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Spina bifida

Spina bifida is a rare neurological disease or cats characterized by the presence of a midline cleft in the vertebral arch of one or a few vertebrae[1].

The cleft may involve most of the vertebral arch or only the spinous process. This anomaly results from failure of fusion of the two vertebral laminae and may be accompanied by protrusion of the meninges to form a meningocele or spinal cord and meninges to form a meningomyelocele. Myeloschisis is a failure of the neural plate to fold into a neural tube leaving it exposed on the surface of the embryo. This causes a failure of vertebral arches to form over a number of vertebrae which is called rachischisis. Spina bifida occulta is characterized by a bony defect without visible protrusion of enclosed vertebral canal structures and is usually associated with smaller defects in the lamina. Most meningoceles and meningomyeloceles occur in the lumbosacral area and mainly involve nerve roots and spinal nerves of the cauda equina rather than spinal cord itself. In such conditions, the meninges and their associated subarachnoid space extend through the vertebral defect to attach to the overlying skin from which CSF may leak. Subdermal or epaxial accumulation of CSF may also be found. Various forms of myelodysplasia may occur in the associated spinal cord parenchyma. This includes: a dilated central canal (hydromyelia), cavities in the parenchyma (syringomyelia), abnormal position of the grey matter, excessive formation of grey matter and gliosis.

As a consequence of the meningeal attachment in the meningomyelocele, abnormal tension may be exerted on the spinal cord. This rare event has been termed tethered cord syndrome[2]. The degree of spinal cord dysfunction in tethered cord syndrome appears to be related to both the force and duration of traction. Other anomalies, such as hydrocephalus, multiple thoracic and/or sacral hemivertebra, may also be present in affected animals.

The embryonic pathogenesis of this anomaly is controversial: it may represent overgrowth of cells of the dorsal neural tube that, in turn, interferes with fusion of the neural tube and vertebral arches; or the vertebral arches may fail to fuse as a result of a neuroschistic bleb [247]. Developmental arrest and hydrodynamic theories have also been suggested. Spina bifida (involving cervical vertebrae C1 to C4) was found among multiple congenital malformations in kittens of cats treated during gestation with griseofulvin[3]. Spina bifida/meningomyelocele has also been observed in kittens following methylmercury and ethylenethiourea toxicity studies in pregnant queens.

While spina bifida has been reported in a wide variety of cats, there is a high incidence of this condition in Manx cats with sacrocaudal dysgenesis. Spina bifida may occur anywhere along the vertebral column but is most common in the lumbar region. In some instances, the defect can be extensive resulting in rachischisis where it involves most of the thoracic, lumbar, sacral and caudal vertebrae. Spina bifida is also often a subclinical condition and an incidental radiographic finding[4].

Clinical signs

Video of myelodysplasia

Clinical signs in animals with spina bifida usually indicate an associated myelodysplasia or meningocele or meningomyelocele and are usually noticed when affected animals begin to ambulate. Signs may include pelvic limb ataxia and paresis, simultaneous protraction of the pelvic limbs (bunny hopping), fecal and urinary incontinence, perineal analgesia, and flaccid anal sphincter. The analgesia may extend to the most proximal part of the caudal surface of the thighs and include the scrotum and prepuce in males, and the tail if present. The site of the bony defect may be marked by dimpling of the overlying skin, streaming of hair coat, and palpable cavitation in the spinous process[5].

Meningocele alone can be present without neurological deficits. Decreased serum and CSF chloride concentrations were documented in a 5 year old Manx cat with spina bifida associated with chloride loss through a fistulated meningomyelocele. In an 8 month old Manx-type cat with neurological deficits and CSF draining from a skin mass dorsocaudal to the sacrum, exploratory surgery and histopathology confirmed a tethered spinal cord and an intradural lumbosacral lipoma associated with a meningocele[6][7].

Diagnosis

Plain radiographs will demonstrate abnormalities ranging from non-fusion of dorsal laminae to a cleft spinous process; however, myelography or advanced imaging techniques (e.g., ultrasonography, CT, or MRI) may demonstrate protrusion of spinal cord, nerve roots, and/or meninges through the sacral defect to the skin or subcutaneous spaces.

Treatment

Prognosis is guarded to poor, particularly when myelodysplasia is present.

In some animals with a fistulated meningocele/ meningomyelocele, surgical ligation of the meningocutaneous tract can correct problems associated with loss of CSF and surgical untethering may reverse some of the neurological dysfunction caused by the tethered cord syndrome and prevent further deterioration of neurologic function.

References

  1. ↑ Vite, Ch (2004) Developmental disorders. In: Braund’s Clinical Neurology in Small Animals: Localization, Diagnosis and Treatment. IVIS, Ithaca, New York, USA
  2. ↑ Yamada S et al (1995) Pathophysiology of tethered cord syndrome. Neurosurg Clin N Am 6(2):311-323
  3. ↑ Scott FW, Lahunta A, Schultz RD, et al (1975) Teratogenesis in cats associated with griseofulvin therapy. Teratology 11:79-86
  4. ↑ Bailey CS (1975) An embryological approach to the clinical significance of congenital vertebral and spinal cord abnormalities. J Am Anim Hosp Assoc 11:1975
  5. ↑ Bailey CS & Morgan JP (1992) Congenital spinal malformations. Vet Clin North Am Small Anim Pract 22(4):985-1015
  6. ↑ Plummer SB et al (1993) Tethered spinal cord and an intradural lipoma associated with a meningocele in a Manx-type cat. J Am Vet Med Assoc 203(8):1159-1161
  7. ↑ Newitt A et al (2008) Congenital abnormalities of the feline vertebral column. Vet Radiol Ultrasound 49(1):35-41

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