.

Take away the large sensory fibres and every movement must be steered by sight.

Questions: How can someone lose the sense of where the limbs are and still feel pain? Because the two travel on different fibres. Position and movement ride the thick myelinated fibres (Ia, II and Ib from muscles and tendons, Aβ from the skin); pain and temperature ride the thin Aδ and C fibres. A disease that destroys only the large fibres, as a viral illness did to Ian Waterman in 1971, takes position sense and touch and leaves pain and temperature. Why does a patient with sensory ataxia fall when closing the eyes, while one with cerebellar ataxia is unsteady with them open too? Because the first one has lost information and the second one coordination. Without proprioception, vision stands in for the lost sense of where the body is, so the patient is steady with eyes open and sways or falls once they close (a positive Romberg sign). In cerebellar ataxia the information arrives but the timing and aim of the movements fail, so the patient is unsteady with eyes open too.