The levodopa test is a highly reliable diagnostic test used to determine whether motor symptoms such as tremors in the limbs, rigidity in the muscles, and problems with walking, balance and fine motor coordination, are caused by Parkinson’s disease.
Clinical basis of the levodopa test
Parkinson’s disease is difficult to diagnose, especially in the early stages. Several neurological diseases such as atypical Parkinsonian disorders show similar symptoms to Parkinson’s disease, but have different causes. There is no single test that can definitively identify Parkinson’s disease, and 5 to 10 percent of patients are misdiagnosed. Up to 20 percent of patients who are diagnosed with Parkinson’s disease are later found to have other conditions.
Parkinson’s disease is characterized by low levels of a neurotransmitter (a cell-signaling molecule) called dopamine in the brain. This is caused by the death of dopaminergic (dopamine-producing) neurons in a region of the brain called the substantia nigra, which regulates motor functions and normal physical activity. This causes progressive deterioration of motor functions in Parkinson’s disease patients resulting in typical motor symptoms (tremors, rigidity, problems with walking or balance, and fine motor coordination).
Levodopa is the precursor of dopamine that, unlike dopamine, can cross the blood-brain barrier. Once inside the brain, it is converted into dopamine by the enzyme, dopa decarboxylase (also called amino acid decarboxylase) and replenishes dopamine levels, thereby restoring or improving motor function. That is why patient response to levodopa can be used to distinguish Parkinson’s disease from other neurological conditions.
How the levodopa test is conducted
The levodopa test is given at least eight hours after the patient’s last dose of any medication to boost dopamine levels in the brain (levodopa or dopamine agonists) and usually takes place in the morning. Motor functions are analyzed before the test and again 60 to 90 minutes after taking levodopa using part 3 of the unified Parkinson’s disease rating scale (UPDRS).
Motor function parameters analyzed include:
- Speech (volume and diction)
- Facial expressions
- Finger and toe-tapping
- Hand movements
- Leg agility
- Degree of difficulty while rising from a chair
- Gait (shuffling, difficulty walking, or freezing of gait)
- General posture
- Postural instability (difficulty recovering balance)
- Global spontaneity of movement
- Tremor at rest.
If the scores of these parameters show significant improvement after taking levodopa, this indicates Parkinson’s disease. If the scores remain the same before and after taking levodopa, it suggests a non-Parkinson’s neurological disease.
Evaluation of the levodopa test
Patient symptoms before and after administration of levodopa are scored using the UPDRS scoring system. The UPDRS evaluation has four parts:
- intellectual function
- mood, and behavior
- activities of daily living
- motor functions and complications
Each of the four parts has multiple parameters that are scored individually, using zero for normal or no problems, 1 for minimal problems, 2 for mild problems, 3 for moderate problems, and 4 for severe problems. These scores are added to determine the severity of the disease. A score of 199 points is the worst and means total disability; a score of 0 means no disability. In the case of the levodopa test, motor improvement observed as a change in part 3 of the UPDRS is commonly referred to as the “levodopa response.”
The levodopa test has an accuracy of 80-83 percent according to several clinical studies. The diagnostic accuracy can be further improved by taking into account the medical history and the results of a physical or neurological examination.
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