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.
The limbs of neonates with increased dynamic tone offer increased resistance to the examiner’s effort to quickly extend them, and have a quick recoil once the limb is released. Moro reflex is exaggerated, muscle stretch reflexes are increased, radiating stretch reflexes (cross adductor response) are evident, and clonus may be present in the limbs and jaw of neonates with hypotonia and increased dynamic tone. Neonates with hypotonia and increased dynamic tone do not give the appearance of weakness because of the forceful displacement of the limbs during these maneuvers. Neonates with hypotonia and increased dynamic tone often have cortical thumbs.
The site of pathology in neonates with hypotonia and increased dynamic tone is the upper motor neuron: either in the brain, brainstem, rostral spinal cord, or a combination of these sites. The distinction among brain, brainstem, or spinal cord hypotonia can often be made based on associated neurological findings such as seizures, weakness of facial muscles, increased facial dynamic tone, parasympathetic pupil abnormalities, lack of bowel movements, and anal sphincter weakness (Figure 98.1).

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.