---

MRI IN GIDDINESS

Shrinivas B Desai, Sharad Maheshwari,Anirudh Kohli, Mohit Shah
Jaslok Hospital and Research centre, Mumbai.

---

INTRODUCTION

Giddiness is an unpleasant sensation of disturbed relation to surrounding objects in space. It differs from vertigo that there is no experience of the external world, or the, patient being in motion. Dizziness is a lay term, which encompasses a number of symptoms of disequilibrium, including light-headedness, giddiness, imbalance, ataxia, vertigo, and minor episodes of mental confusion or loss of consciousness.

PHYSIOLOGY

Human body has a perfect system to maintain equilibrium. It’s a symphony between the sensory information from the eyes and vestibular system and proprioceptive information from the neck and limbs, which passes into the central nervous system. At the level of vestibule nuclei it is integrated and modulated by activity arising in the cerebellum, the extra pyramidal system and cortex.

CAUSES OF GIDDINESS

Giddiness is an extremely common symptom, and can be due to multiple pathologies. The cause can be vestibular, neurological or general medical disorder. Vertigo per se is associated with vestibular disorders, whereas vague dizziness is more related to general medical disorders.

I. GENERAL MEDICAL DISORDERS

1. Haematological

2. Cardiovascular

3. Metabolic ( hypoglycaemia)

II. NEUROLOGICAL

1. Syncope

2. Epilepsy

3. Psychogenic

4. Vascular : vertebrobasilar insufficiency, subclavian steal, Wallenburg’s synd, AICA syndrome.

5. Infective disorders

6. Degenerative disorders

7. Tumours

8. Craniovertebral disorders

9. Cerebellar disorders

10. CP angle pathologies

III. OTOLOGICAL

1. Meniere’s disease

2. Vestibular neuritis

3. Infection

4. Vascular accidents

5. Acoustic neuromas

6. Vascular loops

IV. MISCELLANEOUS

1. Cervical

2. Ocular

3. Odontogenic

4. Functional

ROLE OF MAGNETIC RESONANCE IMAGING

Evaluation of patient with giddiness requires a thorough knowledge of various systems. The origin may be located in any specific area or combination of various areas. The cause may also be functional. After a proper history and preliminary examination, including CNS and ocular examination one needs to localize the probable cause to a particular system. If the probable cause appears to be CNS or otologic, imaging plays important role.

In most of the conditions MRI is the primary modality or can be used as a secondary modality for further characterization after the lesion has been located by CT scan. MRI has experienced many advances in recent years. MR resolution has surpassed that of CT in certain situations. MRI is quite versatile, with many different techniques for studying anatomy, physiology and pathology. It has the advantage of better resolution, no radiation exposure, multiplanar capability, shows blood vessels without the need of contrast injection and capability of tissue characterization. However MRI does not show signal from cortical bone, so CT would be the preferred modality to study bony details of the temporal bone.

MRI characterizes CSF, brain, cranial nerves and blood vessels much better than CT. MRI is outstanding in early diagnosis of hyper acute infarcts, study of cranial nerves, especially eighth nerve with detection of small intracanalicular tumours without need of contrast injection. Contrast enhanced MRI is sensitive to detect abnormalities that alter the blood brain barrier, or to detect vascular lesions. It can further characterize tumours detected on CT. High resolution MRI can detect fluid in the inner ear structures (cochlea , vestibule and semicircular canals) ( Figs. 7 and 8).

BASIC PRINCIPLES IN MRI

MRI is an interaction between an external magnetic field, radiowaves, and hydrogen nuclei in the body. When placed in an external magnetic field these hydrogen atoms act like tiny magnets. There is enhanced absorption of energy when these tiny magnets are acted upon by radio-frequency pulse. When the RF pulse is switched off, there is exchange of the absorbed energy between the hydrogen atoms and surrounding environment (lattice) which accounts for the MR signal.

       MRI TECHNIQUES

       The various sequences used for characterization are :

T1 W1 spin-echo : Fat shows a high signal, brain is intermediate in signal intensity, CSF is low in signal intensity.

T2 W1 turbo-spin-echo : CSF appears bright. Most of the pathologies also appear bright.

Flair : Signal from the fluid is suppressed while pathology appears bright.

Diffusion : This is an excellent technique to detect ischaemia and infarcts within minutes. Early detection in less than 3 hrs is important because intraarterial thrombolysis can be administered with salvage of the ischaemic tissue. Acute infarcts appear bright on diffusion (Fig. 1).

Fig. 1 : Diffusion weighted MRI showing hyper acute infarct.

CISS (Constructive Interference in Steady State) : This is a 3-D gradient technique, where signal from brain parenchyma is suppressed. It gives excellent demonstration of cranial nerves, including the intracanalicular components of eighth nerve. Fluid appears bright. It shows the cochlear turns and vestibular system brilliantly.

MR Angio (3D TOF) : Non-invasive way to detect flow in intracranial vessels without any injection of contrast agent. The MIP reconstructed images show DSA like pictures. This is an excellent technique to demonstrate vascular loops compressing the nerves (Fig. 6)

IMAGING FINDINGS IN VARIOUS PATHOLOGY

OTOLOGICAL CONDITIONS

1. Eighth Nerve : The evaluation of eighth nerve would be required to rule out vascular loops, acoustic neuromas and extension of infection. We use the combination of T2 W1 and 3D-CISS sequence, which shows the complete course of eighth nerve (Figs. 2 and 3) with its cochlear and vestibular component (Fig. 4). Inaddition it shows tortuous vascular loops abutting or compressing the nerve in its course (Fig. 5). In addition to the above sequences we also perform 3D angio sequence for evaluation of vascular loops (Fig. 6) When infection is suspected post contrast T1 W1 thin images are performed to look for nerve enhancement.

Fig.2 : T2 W1 axial image showing both eighth nerves.

Fig.3 : T2 W1 coronal image showing both eighth nerves.

Fig.4 : CISS image showing the cochlear and vestibular components of the eight nerve

Fig.5 : CISS image in a patient with tinnitus and giddiness,showing ectatic basilar artery compressing the left eighth nerve.

Fig.6 : Source images from a 3D angio showing ectatic basilar artery.

Fig.7 : T2 W1 coronal showing cochlear turns.

Fig.8 : T2 W1 coronal showing vestibule with semicircular canals.

 

2. Inner Ear : Most of the inner ear pathologies would require a combination of high resolution CT scan (1 mm thin slices in both axial and coronal planes) for bony and air space visualization with high resolution MRI. In addition to T1 W1 and T2 W1 sequences (Figs. 7 and 8), a 3D CISS should be done to study the nerve course.

Fig. 9 : MR angio showing thinning of basilar artery in a patient with giddiness.

Fig. 10 : T2 W1 axial image showing pontine infarct.

 

NEUROLOGICAL CAUSES

1. CEREBRO-VASCULAR DISORDERS

A. Vertebro Basilar Ischaemia and Stroke

Vertebro-basilar ischaemia may give rise to end organ or eighth nerve dysfunction, but frequently gives rise to vestibular symptoms, as a result ischaemia of the vestibular nuclei, which occupy a large area in the lateral zone of the brain stem. These are particularly susceptible to a reduction in the blood flow of the main basilar artery (Fig.9). Dizziness is the first and most frequent symptom of vertebro-basilar ischaemia.

Vertebro-basilar insufficiency is defined as astate of transient decrease in the cerebral blood flow, without actual infarction, resulting in transient inability to meet the metabolic requirements of the brain.

Fig.11 : T2 WI sagittal image of the another patient showing pontine infarct

Fig.12 MR angio of the same patient shows tight narrowing of the mid basilar segment

 

Completed strokes in the vertebro-basilar territory are associated with a number of well recognized syndromes. Wallenberg, or lateral medullary syndrome may be the result of occlusion of the PICA (posterior inferior cerebellar artery) or primary disease of the vertebral artery. Vertigo / dizziness is also a feature of pontine and cerebellar infarct (Figs. 10, 11 and 12 ).

Fig.13 T2 W1 image showing cerebellar infarct. Patient presented with gait ataxia and giddiness. Intracranial angio showing absent flow in right ICA, in patient presenting with recurrent attacks of light-headedness.

Fig.14 : Same patient shows block of right ICA at its origin

 

T2 W1, Diffusion W1 and 3D TOF (time of flight) are the most important sequences to study a patient with suspected vertebrobasilar ischaemia or stroke. Diffusion W1 images can detect infarcts within minutes. Incomplete ischaemia can be detected by a combination of diffusion-perfusion studies.

Fig.15 : T2 W1 axial showing brain stem glioma

Diffusion is highly sensitive to slowing of intracellular water movement that occurs within minutes of onset of ischaemia. Acute infarct shows as bright lesion (Fig. 1 ), while chronic infarct looks as hypointense. It provides information that is not available with a standard T1 and T2 weighted images. Thus diffusion weighted imaging has attained importance in assessment of stroke by showing cerebral ischaemia early and with a high degree of specificity.

Perfusion assesses flow at capillary level. It is performed with administration of I.V. contrast medium. Intravascular gadolinium results in a proportionate decrease in signal intensity on T2 * W images, because of magnetic field inhomogenity. It shows diminished perfusion within minutes of insult as a delay in peak signal intensity attenuation if ischaemia is incomplete or as an absence of signal intensity attenuation if ischaemia is complete.

MR angio will detect narrowing and plaque in the intracranial as well as extra-cranial circulation. The study should include the origins of great vessels from the arch of aorta.

Fig.16 : CISS axial shows left acoustic neuroma with intracanalicular extension.

B. Supratentorial Vascular Lesions

About 8% of patients with internal carotid or middle cerebral artery ischaemia complain of dizziness. The pathophysiological mechanism remains unclear.

2. MULTIPLE SCLEROSIS

5% of patients with multiple sclerosis have dizziness as their first complaint or during the course of illness. Plaques of multiple sclerosis are commonly situated in the periventricular location, internal capsule, corpus callosum, pons, periaqueductal region and brachium pontis. The corpus callosum is a region that is especially sensitive to demyelination. These lesions appear hyperintense (bright) on T2 W1 images. Enhancement of plaque is a sign of active plaque.

DETERMINING THE AGE OF LESION

1. Magnetization Transfer Ratio : It’s a new technique to image brain. Tissues such as white matter, show large decrease in signal intensity after on the MT image. Because ofmyelin loss in MS plaques, the magnetization ratio is significantly lower than in normal white matter. The current role of MTR is to evaluate the age of the lesion. Chronic lesions have much lower ratio than acute lesions.

2. Diffusion Weighted Imaging : Acute lesion appears bright on diffusion.

3. MR Spectroscopy : acute lesions show raised choline peaks. Whereas chronic lesions have low NAA. MRI is an extremely valuable tool in confirming the clinical diagnosis of MS. T2 WI remain the standard diagnostic tool. Serial MR and MRS are helpful in understanding the natural history and pathophysiology of the disease .

Prognostic implication of MRS in MS : monitoring treatment efficacy
Stage	Pathology	Ratios	Response to Steroids
Hyperacute plaque	Oedema	Unchanged	Good
Acute plaque	Demyelination	Increase Cho / Cr
Sub acute to	Demyelination	Decrease	Do not
chronic plaque	with gliosis	NAA / Cr	respond

3. NEOPLASIA (CEREBELLAR / CP ANGLE / BRAIN STEM )

Dizziness and/or vertigo are early or initial symptoms of brainstem tumours (Fig. 15), cerebello-pontine lesions, in particular acoustic neuromas (Figs. 16, 17 and 18). Other lesions like meningioma, epidermoid, metastasis (Fig. 19) and Vth nerve neuroma are well evaluated on MR Imaging. Primary tumours or metastasis in cerebellum can present with dizziness, truncal ataxia and oculomotor abnormalities. Temporal lobe lesions can present with similar complaint of dizziness in addition to other symptoms.

Acoustic neuromas can be easily diagnosed on CISS images showing it along the nerve course(Fig. 16). It appears iso-intense on T1 W1 (Fig. 17) and hyperintense on T2 W1 (Fig. 18) images. Meningiomas appear iso to hypointense on T2 W1 images. Post contrast scans show dural tail in meningiomas, which is diagnostic. Epidermoids are characteristically bright on diffusion, thus can be differentiated from arachnoid cysts and other CP angle lesions, which appear hypointense on diffusion weighted images.

Fig.17 : T1 W1 axial of the same patient showing lesion to be hypointense.

Fig.18 : T1 W1 axial of the same patient showing lesion to be hyperintense.

Fig.19 : T2 W1 axial shows left CP angle hypointense lesion(metastatis). Patient presented with imbalance and giddiness.

4.INFECTION (BACTERIAL / VIRAL ENCEPHALITIS)

Contrast enhanced MRI would be mandatory in suspected meningitis as an intracranial complication of inner ear infection. Viral encephalitis may involve vestibular nuclei. Meningeal enhancement is seen in meningitis. Labyrinthitis refers to inflammation of the membranous labyrinth. MRI is the only modality able to detect this abnormality.

Areas of enhancement are seen within the involved portion of the labyrinth on post-contrast images.

5. CEREBELLAR DISEASE

Apart from ischaemia / stroke and tumours other cerebellar disease like degenerative disease can be evaluated with MRI. Dilated fourth ventricle and widened cerebellar folia can be appreciated in atrophy.

6. EPILEPSY

It may be a manifestation of an aura, or part of a temporal lobe seizure

MRI plays a big role in screening patients with epilepsy to rule out mesial temporal sclerosis or focal heterotopia. MTS shows focal atrophy and hyperintense signal in the head of hippocampus.

MR SPECTROSCOPY

MR spectroscopy is a non invasive potentially risk free method to monitor the biochemistry of acute and chronic stages of disease. MRS provides measurements of intermediate biochemistry. It samples the relative levels of mobile metabolites from a volume of tissue defined by an MR image. Similar to MRI, the greatest clinical application of MRS has been in the CNS. It has a wide clinical applications especially in tumours, infarcts, localizing epileptic focus, characterizing MS plaques. Tumours show high choline peak, whereas infarcts show raised lactate peaks.

CONCLUSION

Magnetic resonance imaging represents a revolution in medical technology. Because of the innate versatility of this modality, tissue anatomy, pathology, metabolism and flow are all amenable to non-invasive evaluation. MRI has already consolidated itself as a major modality for diagnosis.

---

REFERENCES

1. Adult Audiology, Scout-brown’s otolaryngology, fifth edition. Buterworth international.
2. Sandra desa souza, Shrinivas B Desai. Modern concepts of neuro-otology.
3. Clinical magnetic resonance imaging. Edelman and hesselink saunders.
4. David D Stark and William G Bradley. Magnetic resonance imaging Mosby.
5. High power gradient MR imaging, advances in MRI. Matthijs oudkerk and edelman. Blackwell Science.
6. Magnetic Resonance imaging of the brain and spine. 2nd ed. Scott W.Atlas.
7. Prognostic implication of MRS in monitoring treatment efficacy in MS. Lancet 337 : 58.

---

To Section TOC