Ataxia is incoordination caused by dysfunction to sensory nerve inputs, motor nerve outputs or the processing of them. It is not the result of muscular weakness. Ataxia is most often applied to unsteadiness in walking but it also refers to upper body incoordination and dysfunction in eye movements and speech.

Ataxia is common in multiple sclerosis but is also seen in several other conditions including diabetic polyneuropathy, acute transverse myelitis, vacuolar myelopathy, tumor or cord compression and hereditary forms of ataxia.

There are three types of ataxia, all of which are seen in MS:

1. Cerebellar Ataxia

This is caused by lesions in the cerebellum or in the parts of the brain that connect to it, such as the cerebellar peduncles, the pons or the red nucleus

 
Because the cerebellum is responsible for synchronising voluntary muscle movement throughout the body, cerebellar ataxia can result in:
 
• Uncoordinated walking - gait ataxia.
• Reduced control of range of movement such as over- or under-shooting targets - dysmetria.
• Inability to maintain a steady posture - hypotonia.
• An inability to maintain a steady rhythm - dysdiadochokinesia.
• Shaking when attempting fine movements - intention tremor.
• An inability to coordinate the muscles involved in speech - dysarthria.
• Jittery eye movements - nystagmus.
2. Vestibular Ataxia

In multiple sclerosis, vestibular ataxia is caused lesions to brainstem and the vestibular nuclei. In other conditions, it can also result from damage to the eighth cranial nerve leading from the balance organs in the inner ear.

Vestibular Ataxia can result in:
 

• Loss of balance.
• Dizziness, nausea and vomitting - vertigo.
• Jittery eye movements - nystagmus.
3. Sensory (Proprioceptive) Ataxia

This results from dysfunction to position sensing (proprioceptive) nerve inputs. This means that the brain is confused as to the position of limbs.

Sensory Ataxia results in:
 

• Loss of position sense - not knowing exactly where your limbs hands and feet are.
• The inability to detect vibrations.
• An unstable stance - Romberg's sign.

Ataxia Links:

MedStudents - Neurology
Ataxias: Classification
Multiple Sclerosis Glossary: Ataxia
GeneClinics: Ataxia Overview
Movement, uncoordinated - Overview
Sensory: Large Fiber
Effect of Cannabinoids on Spasticity and Ataxia in Multiple Sclerosis

MS Glossary
All About Multiple Sclerosis