Hypotonia due to nerve involvement has few distinguishing characteristics (Figure 134.1). Dynamic tone is usually decreased. Hypotonia and weakness occur to a similar degree (Figure 134.1). Large thick nerves (auricular nerve hypertrophy) may be present. Signs of peripheral nervous system sympathetic autonomic dysfunction may be present. Laboratory investigations are often needed to diagnose hypotonia due to nerve problems. Nerve conduction velocity, especially sensory nerve conduction velocity, may be decreased. H-reflexmay demonstrate slow proximal nerve conduction prior to the presence of distal slow nerve conduction velocity in routine nerve conduction studies. Slow sensory nerve conduction velocity is the major electrophysiologic clue to the diagnosis of peripheral nerve disease but it is not always present. Electromyography shows fibrillations. Motor unit potentials tend to be normal or slightly prolonged and large. Cerebrospinal fluid protein may be elevated. Autonomic responses to electrical and chemical stimulation may be abnormal. Nerve biopsy shows myelin or axonal abnormalities or both.
Figure 134.1.— Salient features of generalized hypotonia due to nerve disease. Arrow indicates the anatomical location of the injury; S & M: sensory and motor; SNAPS: sensory nerve action potentials; EMG: electromyogram; CSF: cerebrospinal fluid; PR: protein; WBC: white blood cells; IC: intracutaneous.
Diseases that involve the peripheral nerve in the neonatal period are:
(1) neuronal-axonal disease not associated with Werdnig-Hoffmann disease,
(2) giant axonal neuropathy, (3) infantile porphyria, (4) congenital
sensory neuropathy with anihidrosis, (5) congenital hypomyelinative
neuropathy, (6) Riley-Day syndrome, (7) acute polyneuropathy, (8) chronic
inflammatory demyelinating polyneuropathy, and (9) congenital sensory
neuropathy. Neuropathy can be classified as axonal, hypomyelinative,
or demyelinating, and as sensory, motor, sensory-motor, or sensory-autonomic.