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Primary acquired melanosis (PAM) presents as a unilateral
neoplastic
melanocytic
proliferation within the conjunctival epithelium
characterized by patchy area
of conjunctival pigmentation (yellow
brown stippling
within the epithelium).
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It has
been suggested by some authors that
""primary
acquired
melanosis"
(PAM) of the conjunctiva and its analogs, namely, lentigo
melanosis (Hutchinson), melanotic freckle (Hutchinson),
melanose
circonscrite precancereuse (Dubrueilh), melanotische
precancerose
(Miescher), lentigo maligna (Clark), precancerous
melanosis (Reese),
benign, precancerous, and cancerous melanosis
(Zimmerman),
atypical melanocytic hyperplasia (Silver et al.), and
benign acquired
melanosis
(Zimmerman), are synonyms for melanoma in situ.
"Primary acquired melanosis," is not simply a condition
of
blackening by melanin, but a flat melanoma that, if not
removed
completely, may give rise one day to metastases that cause
death. |
"Primary
acquired melanosis," is mostly found in middle-aged or
elderly
white
patients.
It is
predominantly a proliferative condition of the
melanocytes that
normally populate the conjunctival epithelium.
Although bulbar
conjunctival involvement is most common, any part
of the
conjunctiva, including the non-exposed fornical, palpebral
and canthal
regions, may be affected.
Because PAM has
the potential of becoming malignant, it is important
to recognize PAM and
to rule out other causes of pigmented lesions.
The presence or
absence of atypia is helpful in
determining the potential
for malignancy, because PAM without atypia
is usually benign, whereas
PAM with atypia may convert into a
conjunctival melanoma.
If atypia is present, the presence or absence
of epithelioid cells and the
pattern of
intraepithelial growth are the main factors in determining
the likelihood of neoplastic
transformation.
The lesion is
usually flat, which distinguishes it from nevi and melanomas
which are
elevated.
When elevated
lesions develop in PAM they are usually a sign of
malignancy.
The elevation
of PAM is unpredictable. It often progresses slowly, but
may wax or
wane.
Clinical
evaluation by simple inspection and slit-lamp examination may
not reveal
the full extent of the melanosis.
Using
ultraviolet light, areas of subclinical melanosis may be
identified,
extending far
beyond the edges of the obvious lesion.
The
histologic features of PAM are extremely variable, not only in
different
cases but in different areas within a lesion.
The
melanocytes vary in size, shape, and degree of atypia.
They may be
small polyhedral cells with no atypia, or large, highly
atypical
epithelioid cells. The pattern of involvement also varies.
Individual
melanocyte may line up along the basal lamina of the
epithelium,
form nests of melanocytes extending into the epithelium
in a pagetoid
fashion, or completely replace the epithelium
with atypical
melanocytes mimicking an in situ-carcinoma.
When
melacocytes invade the epithelium in a pagetoid fashion or
replace the
epithelium, the risk of melanoma is approximately 90%.
When they are
confined to the basal layer of the epithelium, the risk
of that
lesion developing into a melanoma is around 20%.
75 % of
tumours that contain epithelioid cells progress to melanoma.
Lesions in
which no or only minimal atypia can be detected seem not
to progress
to melanoma.
Primary
acquired melanosis in an adult should not be
confused with "a
junctional
nevus," which is almost always restricted to childhood.
A
combination of surgical excision and cryotherapy is the most widely
recommended
treatment for conjunctival primary acquired melanosis
with atypia.
Recently, topical chemotherapy with mitomycin-C eye drops has been
established as a therapeutic option.
Visit:
Conjunctival Melanocytic Tumours
;
Conjunctival Nevus
;
Conjunctival Melanoma
;
Uveal Melanoma
.
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