Neuroscience Clerkship at UH/VA



The Floppy Infant Syndrome (FIS) is a common problem encountered in pediatric neurology. It is characterized by prominent generalized hypotonia.

The examination for hypotonia is first carried out with the infant at rest, the head in midline position and the limbs moved passively. The truncal tone is assessed by suspending the infant with the abdomen held on the examiner’s hand (see figure above - right insert). While a normal full-term infant assumes a posture with straightened back, flexed limbs, and straight head, the hypotonic infant will drape over the examiner’s hand. Tone in the neck can be further assessed with the baby in the supine position and gently lifting up the baby by pulling both arms at the same time (see figure above). The hypotonic infant will generate little or no resistance to gravity and the head will assume a fully extended position. The shoulder girdle tone may be assessed by suspending the infant vertically while supporting the axillae. Full-term infants will adduct the arms and fix the shoulders, whereas hypotonic infants tend to slip through the examiners’ hands.


Differential Diagnosis

The differential diagnosis of FIS includes a wide variety of causes, which may be related to central nervous system (CNS), anterior horn cell, peripheral nerve or muscle disorders, as well as connective tissue disorders or systemic illness.

Common causes of the floppy infant syndrome

● Central nervous system disorders

● Neuromuscular Disorders

● Connective tissue disorders

● Systemic illnesses

● Maternal medications

● Benign congenital hypotonia

Among neurologic causes, both central and peripheral nervous system disorders may be responsible (Algorithm). Approximately 80% of floppy infants have primary acute or chronic central nervous system (CNS) disorder. Increased tendon reflexes, seizures, obtundation, or delayed intellectual and language milestones suggest a CNS disorder.

Acute CNS diseases causing FIS include perinatal hypoxia, sepsis and meningitis, birth trauma, intracranial hemorrhages, intoxication, drug withdrawal, inborn errors of metabolism, and acquired metabolic disturbances of glucose, calcium and electrolytes. Hypoxic-ischemic encephalopathy is by far the most common cause of all hypotonia in newborns. Acute CNS diseases such as birth trauma, intraventricular hemorrhage or asphyxia may have prominent hypotonia and decreased reflexes early in life, which change to hyperreflexia in early childhood.

Chronic CNS diseases associated with FIS include neural tube defects, microcephaly, cerebral and cerebellar malformations, congenital infections, cerebral lipidoses, chromosomal disorders and dysmorphic syndromes. Spinal cord malformations or injuries such as seen in a difficult breech delivery may be associated with hypotonia, weakness and a sensory level, with normal muscle strength and tone above the involved level. Among the dysmorphic syndromes known to be associated with hypotonia are Prader-Willi syndrome, Cohen syndrome, Zellweger syndrome, Lowe syndrome and Down syndrome. In many of these syndromes, hypotonia is an early manifestation at birth, and only with time will children develop the more classic signs of the syndromes.

Approximately 20% of patients with FIS have neuromuscular diseases including anterior horn cell disorders (e.g., spinal muscular atrophy), neuropathies, neuromuscular junction disorders and myopathies. The infant with lower motor neuron disorder usually has profound weakness proportional with the severity of hypotonia, decreased or absent tendon reflexes, and abnormalities on muscle enzymes, EMG studies, or muscle biopsy.

Neuromuscular Disorders Associated with Floppy Infant Syndrome

● Anterior horn cell disorders

Spinal muscular atrophy (SMA)
Glycogen storag (Pompe’s disease)
Enterovirus Infection (e.g., coxsackie, polio)

● Peripheral neuropathies

Guillain-Barré Syndrome
Inherited Neuropathies (e.g., Dejerine-Sottas disease)

● Neuromuscular junction disorders

Infantile botulism
Neonatal myasthenia
Congenital myasthenic syndromes

● Myopathies

Congenital myopathies
Metabolic myopathies
Mitochrondrial myopathies
Muscular dystrophies

Clinical features of Upper and Lower Motor Neuron Etiologies for the Floppy Infant Syndrome

Upper Motor Neuron Lower Motor Neuron
Hypotonia in infancy and spasticity in childhood Hypotonia
Mild weakness or normal strength Significant weakness
Increased tendon reflexes Decreased or absent tendon reflexes
Abnormal reflexes (Babinski signs, ankle clonus) No abnormal reflexes
No muscle atrophy Muscle atrophy (may not be present in infancy)
Delayed developmental milestones Only motor skills delayed
Signs and symptoms of CNS involvement No CNS involvement