Paramount to determining
the anatomical diagnosis is the distinction between upper motor neuron
system and motor-sensory unit dysfunction. This distinction can often
be made based on the evaluation of the dynamic tone. Dynamic tone refers
to the response of the striated muscles to being stretched by a brief
high-intensity force. The dynamic tone is evaluated
by determining: (1) the resistance the limbs offer to the examiner’s
effort to quickly extend them, and their speed of recoil after such
a maneuver; (2) the characteristics of the Moro and stretch muscle reflexes;
and (3) the presence of clonus.
Figure 98.1.— Schematic representation of the possible sites of neuromuscular damage in neonates with generalized hypotonia and increased dynamic tone. The symbols in the rectangular box depict the presence (in green background) and the absence (in red background) of important neurological findings. These findings help to localize the site of damage in hypotonic neonates: (1) brain; (2) brainstem; and (3) rostral spinal cord.
The limbs of neonates with decreased dynamic tone offer little resistance to the examiner’s effort to quickly extend them and they have poor recoil. Moro and muscle stretch reflexes are diminished in neonates with hypotonia and decreased dynamic tone. Clonus is not present. Neonates with decreased dynamic tone are weak. Cortical thumbs are not present.