We need three systems to help maintain balance: the visual system, the vestibular sytem, and the proprioceptive system. The proprioceptive system consists of the nerves that begin in our limbs, travel through larger nerves, enter the spinal cord, and finally connect to our brains to convey information that helps maintain awareness of the position and posture of our bodies and limbs in space.
Proprioception is controlled by specialized neurons called proprioceptors located within muscles (muscles spindles), joints (joint receptors), and tendons (Golgi tendon organs). Proprioceptive information is sent from these receptors, through the peripheral nerves, into the spinal cord before being conveyed to the brain.
Diseases that affect the proprioceptive system cause sensory ataxia – a balance disorder characterized by difficulty maintaining an upright posture or walking when visual input is removed. As such patients with sensory ataxia have trouble maintaining their balance in dark conditions or when their eyes are closed.
Conditions that cause sensory ataxia
There any many conditions that can affect the peripheral nerves. These include:
- Diabetes mellitus
- Alcohol abuse
- Nutrient deficiencies (e.g. vitamin B12, vitamin E, copper)
- Vitamin B6 toxicity
- Infections (e.g. leprosy, syphilis, HIV)
- Medications (e.g. platinum-based chemotherapy drugs)
- Autoimmune diseases (e.g. Guillian-Barre syndrome)
- Abnormal proteins (e.g. cryoglobulinemia, monoclonal gammopathy of undetermined significance)
- Inherited conditions (e.g. Charcot-Marie-Tooth disease)
Dorsal Root Ganglia Lesions (Sensory Neuronopathy)
The dorsal root ganglia are the collection of sensory neurons just before it forms the nerve root that enters the spinal cord. Certain rare diseases may affect the dorsal root ganglia:
- Paraneoplastic (immune-mediated conditions triggered by a hidden tumor) syndromes (e.g. anti-Hu antibodies)
- Sjogren’s syndrome
- Cerebellar Ataxia, Neuronopathy, Vestibular Areflexia Syndrome (CANVAS)
Spinal Cord Lesions
Diseases that affect the dorsal columns of the spinal cord typically cause sensory ataxia:
- Infections (e.g. syphilis, HIV)
- Nutritional deficiencies (e.g. vitamin B12, folate, vitamin E, copper)
- Multiple sclerosis
- Spinal cord trauma
Connective tissue disorders
Inherited that affect joint integrity (Ehlers-Danlos syndrome, Marfan’s syndrome).