Here is a scan from the Merck Manual 17th ed. IBSN 0911910-10-1

DISORDERS OF SMELL AND TASTE

Because distinct flavors depend on aro-
mas to stimulate the olfactory chemorecep-
tors, taste and smell are physiologically in-
terdependent, and dysfunction of one often
disturbs the other. Disorders of smell and
taste are rarely incapacitating or life threat-
ening, so they often do not receive close
medical attention. However, the inability to
detect certain odours, such as gas, may be
dangerous, and several systemic and intra-
cranial disorders should be eliminated
before dismissing symptoms as harmless.
Whether brain stem disease (involvement of
the nucleus solitarius) can cause disorders
of smell and taste is uncertain because other
neurologic manifestations are usually over-
shadowing.

Anosmia (loss of the sense of smell) is
probably the most common abnormality (see
below). Hyperosmia (increased sensitivity
to odors) usually reflects a neurotic or his-
trionic personality. Dysosmia (disagreeable
or distorted sense of smell) may occur with
infection of the nasal sinuses, partial damage
to the olfactory bulbs, or psychologic de-
pression. Some cases, accompanied by a dis-
agreeable taste, result from poor dental hy-
giene. Uncinate epilepsy can produce brief,
vivid, unpleasant olfactory hallucinations.
Hyposmia (diminished sense of smell) and
hypogeusia (diminished sense of taste) can
follow acute influenza, usually temporarily.
Drying of the oral mucosa from heavy
smoking, Sjogren's syndrome, radiation ther-
apy of the head and neck, or desquamation
of the tongue can impair taste, and various
drugs (eg, amitriptyline, vincristine) alter
taste. In all instances, the gustatory recep-
tors are diffusely involved. When limited to
one side of the tongue (eg, in Bell's palsy),
ageusia (loss of the sense of taste) is rarely
noticed.

Rarely, idiopathic dysgeusia (distorted
sense of taste), hypogeusia, and dysosmia
respond to zinc supplementation.

ANOSMIA
Loss of the sense of smell.

Anosmia requires thorough evaluation for
intranasal and intracranial diseases. Loss of
smell occurs when intranasal swelling or
other obstruction prevents odors from gain-
ing access to the olfactory area; when the
olfactory neuroepithelium is destroyed, as in
viral infections, atrophic rhinitis, or the
chronic rhinitis of granulomatous diseases
and neoplasms; or when the olfactory nerve
fila, bulbs, tracts, or central connections are
destroyed, eg, by head trauma, intracranial
surgery, infections, or neoplasms. Head
trauma is a major cause of anosmia in young
adults. Viral infections are a major cause in
older adults. Anosmia occurs congenitally in
male hypogonadism (Kallmann's syn-
drome). Most patients with anosmia have
normal perception of salty, sweet, sour, and
bitter substances, but they lack flavor dis-
crimination, which is largely dependent on
olfaction; therefore, they often complain of
losing the sense of taste (ageusia). If unilat-
eral, anosmia is often unrecognized.

Diagnostic evaluation requires examina-
tion of the cranial nerves (see NEUROLOGIC
EXAMINATION in Ch. 165) and of the upper
respiratory tract (particularly the nose and
nasopharynx), psychophysical assessment
of odour and taste identification and threshold
detection, and enhanced CT of the head to
rule out neoplasms and unsuspected frac-
tures of the floor of the anterior cranial fossa.

Treatment of allergic or bacterial rhinitis
and sinusitis or removal of nasal polyps and
benign neoplasms may result in recovery of
the sense of smell. Conditions causing de-
struction of the olfactory neuroepithelium or
its central pathways do not lend themselves
to effective treatment, although spontane-
ous recovery may follow regeneration of
these tissues.