The Balance System
Its functioning and the assessment of its functional integrity
Giddiness, dizziness, instability and vertigo all indicate a disorder of the balance system. Technically, the balance system is known as the Vestibular system. It is one of the most complicated and rather fragile systems of the body and derangements of the balance system are quite common.
A Brief presentation on Balance System for non professionals
It has been found that 5% of all patients going to the general practitioner and 10% of patients going to the ENT specialist or neurotologist suffer from balance disorders. Giddiness / vertigo / instability is one of the most common complaints heard in a doctor’s chamber. All of them indicate a disorder of the balance system. Exact figures of prevalence of balance disorder in our country is not available, but the American National Institute of Health Statistics has reported that 42% of the nation’s population presents to the doctor’s clinic with vertigo / imbalance sometimes in their lives. According to the National Ambulatory medical Care Survey from1991, balance related problems (giddiness – instability) are among the 25 most common causes for which Americans visit the doctor. There is more than 5 million vertigo – instability related visits to the doctor every year in USA. Instability and unsteadiness are very common in old aged persons and the apprehension of falling or losing balance is a very common health concern in people above 65 years of age. The American National Institute of Health Statistics has reported that balance related falls account for 50% of accidental deaths in the population above 65 yrs. All these statistics show the high incidence of balance disorders in GPs practice.
Balance Disorders are Curable
Balance disorders are a very common medical problem but fortunately diseases of the balance system are very rarely life – threatening conditions and most of them are self – limiting. More than 90% of patients suffering from balance disorders are cured totally only by medication and some exercises. Rarely however, surgery is required, and in a very few, the disorder is of unknown origin (idiopathic) and refractory to treatment. It is very essential that the vertiginous patient gets proper medical treatment. General practitioners can very efficiently manage most cases of vertigo but specialized management by a neurotologist is occasionally required. Those patients who have associated features like ear problems e.g. deafness / tinnitus / fullness / aural discharge, headache, visual disturbances e.g. blurring of vision / diplopia and neurological problems like motor / sensory disturbances require referral to neurotologist. The other balance disorder patients who require referral are those in whom the giddiness or instability is persistent after 4 weeks or in whom the symptoms are progressively deteriorating with no signs of recovery. A neurotologist is a medical person who is specialized in the management of balance disorders. The patient usually requires some specialized investigations by which the neurotologist can identify and document the patient’s disability and also establish the cause and localize the site of disorder in the balance system. After identifying the pathology, the neurotologist treats the patient according to the nature and site of disorder. Though most balance – disorders are benign, self limiting conditions, yet the medical practitioner should not take it lightly as sinister conditions like a tumour in the brain or a disabling condition like multiple sclerosis or even a cerebral stroke localised to the prosterior part of the brain may present with vertigo / instabiliy only. Hence patients presenting with vertigo / instability should be very thoroughly investigated by a neurotologist if the patient does not recover within a maximum of 4 weeks time or if the patient has other concomitent symptoms enumerated above.
The Maintenance of the Body’s BalanceThe maintenance of balance is a function of the body’s nervous system. It comprises of parts of the brain, some nerves and muscles and certain special sensory organs. The parts of the brain and the nervous system concerned with the maintenance of the balance, and the sensory organs are collectively known as vestibular system. The brain collects information about the body’s position in relation to the ground,the surroundings and from 3 sources (which are special sensory organs) viz. –
· The vestibular labyrinth which is a very small organ located deep inside our ears,
· The two eyes, and
· Some special receptors called proprioceptors located in the muscles of the legs, soles of the feet, the buttocks. our backs etc.
By integrating the information obtained from these 3 sources, the brain recognizes changes in the subject’s position in relation to the surroundings and the ground and also the changes in the position of the head in relation to the body. Of the 3 sensory inputs to the brain by which the brain recognizes the subject’s position; the vestibular labyrinths (which is the internal sensor of the head) tells the brain how the subject’s head is oriented is space, i.e. the position of the head in relation to the body; the visual system (which is an external sensor) tells the brain whether the surroundings (the world around us) are moving and the proprioceptive system (which also is an external sensor) tells the brain whether the ground on which subject is standing or moving or is unstable.
The harmonious integration of these inputs in the brain is essential to maintain balance. As soon as the brain is informed about any change of position of the subject, it immediately brings about a coordinated movement of different parts of the body (mainly the eyes and the limbs) such that the subject’s position is maintained and a fall is prevented. The corrective movement is generated by the brain.
A proper coordinated movement of the limb and the trunk prevents the subject from falling down and a precise movement of the eyes prevents images of the surrounding objects slipping in the retina. Slippage of the image of surrounding objects in the retina (called retinal slip) occurs whenever there is a relative movement of head and surroundings. A perfectly measured and precisely timed movement of the eyes prevent this retinal slip by moving the eyes in such a manner that the image of the surrounding objects remains fixed in the most sensitive part of the retina (called fovea). Whenever a slip of the image occurs, the patient feels a hallucination of movement, i.e., vertigo. The corrective movement of the eye is brought about by the vestibulo – ocular reflex system and the corretive movement of the limbs and trunk is brought about by the vestibulo – spinal reflex system. In disorders of the balance system, these mechanisms are jeopardized and the patient not only becomes unstable whenever there is any change in the relative positions of the body and the surroundings, but also experiences hallucination of movement (called vertigo) even when there is no relative change in the position of the body and the surroundings.
A disorder of the balance system may occur whenever there is any defect in one or more of the different organs, which together comprises the balance system. The commonest site of the disorder is the vestibular labyrinth inside our ears. It is a very fragile organ and is susceptible to damage by numerous factors like medicines (e.g. gentamycin, streptomycin, antimalarial drugs, etc.), infections, degenerative changes of aging, trauma to the head etc. However, the defect may very well lie in the other sectors like the proprioceptors in the legs, the eyes, the muscles of the limb and the small muscles outside the eyes and even in the brain itself. Metabolic and degenerative changes in the brain due to deficient blood supply in the lower part of the brain (technically called vertebrobasilar insufficiency) are also not uncommon cause of the instability. Tumors in the brain especially in the vestibular nerve, cerebellum, and cerebellopontine angle or in the lower brainstem region are rare causes of balance disorders.
The Vestibular labyrinths which are located deep inside our ears (one on each side) need to be described in a little detail because this is the commonest site of lesion in disorders of balance. The Vestibular labyrinth has 2 parts viz.- the semicircular canals and the otolithic organs (utricle and saccule). There are three semicircular canals which are hollow tubes filled with fluid. They are arranged perpendicular to each other so that they may respond to all possible directions of body movement. Attached to the walls of these tubes are special motion sensing cells that are topped by tiny hairs which project into the fluid. During head movement the fluid lags slightly behind the motion of the rest of the body. This causes the hair cells to bend, which in turn stimulates the balance nerve. The balance nerve (8th Cr.nv.) carries this information to the brain which recognises the change in position of the head.
The semicircular canals are especially important for coordinating rotating movements. Diseases which affect them often cause a sensation of spinning or turning.
The two otolithic organs also have tiny hair cells. On their surface are minute crystals of calcium carbonate known as otoliths. During head movement these heavy crystals lag slightly behind the motion of the rest of the body due to inertia, causing the hairs to move and thereby stimulate the balance nerve. The otoliths are especially important for monitoring linear (straight line) movements and up and down movements. Disorders of this part of the inner ear usually cause a sense of the ground rolling or a feeling like that of dropping in an elevator. Diseases of the Vestibular labyrinth are known as peripheral disorders and include diseases like Meniere’s disease, infective labyrinthitis, perilymph fistula, benign paroxysmal positional vertigo etc. Disorders may also involve the balance nerve which carry the sensation from the vestibular labyrinth to the brain. These include diseases like vestibular neuronitis and acoustic neuroma.
A subject gets the sensation of vertigo or imbalance when one of the following things happen-
a ) any one or more of the sensory receptors for maintenance of balance viz.- the vestibular labyrinth (s), the eyes, or the proprioceptors in the muscles is defective,
b) there is a mismatch between the information about spatial orientation received from the vestibule with the information received from the other sensory systems viz.- the proprioceptive and visual systems,
c) the central nervous system (CNS) fails to integrate the information received from the vestibular, proprioceptive and visual systems,
d) the CNS fails to effectively generate the motor output and
e) the motor output system for the maintenance of balance is defective. The investigations for vertigo are aimed at ascertaining which of these mechanisms are at fault.
The treatment as well as the prognosis (i.e. the outcome of treatment) varies according to the nature and site of the disorder. The neurotologist diagnoses the disorder by the help of some specialized clinical tests (called neurotological tests) and investigations (called vestibular function tests). The management of the patient is tailored in accordance to the findings of these clinical tests and investigations. Imaging studies are sometimes helpful especially if there are associated neurological symptoms or if tumor is suspected.