Two approaches to the neonatal metabolic disorders are possible—one, to screen every newborn, using a battery of biochemical tests of blood and urine, and the other, to undertake in the days following birth a detailed neurologic assessment that will detect the earliest signs of these diseases. Unfortunately, not all the biochemical tests have been simplified to the point where they can be adapted to a mass screening program, and many of the commonly used clinical tests at this age have yet to be validated as markers of disease. Moreover, many of the biochemical tests are costly, and practical issues, such as cost-effectiveness, insinuate themselves, to the distress of the pediatrician. The introduction of tandem mass spectrometry for the evaluation of blood and urine has allayed some of the latter concerns. The extent of screening varies across countries. In the U.S. screening includes several disorders relevant to neurologic development including the categories of endocrine (particularly hypothyroidism), amino acid, organic acid, fatty acid, mucopolysaccharide, and other disorders. A hearing screening test is also usually mandatory before an infant is discharged. These tests are organized and operated at the state level.
Neurologic Assessment of Neonates With Metabolic Disease
As pointed out in Chap. 27, the neonate’s nervous system functions essentially at a brainstem–spinal level. The pallidum and visuomotor cortices are only beginning to be myelinated and their contribution to the totality of neonatal behavior cannot be very great. Neurologic examination, to be informative, must therefore be directed to evaluating diencephalic–midbrain, cerebellar–lower brainstem, and spinal functions. The integrity of these functions in the neonate is most reliably assessed by noting the following, as was described in Chap. 27: 1. Control of respiration and body temperature; regulation of thirst, fluid balance, and appetite–hypothalamus–brainstem mechanisms 2. Certain elemental automatisms, such as sucking, rooting, swallowing, grasping—brainstem–cerebellar mechanisms 3. Movements and postures of the neck, trunk, and limbs, such as reactions of support, extension of the neck and trunk, flexion movements, and steppage—lower brainstem (reticulospinal), cerebellar, and spinal mechanisms 4. Muscle tone of limbs and trunk—spinal neuronal and neuromuscular function 5. Reflex eye movements—tegmental midbrain and pontine mechanisms (a modified optokinetic nystagmus can be recognized by the third day of life) 6. The state of alertness and attention (stimulus responsivity and capacity of the examiner to make contact) as well as sleep–waking and electroencephalographic patterns—mesencephalic–diencephalic mechanisms 7. Certain reflexive reactions such as the startle (Moro) response and placing reactions of the foot and hand—upper brainstem–spinal mechanisms with possible cortical facilitation
Derangements of these functions are manifest as impairments of alertness and arousal, hypotonia, disturbances of ocular movement (oscillations of the eyes, nystagmus, loss of tonic conjugate deviation of the eyes in response to vestibular stimulation, i.e., to rotation of the upright infant), failure to feed, tremors, clonic jerkings, tonic spasms, opisthotonos, diminution or absence of limb movements, irregular or chaotic breathing, hypothermia or poikilothermia, bradycardia, circulatory difficulties, poor color, and seizures.