Hello everybody!
So today let's learn about examination of subtle hemiparesis, a very important inspectory finding.
Sometimes patients with mild CST (Corticospinal Tract) lesions may have normal strength to routine testing, but the deficit may be brought out using ancillary maneuvers like the examination for pronator drift (Barré’s sign).
With the patient’s upper extremities outstretched to front, palms up and with the eyes closed, we have to observe the position of hands.
Normally patient should hold this position for at least 20 to 30 seconds and the palms will remain straight with the elbows straight, and the limbs horizontal.
Any deviation from this position should be similar on the two sides.
(One exception to the usual symmetry is that the dominant hand occasionally may pronate slightly more than the nondominant, perhaps because the nondominant extremities tend to be more flexible than the dominant extremities, making it more difficult to stretch the dominant hand to a horizontal position.)
However, greater pronation of the nondominant arm is sometimes an indication of subtle hemiparesis.
Three types of drifts may occur when the patient attempts to hold the arms outstretched with eyes closed: pyramidal drift, parietal drift, and cerebellar drift. In pronator drift (Barré’s sign) due to a pyramidal lesion, the arm sinks downward and there is accompanying pronation of the forearm.
In parietal drift, the arm usually rises and strays outward (updrift).
With cerebellar drift, the arm drifts mainly outward, either at the same level, rising, or less often sinking.
The patient with a mild CST deficit may demonstrate “pronator drift” to varying degrees.
Mild drift : there is slight pronation of the hand and slight flexion of the elbow on the abnormal side.
Severe drift : there is more prominent pronation and obvious flexion of the elbow, and there may be downward drift of the entire arm.
Mechanism: Because of the innervation pattern of the CST, the minimally weak CST innervated muscles are overcome by the non-CST muscles.
With a mild CST lesion, the minimally weak muscles in the upper extremity are the extensors, supinators, and abductors. These are overcome by the uninvolved and therefore stronger muscles: the pronators, biceps, and internal rotators of the shoulder. As these overcome the slightly weakened CST innervated muscles, the hand pronates, the elbow flexes, and the arm drifts downward.
The tendency to pronation and flexion in mild hemiparesis has also been attributed to subtle hypertonicity in the pronator and flexor muscle groups.
Imagine what would occur if this motion continued to the extreme: the hand would become hyperpronated, the elbow fully exed, and the shoulder internally rotated, that is, the position of spastic hemiparesis.
The abnormal upper limb positions in minimal pronator drift and in severe spastic hemiparesis are due to the same underlying phenomenon: strong non-CST muscles overcome variably weak CST muscles involved by the disease process.
The examination for pronator drift is a very important part of the neurologic examination. If only one motor test could be done on a patient, the best single test to use would probably be examining for drift.
Hope this was informative!
Let's learn Together!
-Medha.
