J Manipulative Physiol Ther 2003 (Sep); 26 (7): 443–447 ~ FULL TEXT
Thomas W Jensen
Jensen Chiropractic Clinic, LTD,
2002 East Fifth Street,
Sterling, IL 61081, USA
OBJECTIVE: To examine cerebral arterial blood flow in 2 patients exhibiting signs of vertebrobasilar arterial ischemia (VBI) before and after spinal manipulative therapy.
CLINICAL FEATURES: Two patients had a repetitive/resting tremor, one from a spastic torticollis with the onset immediately after self-manipulation by the patient 6 months earlier, and the second one with a generalized resting tremor, hip clonus, dizziness, and presyncope. The diagnosis of vertebrobasilar ischemia was established by continuous wave Doppler ultrasound and physical examination.
INTERVENTION AND OUTCOME: Nonrotary cervical manipulation and diversified technique to the thoracic spine were performed. In the first patient, the spastic tremor improved by 80%. The repeat Doppler performed 13 months later showed an improvement in the arterial flow in the right external carotid artery peak flow from 0.7 kHz to 1.75 kHz. In the second patient, the resting tremor diminished in 4 days, with the right common carotid artery peak systolic flow improving from 1.0 kHz to 1.9 kHz and the left vertebral artery flow improving from 0.175 kHz to 0.5 kHz. The symptoms of VBI and objective Doppler findings improved following spinal manipulation. Both cases had impaired vertebral arterial flow.
CONCLUSIONS: Spinal manipulation may have a normalizing effect on the sympathetic nervous system, allowing for a change in vasospastic cerebral vascular arteries.
From the Full-Text Article:
Vertebrobasilar insufficiency is clinically manifest by a broad range of symptoms. The most common cause of this symptom complex is atheromatous involvement of the arteries supplying the brain.  The age of the patient would suggest against this as a cause but not necessarily rule it out.
A single blocked vertebral artery by cervical rotation to the point of occlusion does not necessarily produce vertebrobasilar insufficiencies. However, when a second artery is compromised to a level that is not normally considered to be stenotic, vertebrobasilar ischemia is produced.  In addition, neck rotation can induce vertebral artery insufficiency and neurological sequelae if there are preexisting factors or cofactors that have further reduced the blood flow and induced stenosis or occluded other cerebral vascular structures. 
The significance of the 2 cases relates to the fact there was a decrease in peak systolic flow in both cases. In the first patient (Fig 1), with a left external carotid peak systolic of 1.8 kHz and a right external carotid peak systolic of 0.5 kHz, the left was decreased by approximately 72%.
The follow-up examination of the first patient 13 months later showed the right external carotid peak systolic flow to have improved to 1.75 kHz with the left approximately 1.75 kHz. The right vertebral artery still remained substantially insufficient, and we were unable to obtain a signal on the paper. In the second patient, there was a right common carotid artery peak systolic value of 1.0 kHz initially, which improved 4 days later to a value of 1.8 kHz. The left common carotid artery initially was 1.0 kHz and remained approximately 1.0 kHz.
Both of these cases showed decreased blood flow in the vertebral artery, followed by a subsequent decrease in either a carotid or external carotid artery. Improvement followed in the first patient 13 months later and in the second patient 4 days later. The manipulative procedure that was preformed encompassed rotation under 45° without extension. The technique employed a mid-digit contact on articular facets, which palpated prominently. The thrust was short with high velocity. The direction was in the plane of the articular facets of C4, C5, and C6. Substantial audible releases were noted in both cases with several treatments to the cervical spine.
The etiology is established by history and Doppler examination. The history revealed the length of time present for the first exam would rule against an emboli or thrombus formation causing occlusion, specifically it being present for 6 months. The Doppler rules out occlusive disease via waveform analysis. (Examples of stenotic and normal waveforms are provided in Fig 4, Fig 5). An occlusive waveform has a dome shape (Fig 6), and acute occlusion without collateral circulation demonstrates a silent curve (Fig 7).  This leads to the cause of constriction of the external carotid artery in the first patient and the common carotid artery constriction in the second patient. Both cases had decreased blood flow in 1 vertebral artery.
Fig 5. Normal Doppler sound wave.
Fig 6. Occlusive Doppler sound wave with good collateral circulation.
Fig 7. Acute occlusion without collateral circulation demonstrating a silent curve.
Spasticity and tremors are abnormalities in tone and they can occur in the presence of disease of any portion of the motor system. Clonus is a rhythmical contraction of a muscle on stretching. 
Perhaps an angiogram would have further aided understanding of the etiology of the decreased flow of the vertebral and carotid arteries. A functional vascular test is frequently thought to be a reliable indicator or predictor of vertebrobasilar ischemia; however, these tests may elicit false-positive results. [11, 12] False positives may also inhibit patients from obtaining needed therapy.  Negative findings may be caused by altered afferent proprioceptive transmissions from the cervical spine joint capsule.
Continuous wave Doppler sonography has been the most important tool for noninvasive investigation of the external cranial vertebral arteries when combined with duplex Doppler. 
In a study by Yuan et al,  peak systolic frequency averages on 60 vertebral arteries were 1.4 ± 0.4 kHz and 1.6 ± 0.5 kHz, right and left, respectively. The decreases are generally insufficient and usually asymptomatic up to the point of 60% occlusion. 
The return of symptoms was a result of the suspected etiology of the neurological embarrassment arising from an ischemic process. The tremor resulted in vasoconstriction of the artery due to sympathetic stimulus. Gayral et al16 noted lesions of the cervical spine might compress and stimulate the cervical sympathetic nervous system and thus provoke a vasomotor response (vasoconstriction) in the vertebral vasculature. Gayral et al  specifically noted a Barre-Lieou syndrome resulting from cervical nerve involvement of cranial nerves V through XI. I believe that the sympathetic nervous system can provoke a sympathetic response with resulting symptoms dependent on the site of ischemia.
I have presented the diagnosis and evaluation of vertebrobasilar ischemia. One case resulted from self-manipulation to the cervical spine, and a second case was due to a spinal strain. Cerebral vascular Doppler examination, a thorough history, and a physical examination indicated a need for spinal manipulative therapy, even though the functional vascular test did not.
Serial cerebrovascular Doppler sonography documented the improvement of peak systolic flow; however, because of the 13-month duration in the first patient, there is some question about the amount of time, even though the symptomatic picture improved considerably within 24 hours. In the second patient, the repeat study was performed within 4 days.
I am not recommending the use of cervical spine manipulation in patients that exhibit vertebrobasilar ischemia. I do, however, feel chiropractic physicians in general successfully treat substantial amounts of dizziness and light-headedness, which have resulted from irritation to the apophyseal joints and possibly stimulus to the sympathetic nervous system resulting in ischemia.
Further, a nonrotator type of manipulative technique, which has been described, should be employed in high-risk patients. In addition, continuous wave Doppler and duplex Doppler sonography were shown to be an excellent method to rule out possible vertebrobasilar arterial ischemia that can result from something more serious than a spasm of the vertebral or carotid arteries.