Overview of peripheral nerve and muscle disorders causing hypotonia in the newborn
- Olaf A Bodamer, MD, PhD, FAAP, FACMG
Olaf A Bodamer, MD, PhD, FAAP, FACMG
- Park Gerald Chair in Genetics and Genomics
- Associate Chief, Genetics and Genomics
- Boston Children’s Hospital/Harvard Medical School
- Geoffrey Miller, MD
Geoffrey Miller, MD
- Professor of Pediatrics and Neurology
- Yale University
- Section Editors
- Marc C Patterson, MD, FRACP
Marc C Patterson, MD, FRACP
- Section Editor — Pediatric Neurology
- Professor of Neurology, Pediatrics, and Medical Genetics
- Chair, Division of Child and Adolescent Neurology
- Mayo Clinic College of Medicine
- Richard Martin, MD
Richard Martin, MD
- Section Editor — Neonatology
- Professor, Pediatrics, Reproductive Biology, and Physiology & Biophysics
- Case Western Reserve University School of Medicine
Newborns with neuromuscular disorders often present with hypotonia and weakness. (See "Approach to the infant with hypotonia and weakness".)
These disorders are caused by a variety of conditions that affect the central nervous system (brain or spinal cord), peripheral nervous system, or skeletal muscle . Conditions that affect the last two sites are reviewed briefly here (table 1). More complete discussions of these conditions and of disorders caused by upper motor neuron lesions that involve the descending motor tracts within the brain and spinal cord and of other systemic conditions (eg, Prader-Willi syndrome, Angelman syndrome, multiple minor congenital anomaly syndromes, or inborn errors of metabolism) are presented elsewhere. (See "Clinical features, diagnosis, and treatment of Prader-Willi syndrome" and "Inborn errors of metabolism: Classification" and "Inborn errors of metabolism: Metabolic emergencies" and "Inborn errors of metabolism: Epidemiology, pathogenesis, and clinical features".)
ANTERIOR HORN CELL DISORDERS
Many conditions are caused by degeneration of the anterior horn cells of the spinal cord. They include spinal muscular atrophy, traumatic myelopathy, hypoxic-ischemic myelopathy, and neurogenic arthrogryposis.
Spinal muscular atrophy — The most common and severe type that presents in newborns is spinal muscular atrophy (SMA) type 1, also known as Werdnig-Hoffmann disease. This condition should be suspected in infants with diffuse symmetric proximal muscle weakness that is greater in the lower than upper limbs and absent or markedly decreased deep tendon reflexes. SMA types 2 and 3 have later onset and are less severe. (See "Spinal muscular atrophy".)
Traumatic myelopathy — Myelopathy caused by trauma to the high cervical spinal cord is a rare cause of hypotonia in infants. This condition results in a flaccid paralysis, which may be asymmetric, and absent reflexes. Physical examination may reveal evidence of trauma, such as bruising or fractures. If no accompanying brain injury is present, the infant will be alert with no cranial nerve abnormalities. A pin prick on the face will elicit a facial grimace but no response below the neck. A useful sign is withdrawal to a noxious stimulus of a limb with no spontaneous activity. Bladder distension, priapism, and absence of sweating below the level of the spinal lesion typically will appear as the myelopathy evolves over several days.
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- ANTERIOR HORN CELL DISORDERS
- Spinal muscular atrophy
- Traumatic myelopathy
- Hypoxic-ischemic myelopathy
- Arthrogryposis multiplex congenita
- CONGENITAL NEUROPATHIES
- Congenital motor sensory neuropathies
- Hereditary sensory and autonomic neuropathies
- NEUROMUSCULAR JUNCTION DISORDERS
- CONGENITAL MYOPATHIES
- MUSCULAR DYSTROPHIES
- Congenital muscular dystrophies
- Facioscapulohumeral muscular dystrophy
- Congenital myotonic dystrophy
- INBORN ERRORS OF METABOLISM
- Disorders of glycogen metabolism
- Primary carnitine deficiency
- Peroxisomal disorders
- Disorders of creatine metabolism
- Mitochondrial myopathies
- - Cytochrome c oxidase deficiency