The most common documented symptom of basilar impression is nuchal pain and headache, mainly in the occipital region, usually occurring in the morning.[9,13,14] The pain may be triggered by movement and exercise and is sometimes so severe that the patient has to hold the head rigid in order to minimize the pain. Headache-precipitating actions such as coughing, sneezing, and straining can also result in vertigo or imbalance. The exact mechanism of cough headache remains elusive, and some argue against raised intracranial pressure as a cause.[15,16]
The most common neurologic sign in symptomatic basilar impression is horizontal nystagmus, with the fast component in the direction of gaze. A marked bi-directionality to the nystagmus almost always denotes a central cause; however, it has poor localizing value because it can occur in both brainstem and cerebellar lesions.[17] The presence of downbeat nystagmus, on the other hand, is indicative of an abnormality at the craniovertebral junction but is more common in Arnold-Chiari malformations than basilar impression alone.
Involvement of the lower cranial nerves has also been described in basilar impression but is reported to be relatively uncommon.[14,18 ]The most common nerve involved is the fifth nerve, probably because of its large size and extension of the nerve and its root in the pontomedullary region. Sensory symptoms in the distribution of nerve have been reported to be as high as 30%-50% in some case series, but motor involvement is uncommon.[8] Facial nerve palsy is rare, but facial nerve spasm is more common, presumably due to posterior circulation abnormalities induced by the basilar impression.[19,20]
Trigeminal neuralgia has been reported in several cases and has a vascular etiology similar to facial spasm.[19] Bladder disorders are rarely reported in the literature. Raised intracranial pressure has been documented in few case reports. Other symptoms include dysarthria, dysphagia, and periods of confusion, loss of balance, loss of coordination, dizziness, orthostatic hypotension, numbness/tingling in the extremities, paralysis, and even death from brainstem compression. Symptoms may become worse with flexion of the head because it further drapes the spinal cord over the upper portion of C2.
In order to elucidate the probable cause and origin of basilar invagination, researchers have put forward various theories, including genetic abnormalities, mechanical or embryologic dysgenesis, and viral infections.[21,22,23,24] For many years, anatomists were of the opinion that the deformation had a mechanical cause and therefore gave it the name impressio baseos cranii or basilar impression.[25] The pathogenesis of neurologic symptoms and signs can be attributed to a number of factors. First, the direct pressure effect of the clivus, the anterior lip of the foramen magnum, the body of C1, and dens on the anterior brainstem results in pontomedullary region angulation and early involvement of the corticospinal tracts. Second, the interaction of minor trauma from head movements with the abnormal anatomy causes chronic low-grade inflammation, leading to arachnoid adhesions and increased vascularization in the region of the foramen magnum.
Trigeminal neuralgia has reportedly been relieved by removing these adhesions from the trigeminal roots.[26] Finally, mechanical stresses and inflammation also result in vascular abnormalities. The transient nature of some of the symptoms also points toward a vascular etiology. Early angiographic studies of the vertebral system in basilar impression suggested an abnormal dorsal displacement of the terminal segment of the vertebral and initial parts of the basilar artery.[20] This is because of the "kinking" of normal vessels in order to adapt to a posterior fossa shortened by the process of basilar impression. In addition, an abnormal clivus is likely to accentuate this arching of the vessel.[12]
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