We are in the presence of optic ataxia when there is a pronounced imprecision in movements linked to the intent to reach an object positioned at a precise point in the visual field with one or both upper limbs; this occurs in the absence of a motor or sensory deficit that could in some way explain this symptom.
In the presence of this problem, one retains the ability to locate the object in space, but is unable to interact with it from a motor point of view. It is important to emphasize that this type of problem is neither exclusively visual nor purely motorized; rather, we are talking about a lack of coordination between these two fields.
This type of problem can be confused with neglect due to a certain similarity of symptoms, except for one fundamental difference: in optic ataxia the patient, despite not being able to grasp or touch an object, remains aware of its presence, which is not the case with neglect.
A range of dysfunctions may be present, including
- inability to rotate or shape the hand when approaching the object;
- difficulty in correcting voluntary movements by adapting to a change in the target position.
- Forms of ataxia
This problem may involve only one of the hemicampi or the entire visual field According to this distinction we speak of:
- unilateral ataxia;
- bilateral ataxia.
A further distinction can be made to describe whether this disorder involves the limb ipsilateral or contralateral to the lesion:
- in the first case we speak of direct ataxia;
- in the second one of a crossed ataxia.
Causes of optic ataxia
The cause of this problem can be found in a compression of the occipitofrontal fibers directed to the motor areas. In the case of unilateral ataxia, the pressure is only located in one of the two fibrous pathways, and vice versa when we are in the presence of bilateral ataxia, the lesion involves both pathways.
In humans, a lesion of the left hemisphere results in ataxia of the right limb for both visual fields and visual ataxia of the left limb concerning the contralateral hemicampus. Conversely, a right-hemisphere lesion results in bilateral left-hemicampus visual ataxia.
Although lesions to a single hemisphere are the main cause of the onset of this problem, there are cases in which the brain damage is more extensive: this is the case when ataxia arises as part of Balint’s syndrome (a rare neurological disorder in which optic ataxia, ocular apraxia, and simultanagnosia secondary to posterior parietal lobe lesions are present).
Types of deficits caused by optic ataxia
The movement consists of two components:
- proximal: the part that affects the transport phase, that is, that which concerns the level of precision of the movement;
- distal: preparation phase that the hand puts into action during its approach to the object.
Ataxia can affect each of these two components, thus impairing the motor capabilities of the limbs.
How to diagnose optic ataxia
One way to diagnose this problem is to subject the patient to a test: an object (such as a pen) is presented on the right or left side of the patient’s visual field; the patient is asked to grasp it with his or her hand.
Two main errors are highlighted when performing this exercise
- both limbs make significant errors in performing movements on the side contralateral to that of the damaged area;
- the limb contralateral to that of the injured area makes coordination errors on both sides.
If these errors are shown not to be due to specific vision or movement disorders, then optic ataxia is diagnosed.
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