In a previous review of the literature, Reid and Rivett  concluded there was limited (Level 3) evidence for manual therapy in the treatment of cervicogenic dizziness. The current systematic review has identified additional studies published since the previous review, including: four RCTs [76–79], three prospective cohort studies [75, 85, 86], and a long-term follow up  of the intervention group from the RCT published by Karlberg et al. .
The RCT by Reid et al. , which was deemed to be of good methodological quality, assessed the effectiveness of a specific type of spinal mobilisation known as sustained natural apophyseal glides (SNAGs). Reid et al.  found significant improvement in dizziness severity and frequency, lower scores on the Dizziness Handicap Inventory (DHI), and decreased neck pain in the treatment group at both six and twelve weeks post-treatment. In comparison the placebo group had significant changes only at the 12-week follow-up in three outcome measures (dizziness severity, DHI, and neck pain). The remaining four RCTs [26, 77–79] were deemed to be of moderate methodological quality. The findings from the RCT by Karlberg et al.  (including the long-term follow-up by Malmstrom et al.  appear to corroborate the evidence provided by Reid et al. . The RCTs by Kang, Wang and Ye , Fang , and Du et al.  all utilised spinal manipulation in the intervention group and reported improvements in clinical outcomes.
In addition to five RCTs the current systematic review identified eight prospective cohort studies, of which seven [80–86] reported improvements in dizziness following manual therapy. Although these were generally of poor methodological quality they also reported improvements in additional outcome measures, including: neck pain , reduction of pathological vestibulospinal activity , balance , and reduced vertebral displacement and vertebrobasilar artery blood flow velocity  The remaining cohort study  reported improvements in skull spatial offset repositioning ability post treatment. Collectively, these findings provide further rationale for the use of manual therapy in the treatment of cervicogenic dizziness. Overall, the evidence evaluated in the current systematic review suggests that there is moderate (Level 2) evidence in a favourable direction to support the use of manual therapy for cervicogenic dizziness.
Although positive clinical outcomes have been demonstrated, the underlying biological mechanism remains a controversial subject. It has been theorised that disturbances to the afferent input from cervical spine mechanoreceptors may lead to a sensory mismatch between vestibular and cervical input subsequently resulting in symptoms such as dizziness, unsteadiness, and visual disturbances . There is an experimental body of evidence indicating that the biomechanical forces of spinal manipulation and mobilisation impacts primary afferent neurons in paraspinal tissues, which in turn leads to physiological consequences such as gating of nociception at the spinal cord and spinal reflex activity to alter muscle activity [87, 88]. Thus it is believed that manual therapy serves to normalise disturbances to the afferent input from deep neck proprioceptors and their subsequent reflex arcs (e.g. cervico-collic, cervico-ocular, and tonic neck), which in turn restores the ability to utilise internal vestibular orienting information to resolve inaccurate information from the somatosensory and visual subsystems (i.e. reducing sensory mismatch) .
Alas, no experimental or observational studies reporting the effect of combining manual therapy and vestibular rehabilitation in the management of cervicogenic dizziness could be identified. Collins and Misukanis  and Schenk et al.  have published case studies in which they argue that manual therapy combined with vestibular rehabilitation may be superior in the treatment of cervicogenic dizziness. Notwithstanding the paucity of such investigations, consideration of vestibular dysfunction is paramount in patients with dizziness. Unilateral peripheral vestibular dysfunction can be characterised by complaints of dizziness, visual or gaze disturbances and balance impairment . In a recent meta-analysis of vestibular rehabilitation for unilateral peripheral vestibular dysfunction is was concluded that vestibular rehabilitation is a safe and effective therapy .
The original vestibular rehabilitation protocols were developed by Cooksey  in 1946. These included: mental exercise, occupational therapy, physical exercise with the aim of restoring balance and joint position sense, and training of the eyes, to compensate for permanent vestibular dysfunction . More recently, Hillier and Hollohan  stated vestibular rehabilitation may include: learning to coordinate eye and head movements, improving balance and walking skills, learning to bring on the symptoms to desensitize the vestibular system, patient education, coping strategies, and physical activity. There are four mechanisms of vestibular rehabilitation techniques that may contribute to its benefits, namely: (i) the compensatory response, (ii) adaptation, (iii) substitution, and (iv) postural control exercises. The compensatory responses are applied using motion to minimise the responsiveness to repetitive stimuli and to rebalance tonic activity within the vestibular nuclei. Adaptation for visual-vestibular interaction uses repetitive and provocative movements of the head and/or eyes to minimise error and restore vestibulo-ocular reflex gain. Substitution encourages the use of other sensory inputs to compensate for dysfunctional afferent systems. Postural control exercises and functional retraining are applied for movement behaviour and fitness.
The four mechanisms canvas a rationale for the inclusion of vestibular rehabilitation in the management of patients with cervicogenic dizziness. Stability and posture of the cervical spine is achieved by a combination of reflexes mediated by vestibular, visual and cervical sensory input . The cerebellum plays an important role in integrating this sensory information . It can be hypothesised that a well-integrated vestibulo-cerebellar system would be more capable of compensating for the altered cervical sensory input in cases of cervicogenic dizziness. Thus, one can argue that when normal cervical afferent input is compromised, vestibular rehabilitation may strengthen the vestibulo-cerebellar system to improve the ability to adapt to the situation. Further research to elucidate the effectiveness of manual therapy in conjunction with vestibular rehabilitation for cervicogenic dizziness is strongly recommended.
There are insufficient data to provide guidelines on dosage and frequency of manual therapy in general, and spinal manipulation in particular, especially in the context of management of cervicogenic dizziness. With this in mind, it is recommended that caution is taken when delivering any sensory stimulation in the form of manual therapy or vestibular rehabilitation, or both, to affect dysfunctions in the afferent system in patients with cervicogenic dizziness. Further research is necessary to determine appropriate treatment dosage, scheduling of interventions, and which manual therapy and vestibular rehabilitation techniques are most effective in managing patients with cervicogenic dizziness.
Methodological limitations of this systematic review included lack of blinding during the quality assessment and the quality and utility of the quality assessment tool itself. Meta-analysis of the finding was precluded by the lack of robust research methodologies and heterogeneity of outcome measures in the studies included in this systematic review.