Astasis is a lack of motor coordination marked by an inability to stand, walk or even sit without assistance due to disruption of muscle coordination.

The term astasia is interchangeable with astasis and is most commonly referred to as astasia in the literature describing it. Astasis is the inability to stand or sit up without assistance in the absence of motor weakness or sensory loss (although the inclusion of 'the lack of motor weakness' has been debated by some physicians). It is categorized more as a symptom than an actual disease, as it describes a disruption of muscle coordination resulting in this deficit. The disturbance differs from cerebellar ataxia in that with astasis the gait can be relatively normal, with balance significantly impaired during transition from a seated to standing position. This balance impairment is similar to patients with vestibulocerebellar syndrome, which is a progressive neurological disease with many symptoms and effects.

Astasis has been seen in patients with diverse thalamic lesions, predominantly affecting the posterior lateral region of the brain. It is most frequently accompanied by abasia, although not always. Abasia is a symptom very similar to it and is the inability to walk. The two are most commonly seen in astasia-abasia, which is also called Blocq's disease. It is more common for astasia and abasia to be seen together than it is to see either one or the other.

Since astasis itself is more a symptom than a disease, it is more often seen associated with other signs and symptoms. People who have astasis often experience

One study described a patient with astasis as lying in bed with a normal body posture. When the patient was sitting, he tilted his body to the left. When he was asked to stand up, the patient rotated his trunk axis to the left (left shoulder going backwards), and tilted his body to that same side, showing resistance to passive correction of posture in both of these planes. He was unable to stand and fell backwards and towards the left.

There are many speculations as to what is the main cause of astasis. A combination of weakness of the triceps surae muscle, peripheral neuropathy, and irregular postural movements are the leading theories thus far. Diabetes, spinal root or spinal cord lesions, and traumas or injuries to the motor cortex of the brain can also cause similar symptoms that are seen in patients with astasis. However, none of these have been proven definite. Many scientists also believe that this is a conversion disorder, in which patients exhibit symptoms without any neurological cause.

Damage to the peripheral nerves coming from the legs to the somatosensory area is the leading candidate for the cause of astasis. These damaged nerves prevent feedback for stabilization of posture for patients with astasis. This causes a disturbance in postural movements, such as a swaying around the legs and hip joints. This swaying may be seen only when a patient in standing still, and may disappear before walking, indicating that these patients exhibit astasia without abasia. This impairment of sensation is not always required for the sensation to develop. However, impairment often worsens astasia.

Weakness of the triceps surae muscle has been seen in many patients who have been diagnosed with astasis. This weakness can be caused by a myopathy to that muscle group. The bilateral triceps surae muscle, made up of the gastrocnemius and the soleus, is essential to maintain a straight posture while standing. This indicates that weakness to this muscle is the cause of the swaying and impaired posture in patients with astasis. This weakness is seen regardless of whether somatosensory feedback from the legs is impaired, suggesting it is one of the main causes of astasia without abasia.