Smell and Taste after TBI

Examination of changes in the smell

The methods of examining smell disorders in patients with traumatic brain injuries are divided into objective and subjective, as well as into qualitative and quantitative. Severe patients with acute TBI are examined by means of the Borchtein’s method. The procedure is quite simple and the patient should be conscious when undergoing this examination. The test involves the use of the following set of odorous substances:

  • Pink water;
  • Planed laundry soap;
  • Bitter almond water;
  • Turpentine;
  • Tar;
  • Ammonia;
  • Acetic acid;
  • Chloroform.

Before examining the patient’s smell, doctor needs to make sure that the olfactory cleft is patent. If there is edema of the nasal mucosa, it should undergo anemisation. The sense of smell is examined separately for each half of the nose. Traumatic brain injuries may be characterized by both rhinogenic (caused by traumatic injury of the nose and subsequent swelling of its mucosa) and neurogenic impairments of the sense of smell.

Damage to the central olfactory structures in the middle and anterior cranial fossae can give rise to various symptoms. The localization of pathologies in the anterior cranial fossa results in a loss or decrease in the sense of smell from one or both sides. When it comes to the affected primary olfactory cortex in the mediotemporobasal regions, the patient experiences olfactory hallucinations and impaired odor recognition.

After a fall on the back of the head, patients often suffer a loss of smell due to the countercoup, which is a consequence of the damage to the olfactory threads in the frontal basal areas. In traumatic brain injury, neurogenic smell disorders directly depend on the severity of the injury:

  • Minor injury. It does not cause any smell disorders;
  • Moderate injury. Smell disorders occur in 15% of cases;
  • Severe injury. In 48% of cases in the residual period the injuries are characterized by smell disorders due to foci of lesions in the mediotemporalbasal and mediofrontalbasal regions of the brain, which include both secondary and primary olfactory structures.

As a rule, open TBI with lesions in the anterior cranial fossa leads to the loss of smell. Considering the impairments of the sense of smell typical of an open TBI, one can to some extent judge the nature of the sustained injuries: accompanied by the bilateral loss of smell, the injuries of any area are 2 times more likely to be accompanied by damage to the dura mater.

Examination of changes in the taste

Located in the medulla oblongata, the nucleus of the glossopharyngeal nerve is the area where all peripheral gustatory fibers end. However, for the posterior third of the tongue and the front 2/3, the peripheral taste innervation is different. In the case of traumatic injuries, the course of the taste fibers for the anterior two-thirds of the tongue becomes particularly important, namely a taste disorder often occurs in fractures of the temporal bone accompanied by the affection of these fibers. This is due to the fact that the course of the taste fibers; for this part of the tongue has a direct link with the temporal bone (facial nerve channel, tympanum, internal auditory meatus). The fibers of the anterior 2/3 of the tongue go further to the cerebellopontine angle. They end in the nucleus of the IX nerve (medulla oblongata). The posterior third of the tongue is innervated by the fibers of the glossopharyngeal nerve, and therefore the perception of taste in this area in case of traumatic brain injury varies extremely rarely.

The methods for taste examination are also divided into subjective and objective. Clinics usually use subjective methods: threshold (electrogustometry) and suprathreshold (chemical). The objective examination methods include the formation of conditioned reflexes and recording of evoked potentials. To examine the taste of patients with traumatic injuries, doctors usually use a suprathreshold chemical method.

This test uses a quilted pad to apply solutions of salt (concentration of 2.5%, 4%, 10%) and sugar (4%, 10%, 20%) to the anterolateral surfaces of the left and right half of the tongue. Further, the same solutions are applied to the posterior third of the tongue. The patient is asked about what he/she feels (loss of taste, salt or sugar). The chemical method can check if the patient distinguishes between flavoring substances, but cannot detect the taste thresholds.

Electrogustometry can quickly determine the taste thresholds and obtain their most accurate quantitative assessment. It is important to note that this case does not involve a qualitative assessment of taste sensations (the patient only experiences a metallic or sour taste). When carrying out this examination, the anode of the device is applied to the lateral surfaces of the front part of the tongue, whereas the cathode is compressed in the patient’s hand. The electrodes are placed 1.5 centimeters from the center line. The current strength in this circuit increases smoothly from 0 amp up to the occurrence of a sensation of metallic or acidic taste in the mouth. The smallest current at which the patient experiences these sensations is the threshold of taste (it is recorded in mA).

In cases of traumatic brain injury accompanied by a fracture of the temporal bone pyramid, the taste disorders available in the front 2/3 of the tongue from the side of the crack is the leading local symptom. If we are talking about longitudinal cracks in the pyramid of the temporal bone, the taste is reduced in the front 2/3 of the tongue from the side of the lesion. When it comes to the longitudinal cracks in the pyramid of the temporal bone, loss of taste occurs in the front 2/3 of the tongue from the side of the lesion. This is clearly seen during electrogustometry. In this case, the method is 2 times more effective than chemical testing. The taste disorder is the result of traumatic damage to the chorda tympani. In the future, taste disorders will gradually regress.

The transverse cracks in the pyramid of the temporal bone more often result in a complete and irreversible loss of taste sensitivity in the front 2/3 of the tongue. In this case, both chemical method and electrogustometry show the same efficiency. Such patients most commonly damage taste fibers in the internal auditory meatus.

Without the cracks in the pyramids of temporal bones in case of TBI, any local taste disorders can develop. In minor injuries, the taste is preserved to the fullest extent in both acute and residual periods. In moderate TBI, the thresholds of taste slightly increase throughout the entire tongue on the 7-8th day after injury, while in case of severe injuries such a rise can occur at a later period. It is especially manifested in elderly patients. Electrogustometry is the only method for detecting the disorder. It is caused by a general decrease in the functions of various analyzers after the severe TBI.

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