Fig. 11.1
Sagittal T2 magnetic resonance imaging (MRI) demonstrates presence of a CM I, with herniation of the cerebellar tonsils (shaded in red) below the foramen magnum (red line), compressing the cervicomedullary junction
The vast majority of Chiari I malformations are asymptomatic and do not need intervention [3].
When symptomatic, Chiari I malformations often present with sudden-onset suboccipital headaches classically aggravated by activities that invoke a valsalva maneuver. Less commonly, patients can present with lower cranial nerve dysfunction (especially dysphagia or sleep apnea), cerebellar dysfunction (e.g., ataxia), or spinal cord dysfunction secondary to an associated hydromyelia (weakness, scoliosis).
Treatment (when necessary) is predicated on surgical decompression of the cervicomedullary junction.
Biology and Epidemiology
The etiology of CM I remains controversial. The majority of cases are considered to be developmental and a result of a mismatch between growth of the brain and that of the posterior fossa. Evidence comes from craniosynostosis patients and the observation of the clinical constellation of symptoms developing over time, as well as the observation that tonsillar displacement can change for better or worse over time during childhood, sometimes even resolving altogether [2, 4]. A minority of Chiari malformations can be acquired, presumably secondary to an abnormal pressure differential across the foramen magnum such as from hydrocephalus or a mass lesion from above or a lumboperitoneal shunt from below [5].
Pathophysiology
Symptoms from Chiari malformations are generally considered to be secondary to local compression or irritation . Tonsillar compression and scar-related tethering of the dura (particularly with valsalva maneuvers) irritate pain fibers in the dura, leading to headache [6].
Direct compression of the brainstem may contribute to dysfunction of local tracts and nucleui, leading to problems with swallowing, respiration, phonation, and other lower cranial nerve palsies.
Obstruction of normal CSF flow at the cervicomedullary junction has been implicated in the development of hydromyelia (or syringomyelia). This can cause chronic injury to pain and temperature fibers (which cross centrally in the spinal cord to produce a classic “suspended” sensory loss), compression of anterior horn motor neurons leading to lower motor neuron weakness (typically in the hands), compression of corticospinal tracts leading to upper motor neuron weakness with spasticity (typically in the legs), and possible scoliosis from weakness of the axial musculature [7].
Molecular/Genetic Pathology
The majority of Chiari I malformations are thought to be sporadic, although 3 % of cases are found to be familial [8].
There are no known genes specifically implicated in the cause of CM I, but links have been suggested to craniofacial disorders (including some craniosynostosis syndromes) and some connective tissue disorders [9, 10].
Patients may have other findings, including Klippel-Feil, basilar invagination, or other areas of bony fusion [11].
Incidence and Prevalence
Age Distribution
Geographic Distribution
None
Risk Factors—Environmental, Life Style
None
Relationships to Other Disease States, Syndromes
As noted above, there are several associations between Chiari I and other conditions [11]:
Hydrocephalus
Craniosynostosis
Endocrinopathies (growth hormone deficiency and acromegaly)
Hyperostosis
Bone mineral deficiency
Cutaneous disorders (neurofibromatosis type I, blue rubber bleb nevus)
Spinal defects (Klippel Feil, spondyloepiphyseal dysplasia)
Presentation
Symptoms/Signs
Headache (occipital or suboccipital, tussive, worse with flexion/extension)—most common (about 2/3 of patients).
Scoliosis was the 2nd most common reported finding in symptomatic Chiari I malformations in children [8].
Lower cranial nerve dysfunction (hoarseness, dysphagia, dysphonia, aspiration, swallowing problems, snoring, apnea).
Cerebellar syndrome (dysmetria, ataxia, nystagmus).
Central cord syndrome (loss of pain and temperature sensation, weakness with lower motor nerve injury, scoliosis (often with syrinx)).
Patterns of Evolution
The presentation of symptoms in CM I is often chronic, over a period of months or years.
Evaluation at Presentation
MRI is the current standard for evaluation of the cervicomedullary junction (CMJ).
Discovery of a CM I often includes imaging of the brain to exclude mass lesions rostrally and—on occasion—MR imaging of the spine to assess for the presence and extent of syrinx .
In some cases (such as those with a history of neck trauma), the history or imaging findings may suggest the need to assess the bony anatomy of the CMJ with other diagnostic studies, such as flexion–extension radiographs or computerized tomography.
Standard preoperative laboratory studies (complete blood count (CBC), clotting times Prothrombin Time/Partial Thromboplastin Time (PT/PTT), type and cross (T&C) for blood bank, chemistry panel (Chem 7)) may be considered prior to planned surgery.
Differential Diagnosis
Many patients will be asymptomatic on examination if the lesion is found incidentally. The diagnosis of a CM I is relatively straightforward, with the difficulty primarily arising from the exercise of using clinical judgment to select appropriate surgical candidates. The main issue with differential diagnosis is to ascertain whether a proximate cause for the CM I exists. As such, imaging of the head and spine may be warranted to exclude intracranial mass lesions, hydrocephalus or areas of spinal CSF leak. Furthermore, careful history taking is important to determine if things such as lumbar puncture or intracranial pressure (ICP)-elevating medications (such as retinoic acid) may be contributing to the radiographic findings. Lastly, given the often subjective nature of complaints in CM I, the clinician must carefully evaluate the patient for other causes that may explain presenting symptoms.
Diagnosis and Evaluation
Physical Examination
Many patients will be asymptomatic on examination. However, a detailed neurologic examination and history are always important. Attention should be paid to evidence of neurologic dysfunction in the history.
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