Craniovertebral (CV) Junction Anomalies
Disorders of this type are a result of the anatomical challenges posed by this unique location in the body, where the skull (cranio) meets the spine (vertebral), as well as the disproportion of head to body weight in the developing infant.
At birth, the brain is typically 25% of its adult size, although the body weight of the newborn is only about 5% of its adult weight. Importantly, about half of the postnatal growth of the brain volume occurs during the first year of life, and attains about 75% of its adult size by the end of the second year.
The circumference of the head at birth is about 13–14 inches. It increases by 17% during the first 3 months of life, and by 25% at 6 months of age. Head circumference increases markedly during the first postnatal year due to the progressive and rapid growth of the brain as a whole. The important relation of brain size and cranium size can be demonstrated on a percentage basis, which shows that 70% of the adult brain weight is achieved at 18 months, 80% at 3 years, 90% at 5–8 years and approximately 95% at the 10th year. By contrast, genital organs develop very slowly during this period but, instead, reach their adult size during the second decade of life.
Infants and children are not miniature adults and their anatomy differs from the adult in a number of ways. The growth of a child is dependent upon the normal activity of growth centers and the protection of these centers are vital. Abnormalities of body stature and limb mobility might result from injury to these growth centers.
The brain and spinal cord are protected by bony structures: the skull and spinal column.
There are three main parts of the brain: the cerebrum, cerebellum, and brain stem.
- Cerebrum controls higher brain functions and has four lobes, each with different functions:
- Frontal lobe: executive function, decisions, personality, language, planning, movement
- Parietal lobe: intelligence, reasoning, sensation, reading
- Occipital lobe: vision
- Temporal lobe: language, behavior, hearing, vision, emotions, memory
- Cerebellum controls balance, coordination, and fine muscle control
- Brain Stem controls things we do without thinking, like our heartbeat, breathing, and swallowing
- Foramen Magnum – a funnel-like opening to the spinal canal
- The Spinal Cord starts at the brain stem and goes to the tail bone. It is about as wide as your little finger and extends the length of your back. Messages are carried from the spinal cord to and from the brain to the rest of the body
- Meninges are membranes that cover and protect the brain and spinal cord. There are three layers of meninges: Dura mater (closest to the bone), Arachnoid is loosely around the brain, Pia mater is closely attached to the brain and spinal cord surface
- Cerebrospinal Fluid (CSF) cushions the brain and spinal cord. It is a clear, water-like fluid, also called spinal fluid, found between the arachnoid and pia mater. The brain and spinal cord are bathed and cushioned by this spinal fluid, which flows and circulates around the brain and spine
A variation in the anatomical structure at the craniovertebral junction can lead to compression of the spinal column, which, in turn, may cause damage to the spine. These variations can come about 1. congenitally, 2. develop over time, or 3. due to trauma, tumor or infection.
If the cause of the CV malfunction is congenital, it is generally referred to by its location:
Malformations of Occiput
Occiput is another term for the skull. The basi occiput forms the lower portion and the upper portion is formed by the basisphenoid, separated from the basi occiput by the sphenooccipital synchondrosis. Most occipital anomalies are associated with decreased skull base height and basilar invagination. Occipital malformations cause a variety of visual disturbances and headaches. Early diagnosis may lead to treatment that reduces the risk of hemorrhages, visual field loss and other neurological deficits, and death.
Malformations of the Atlas (C1)
The C1 atlas is the uppermost cervical vertebra of the spine. It supports the head and — along with the C2 axis, the cervical vertebra just below it — allows the head to nod and rotate from side to side. At birth, children have softer bones. That is true of the C1 atlas, as well. C1 ossification (solid, hard bone) begins in fetal life and continues to the third or fourth year of life. Children’s heads are larger, with respect to their body size, than adults. Due to this mismatch, young patients have increased chances of spine injuries throughout the cervical spine, but especially between the skull (occiput) and the upper cervical spine.
Malformations of the Axis (C2)
The axis (C2) cervical vertebra is the second vertebra of the spine. It is unique in that it contains the odontoid process (odontoid means “tooth” and that is what this bone looks like) that forms a pivot point on which the C1 atlas can rotate. Injuries to the odontoid are common in motor vehicle accidents and falls. The C1 atlas and C2 axis form the atlantoaxial joint. The atlas (C1) rotates around the odontoid process of the axis (C2), allowing a person’s head to turn from side-to-side (rotate). This joint accounts for approximately 50% of the head’s ability to turn left and right.
The CV junction is a transition site between the mobile cranium and relatively rigid spinal column. It is also the site of the medullo spinal junction. CV anomalies are defects in development, not necessarily congenital and may not manifest at birth. Similar to the congenital disorders, the underlying cause leads to compression and instability of the spine, with the inherent threat of damage to the spine and surrounding tissues.
Basilar invagination is a developmental anomaly of the craniovertebral junction in which the odontoid abnormally prolapses into the foramen magnum. It is often associated with other osseous anomalies of the craniovertebral junction, including atlanto-occipital assimilation, incomplete ring of C1, and hypoplasia of the basiocciput, occipital condyles, and atlas. In certain diseases of the bone like hyperparathyroidism, Paget’s or osteomalacia, there is softening of the base of the skull which gets invaginated. This is called basilar impression or secondary basilar invagination.
Stenosis at Foramen Magnum
Occasionally, congenital issues lead to a condition called foramen magnum stenosis, in which the opening is not large enough for the spinal cord to easily pass through. This most often happens in conjunction with achondroplasia, a condition in which bones and cartilage do not form properly, causing a form of dwarfism. In all children who have this condition, the opening is smaller than normal. This often does not present a danger to the patient’s overall health, but in about 5% of children with achondroplasia who suffer from an abnormally small passage, the reduced size causes compression to the spinal column, which can be dangerous and even fatal.
Atlantoaxial instability (AAI)
Atlantoaxial instability (AAI) is characterized by excessive movement at the junction between the atlas (C1) and axis (C2) as a result of either a bony or ligamentous abnormality. Neurologic symptoms occur when the spinal cord is involved.
Type of Injuries:
- Type I injuries are stable subluxations.
- Type II injuries may be potentially unstable.
- Type III and IV rotatory displacements that are unstable are treated surgically with a reduction and C1-2 fusion.
The most common type of CV disorder is called the Chiari Malformation:
Chiari malformations (CMs) are structural defects in the cerebellum, the part of the brain that controls balance. Normally the cerebellum and parts of the brain stem sit in an indented space at the lower rear of the skull, above the foramen magnum (a funnel-like opening to the spinal canal). When part of the cerebellum is located below the foramen magnum, it is called a Chiari malformation. CMs may develop when the bony space is smaller than normal, causing the cerebellum and brain stem to be pushed downward into the foramen magnum and into the upper spinal canal. The resulting pressure on the cerebellum and brain stem may affect functions controlled by these areas and block the flow of cerebrospinal fluid (CSF)— the clear liquid that surrounds and cushions the brain and spinal cord—to and from the brain. CM has several different causes. It can be caused by structural defects in the brain and spinal cord that occur during fetal development, whether caused by genetic mutations or lack of proper vitamins or nutrients in the maternal diet. This is called primary or congenital CM. It can also be caused later in life if spinal fluid is drained excessively from the lumbar or thoracic areas of the spine either due to injury, exposure to harmful substances, or infection. This is called acquired or secondary CM. Primary CM is much more common than secondary CM.
Spina bifida (SB) is a neural tube defect (a disorder involving incomplete development of the brain, spinal cord, and/or their protective coverings) caused by the failure of the fetus’s spine to close properly during the first month of pregnancy. Infants born with SB sometimes have an open lesion on their spine where significant damage to the nerves and spinal cord has occurred. Although the spinal opening can be surgically repaired shortly after birth, the nerve damage is permanent, resulting in varying degrees of paralysis of the lower limbs. Even when there is no lesion present there may be improperly formed or missing vertebrae and accompanying nerve damage. In addition to physical and mobility difficulties, most individuals have some form of learning disability.
- Myelocele – In myelocele, the spinal cord is exposed so that nerve tissue lies exposed on the surface of the back without even a covering of skin or of the meninges, the membranous tissue surrounding the brain and spinal cord.
- Myelomenginocele – In a myelomeningocele (the most common type, 0.1% of live births), the neural placode and meninges protrude above the skin; it almost always is seen in the context of a Chiari 2 malformation and is generally sporadic but may affect subsequent siblings.
- Meningocele – Meningocele occurs when these meninges protrude through the vertebral defect, forming a fluid-filled sac.
Closed Spinal Dysraphism
Spinal dysraphism is the term used to describe what happens when the spinal column (backbone) fails to form properly. This happens very early in pregnancy, before the end of the fourth week. There are two types of spinal dysraphism: open, also called spina bifida aperta or myelomeningocele, or closed, also called spina bifida occulta.
In closed spinal dysraphism, the spinal cord (a tube-like structure containing nerves) and its coverings do not protrude through the back unlike in the open type. On its own, closed spinal dysraphism is unlikely to cause any problems, and many people have the condition without ever knowing it.
However, it can be associated with other conditions that could lead to problems with movement and bladder control. These include: lipomyelomeningocele, diastematomyelia, and dermal sinus. Many of these, with the exception of dermal sinus, can lead to ‘tethered cord’, which is the term used when the lower end of the spinal cord is attached to the lower bones of the spinal column rather than hanging freely as it should. If the cord is tethered, the spinal cord and nerves are stretched, reducing their ability to carry messages to and from the brain and causing pain.
A lipomyelomeningocele (lipoma) is a fatty lump that has formed beneath the skin. Some lipomas are in between the spinal cord and the spinal bones, but others may affect the spinal cord itself and the bone and tissue around the bottom of the spinal column. The lump puts pressure on the spinal cord, causing problems with messages being carried to and from the brain, and can lead to the cord becoming tethered.
Dermal sinus is a dimple in the skin in the middle of the lower back (or less commonly at the back of the head), which varies in depth from very shallow to very deep and connected to the spinal cord coverings. If the dimple is very deep, it can give a route for infection to get into the spinal cord and its coverings, which can lead to meningitis or abscesses. In some children, there is a birthmark or patch of hairy skin over the dermal sinus.
Tethered Cord Syndrome
Tethered spinal cord syndrome is a neurological disorder caused by tissue attachments that limit the movement of the spinal cord within the spinal column. Attachments may occur congenitally at the base of the spinal cord (conus medullaris) or they may develop near the site of an injury to the spinal cord. These attachments cause an abnormal stretching of the spinal cord.
The lower tip of the spinal cord is normally located opposite the disk between the first and second lumbar vertebrae in the upper part of the lower back. In people with spina bifida, the spinal cord fails to separate from the skin of the back during development, preventing it from ascending normally, so the spinal cord is low-lying or tethered.
Thus, tethered cord syndrome is closely associated with spina bifida. It fact, according to the American Association of Neurological Surgeons (AANS), it is estimated that 20 to 50 percent of children with spina bifida defects repaired shortly after birth will require surgery at some point to untether the spinal cord.
If a tethered cord is suspected, one or more tests may be necessary to confirm the diagnosis, such as MRI, CT scan, myleogram, or ultrasound. Surgery to untether the cord is generally performed if there are clinical signs or symptoms of a worsening condition. In children, early surgery is recommended to prevent further neurological deterioration. Regular follow-up is important as retethering may occur in some individuals during periods of rapid growth, which can be seen between the ages of five to nine.
While fairly rare, this condition can continue undiagnosed into adulthood. In such cases, the strain on the spinal cord will increase, leading to increasing sensory and motor problems, as well as loss of bladder and bowel control.