Infantile neuroaxonal dystrophy (INAD) is an extremely rare, inherited degenerative disorder of the nervous system characterized by abnormalities of nerve endings (axon terminals) within the brain and spinal cord (central nervous system) and outside the central nervous system (peripheral nerves and terminals). In most cases, infants and children with INAD appear to develop normally until approximately 14 to 18 months of age, when they may begin to experience progressively increased difficulties in walking. In other cases, symptoms may begin at approximately six to eight months of age, at which time infants may experience delays or an arrest in the acquisition of skills requiring the coordination of mental and physical activities (delayed psychomotor development).
The symptoms and physical characteristics associated with infantile neuroaxonal dystrophy are the result of swelling and degeneration of individual nerve endings (dystrophic axonal swellings or "spheroids") within and outside the brain and spinal cord (central nervous system). In most cases, INAD is inherited as an autosomal recessive genetic trait.
Infantile neuroaxonal dystrophy is characterized neuropathologically by abnormalities of nerve endings (axons) within the brain and spinal cord (central nervous system or neuroaxis) and outside the central nervous system (peripheral nerves), resulting in loss of proper nerve function. Nerve terminals represent the sites of neuron-to-neuron or neuron-to-target communication.
In most cases, infants and children with the disorder appear to develop normally until approximately 14 to 18 months of age, when they may begin to experience progressively increased difficulties in walking, demonstrating unsteadiness and/or a tendency to fall. Few affected children attain the ability to walk completely independently before such symptoms appear. In other cases, the disorder may become apparent at approximately six to eight months of age, at which time affected infants may experience delays or an arrest in the acquisition of skills requiring the coordination of mental and physical activities (delayed psychomotor development). Affected children may then begin to lose previously acquired skills (psychomotor regression) such as the ability to sit, stand, or vocalize sounds and words. Affected children also demonstrate progressive neuromuscular impairment characterized by generalized muscle weakness, severely diminished muscle tone (hypotonia), abnormally exaggerated reflex responses (hyperreflexia), and/or unusually weak, depressed, or absent reflexes (areflexia). Affected children experience progressive mental retardation in association with gradual motor impairment. In addition, in infants and children with INAD, the head and brain may cease growing at the normal rate after the onset of symptoms. As a result, although the head circumference is normal at birth, it may appear moderately reduced (microcephaly) after symptoms become apparent.
According to the medical literature, infants and children with INAD rarely experience seizures, such as brief, shock-like muscle spasms of the arms, legs, or entire body (myoclonic seizures) or episodes of uncontrolled electrical disturbances in the brain that cause convulsive seizures (epilepsy). However, some affected infants and children may experience involuntary movements of the face and hands and/or sudden involuntary muscle spasms (spasticity) of the lower arms and legs (limbs). In addition, in some cases, affected children may also experience a diminished sensitivity to pain in the lower limbs and trunk and/or progressive paralysis of the legs and the lower portion of the body (paraplegia). Many also experience abnormal retention of urine.
As the disorder progresses, affected children may exhibit involuntary, rapid, side-to-side movements of the eyes (pendular nystagmus); crossing of the eyes (strabismus); and gradual deterioration of the nerves of the eyes (optic atrophy), progressing to blindness. Visual impairment may occur from approximately 10 months to two years after the initial onset of symptoms. As neurological deterioration progresses, affected children may also experience hearing impairment; progressive disorientation and loss of intellectual function (dementia); impaired response to touch (tactile stimulation); uncontrolled, rigid extensions and rotations of the arms, legs, fingers, and toes due to progressively degenerative brain abnormalities (decerebrate rigidity); and increased susceptibility to repeated infections of the respiratory tract. Life-threatening complications may develop by the end of the first decade.
Researchers have reported rare instances in which symptoms associated with INAD have begun before or soon after birth (prenatal or neonatal). According to the medical literature, such cases are often referred to as “Prenatal or Connatal Neuroaxonal dystrophy.” As opposed to classical INAD, affected infants are more likely to experience seizures and more rapid progression of motor and neurological impairment and life-threatening complications. According to the medical literature, such cases may also be characterized by abnormalities of the hypothalamus, the region of the brain that activates and coordinates certain hormonal (endocrine) functions, body temperature, and several other bodily functions. For example, affected infants may experience chronic fevers, suggesting abnormalities of temperature regulation. In addition, abnormally decreased activity of the thyroid gland and underproduction of thyroid hormones (hypothalamic hypothyroidism) may contribute to generalized weakness, growth delays, and/or psychomotor retardation. The hypothalamus also coordinates the function of the pituitary gland, a small structure at the base of the brain, by controlling the gland’s release of certain hormones. In affected infants with “Prenatal or Connatal Neuroaxonal dystrophy,” the pituitary gland may fail to release sufficient levels of a certain hormone (antidiuretic hormone [ADH]). This hormone helps to regulate normal urine production. As a result, affected infants and children may have diabetes insipidus, a disorder characterized by excessive excretion of urine (polyuria) and excessive thirst (polydipsia).
In most cases, INAD is inherited as an autosomal recessive trait. Researchers have determined that disruptions or changes (mutations) of the PLA2G6 gene cause INAD in a large number of affected individuals. The PLA2G6 gene is located on chromosome 22 (22q13.1).
Chromosomes, which are present in the nucleus of human cells, carry the genetic information for each individual. Human body cells normally have 46 chromosomes. Pairs of human chromosomes are numbered from 1 through 22 and the sex chromosomes are designated X and Y. Males have one X and one Y chromosome and females have two X chromosomes. Each chromosome has a short arm designated “p” and a long arm designated “q”. Chromosomes are further sub-divided into many bands that are numbered. For example, “chromosome 22q13.1″ refers to band 13.1 on the long arm of chromosome 22. The numbered bands specify the location of the thousands of genes that are present on each chromosome.
Researchers believe that the PLA2G6 gene carries instructions to create (encode) an enzyme that breaks down certain fats known as lipids. When the PLA2G6 gene is mutated the breakdown of lipids is affected, which may result in the excess accumulation of membranes in the nerve terminal and, ultimately, in a build up of iron in the brain.
Genetic diseases are determined by the combination of genes for a particular trait that are on the chromosomes received from the father and the mother. Recessive genetic disorders occur when an individual inherits the same abnormal gene for the same trait from each parent. If an individual receives one normal gene and one gene for the disease, the person will be a carrier for the disease, but usually will not show symptoms. The risk for two carrier parents to both pass the defective gene and, therefore, have an affected child is 25 percent with each pregnancy. The risk to have a child who is a carrier like the parents is 50 percent with each pregnancy. The chance for a child to receive normal genes from both parents and be genetically normal for that particular trait is 25 percent. The risk is the same for males and females.
All individuals carry a few abnormal genes. Parents who are close relatives (consanguineous) have a higher chance than unrelated parents to both carry the same abnormal gene, which increases the risk to have children with a recessive genetic disorder.
Pathological findings suggest that the symptoms and physical characteristics associated with INAD are in some complex way related to the characteristic swellings at the end of the nerve fibers (axons). These are known as dystrophic axonal swellings or “spheroids”. These spheroids appear to block transmission of nerve impulses between nerve cells (synaptic transmissions), which leads to the loss of proper nerve function. Both myelinated and nonmyelinated nerve endings (axons) in certain areas (focal axonal swelling) may be affected. Axons carry nerve impulses away from nerve cell bodies. Many have a sheath of myelin, a fatty substance that forms a protective covering around certain nerve fibers, which serves as an electrical insulator that protects the nerve and allows for the effective transmission of certain nerve signals.
In many cases of INAD, such swellings and the degeneration of nerve endings (spheroids) are found to be widely distributed in the outer layer of the brain (cerebral cortex); the lowest part of the brain (brain stem); the spinal cord; and the peripheral nerves that extend from the brain and spinal cord to various areas of the body (e.g., the skin, the conjunctiva of the eyes) The brain stem serves as a “gateway” for messages passing between the spinal cord and other areas of the brain and also contains neurons which project to various structures in the head (for example, eye, jaw and facial muscles, tongue). The basal ganglia help to produce smooth coordinated movements. In some cases, the INAD spheroids have also been widely distributed in nerve endings (axons) of the hypothalamus; the back portion (posterior lobe) of the pituitary gland (neurohypophysis), including the short stalk of nerve fibers that connects the pituitary gland to the hypothalamus (“pituitary stalk”).
Infantile neuroaxonal dystrophy is an extremely rare genetic disorder and the incidence and prevalence is not known with any certainty. One source suggests that the prevalence is 1 per 200,000 children.
INAD is diagnosed during the first years of life by means of a thorough clinical evaluation, a detailed patient history, identification of characteristic physical findings, and a variety of specialized tests. The presence of a defect in the PLA2G6 gene provides a diagnosis in 80% of patients with INAD. In addition, microscopic examination (i.e., light and electron microscopy) of samples (biopsies) of peripheral nerve tissue from the skin or the transparent membrane covering the whites of the eyes (conjunctivae) may reveal swelling and degeneration of nerve endings (dystrophic axonal swellings or “spheroids”).
Advanced imaging techniques may also be used to detect, confirm, and/or characterize specific abnormalities of the central nervous system that may be associated with INAD. Magnetic resonance imaging (MRI), which uses a magnetic field and radio waves to create cross-sectional images of the brain, may reveal progressive, generalized degeneration (diffuse atrophy) of tissue of the outer portion of the cerebellum (cerebellar cortex). Affected infants with prenatal or connatal neuroaxonal dystrophy may tend to exhibit less marked degeneration of the cerebellum. Specialized imaging tests may also reveal degeneration of the portion of the brain that helps pass messages between the spinal cord and other areas of the brain (brain stem) and varying degeneration of the nerve tracts of the spinal cord that pass sensory information up toward the brain (ascending tracts) and pass motor signals downward toward the trunk, arms, and legs (descending tracts).
In infants and children with INAD, specialized testing may also be conducted to evaluate electrical activity (electrophysiological studies) of certain areas of the body. For example, electromyograms (EMG), tests that record electrical activity in skeletal muscles at rest and during muscle contraction, may indicate that, although nerve signals may be transmitted at normal speed to muscles (normal nerve conduction velocities), there is failed communication of such signals to the muscles (denervation). Electroencephalography (EEG), which records the brain's electrical impulses, may reveal abnormally high amplitude and fast activity within brain wave patterns. INAD results in a decreased or absent response when the eye is stimulated by light (visually evoked potential [VEP]), confirming defects in optic pathways in the brain.
The treatment of infantile neuroaxonal dystrophy is directed toward the specific symptoms that are apparent in each individual. Treatment may require the coordinated efforts of a team of specialists. Pediatricians, physicians who diagnose and treat neurological disorders (neurologists), eye specialists (ophthalmologists), and/or other health care professionals may need to systematically and comprehensively plan an affected child's treatment.
The treatment of INAD is symptomatic and supportive. Thorough preventive measures may be taken to help prevent respiratory tract infections. Medications that help treat such infections (e.g., antibiotic therapy) and other supportive measures may be used to aggressively treat such infections should they occur. Genetic counseling will be of benefit for affected individuals and their families.
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Contact for additional information about infantile neuroaxonal dystrophy:
Robert E. Schmidt, M.D., Ph.D.
Professor of Pathology & Immunology
Department of Pathology and Immunology
Division of Neuropathology
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