Chapter 030. Disorders of Smell, Taste, and Hearing (Part 2) ppt

Chapter 030. Disorders of Smell,
Taste, and Hearing
(Part 2)

Disorders of the Sense of Smell
These are caused by conditions that interfere with the access of the odorant
to the olfactory neuroepithelium (transport loss), injure the receptor region
(sensory loss), or damage central olfactory pathways (neural loss). Currently no
clinical tests exist to differentiate these different types of olfactory losses.
Fortunately, the history of the disease provides important clues to the cause. The
leading causes of olfactory disorders are summarized in Table 30-1; the most
common etiologies are head trauma in children and young adults, and viral
infections in older adults.
Table 30-1 Causes of Olfactory Dysfunction
Transport Losses
Allergic rhinitis
Bacterial rhinitis and sinusitis

Congenital abnormalities
Nasal neoplasms
Nasal polyps
Nasal septal deviation
Nasal surgery
Viral infections
Sensory Losses
Drugs
Neoplasms
Radiation therapy
Toxin exposure
Neural Losses
AIDS

Alcoholism
Alzheimer's disease
Cigarette smoke
Depression
Diabetes mellitus
Drugs/toxins
Huntington's chorea
Hypothyroidism
Kallmann syndrome
Malnutrition
Neoplasms
Neurosurgery
Viral infections Parkinson's disease
Trauma
Vitamin B
12
deficiency

Zinc deficiency
Head trauma is followed by unilateral or bilateral impairment of smell in up
to 15% of cases; anosmia is more common than hyposmia. Olfactory dysfunction
is more common when trauma is associated with loss of consciousness,
moderately severe head injury (grades II–V), and skull fracture. Frontal injuries
and fractures disrupt the cribriform plate and olfactory axons that perforate it.
Sometimes there is an associated cerebrospinal fluid (CSF) rhinorrhea resulting
from a tearing of the dura overlying the cribriform plate and paranasal sinuses.
Anosmia may also follow blows to the occiput. Once traumatic anosmia develops,
it is usually permanent; only 10% of patients ever improve or recover. Perversion
of the sense of smell may occur as a transient phase in the recovery process.
Viral infections can destroy the olfactory neuroepithelium, which is then

replaced by respiratory epithelium. Parainfluenza virus type 3 appears to be
especially detrimental to human olfaction. HIV infection is associated with
subjective distortion of taste and smell, which may become more severe as the
disease progresses. The loss of taste and smell may play an important role in the
development and progression of HIV-associated wasting. Congenital anosmias are
rare but important. Kallmann syndrome is an X-linked disorder characterized by
congenital anosmia and hypogonadotropic hypogonadism resulting from a failure
of migration from the olfactory placode of olfactory receptor neurons and neurons
synthesizing gonadotropin-releasing hormone (Chap. 340). Anosmia can also
occur in albinos. The receptor cells are present but are hypoplastic, lack cilia, and
do not project above the surrounding supporting cells.
Meningiomas of the inferior frontal region are the most frequent neoplastic
cause of anosmia; loss of smell may be the only neurologic abnormality. Rarely,
anosmia can occur with gliomas of the frontal lobe. Occasionally, pituitary
adenomas, craniopharyngiomas, suprasellar meningiomas, and aneurysms of the
anterior part of the circle of Willis extend forward and damage olfactory
structures. These tumors and hamartomas may also induce seizures with olfactory
hallucinations, indicating involvement of the uncus of the temporal lobe.
Olfactory dysfunction is common in a variety of neurologic diseases,
including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis,
and multiple sclerosis. In Alzheimer's and Parkinson's, olfactory loss may be the
first clinical sign of the disease. In Parkinson's disease, bilateral olfactory deficits
occur more commonly than the cardinal signs of the disorder such as tremor. In
multiple sclerosis, olfactory loss is related to lesions visible by MRI, in olfactory
processing areas in the temporal and frontal lobes.
Dysosmia, subjective distortions of olfactory perception, may occur with
intranasal diseases that partially impair smell or during recovery from a
neurogenic anosmia. Most dysosmic disorders consist of disagreeable odors,
sometimes accompanied by distortions of taste. Dysosmia also can occur with
depression.