# The Effect of L-dopa on Postural Stability in Parkinson's Disease Patients
## 1 Introduction  
The etiology of [PERSON]'s disease is still not determined. The essence of this disorder is to decrease the level of dopamine in the black matter [1; 2]. Progressive and chronic atrophic changes in the Central Nervous System (CNS) that control motor function and degenerative changes in vision, hearing, proprioceptive systems as well as those related to balance, affect postural stability in [PERSON]'s patients. In [PERSON]'s disease patients, progressive failure of the neuromuscular system causes an increase in the thresholds of sensory system activity and uncontrolled muscle stimulation. The balance control system, which include various structures of the central nervous system, may be treated as a control system with three input locations (vestibular, proprioceptive and visual), determining spatial positioning of the body's center of gravity. The main task of the postural stability control system is to maintain an optimal distance between the stability limit and the body's center of gravity projection. If full symmetry of the body is assumed, the center of gravity projection should fall exactly in the center of the support surface [3; 4]. Patients are prone to falls. They suddenly lose balance, ending in a fall (pulsion), especially with sudden head movements [5; 6]. There may be a preference for backward (retropulsion), forward (propulsion) or lateral (lateropulsion) pulsion. Increases in the time of doublesupport and reduced walking speed occur [7; 8]. The so-called tunnel effect consisting in taking small steps before a narrowing along a path of movement also takes place. Furthermore, the so-called freezing symptom may also appear, i.e., sudden obstruction while walking. Often, the patient cannot lift his/her foot up from the floor to take another step [9; 10]. Sudden immobilization can cause loss of balance [11]. When the movement lock occurs during turning or turning around, it may cause a fall. Walking disorders are the most common cause of falls [12]. L-dopa (Lat. _Levodopum_) is a drug alleviating the symptoms of Parkinson's disease--a natural amino acid, a catecholamine produced in the process of tyrosine hydroxylation which is the resultant of the reaction led by tyrosine hydroxylase [13]. It is a dopamine precursor increasing this neurotransmitter's concentration in the brain. History has confirmed that L-dopa is a unique drug, and up to this day, no other invention has been created that would surpass its power of action [14]. However, in pharmacotherapy, the prototype is still being improved by creating its new form, for example: enteral, inhaled, long-acting L-dopa, etc. The introduction of L-dopa has caused many previously unknown concepts, such as fluctuations, dyskinesias and the spectrum of neuropsychiatric disorders to be discovered. It has definitely extended the survival time of patients [15]. The first stage of [PERSON]'s disease, lasting about 5 years, is mainly characterized by the presence of motor disorders, which are its axial symptoms (bradykinesia and resting tremor and/or muscle stiffness, and/or postural disturbances). In the later period, disabilities and so-called motor complications intensify in the form of shortening the proper response time to dopaminergic treatment (movement fluctuations) and movements in the form of dystonic dyskinesia (associated with lack of drug action) which are involuntary or chorea (associated with excessive drug activity). The therapeutic window of opportunity is narrowed and the time of good motor functioning is shortened. Fluctuations in movement and dyskinesia have complex mechanisms connected both with disturbed pharmacokinetics and pharmacodynamics. Disturbed pharmacokinetic mechanisms are mainly problems with the absorption of L-dopa and competition with amino acids during its passage from the intestines to the blood and across the blood-brain barrier. At a pharmacodynamic level, these are changes in the amount and sensitivity of dopamine receptors in the striatum.