Page 775 - Small Animal Internal Medicine, 6th Edition

CHAPTER 46 Disorders of the Hypothalamus and Pituitary Gland 747

compulsive water consumption has not been reported in the
dog or the cat, although an abnormal vasopressin response
VetBooks.ir to hypertonic saline infusion has been reported in dogs with
suspected primary PD. A psychogenic or behavioral basis for
compulsive water consumption has been described in the
dog, typically in young hyperactive dogs that have been
placed in exercise-restricted environments but has not been
reported in the cat. Psychogenic PD may be induced by
concurrent disease (e.g., hepatic insufficiency, hyperthyroid-
ism) or may represent a learned behavior after a change in
the pet’s environment. PU is compensatory to prevent over-
hydration. Psychogenic PD can be diagnosed in dogs of any
age, either gender, and numerous breeds.
Dogs (and presumably cats) with primary or psychogenic
PD have an intact hypothalamic-pituitary-renal axis for con- FIG 46.3
trolling fluid balance and variable severity of renal medullary Endocrine alopecia, thin skin, and severe obesity in a
solute washout. Because AVP production and renal tubular 7-year-old male castrated Pomeranian with iatrogenic
response to AVP are normal, these dogs can concentrate hyperadrenocorticism caused by long-term administration of
urine in excess of 1.030. Depending on the severity of renal prednisone for a seizure disorder. Note the symmetric
medullary solute washout, a period of 24 hours or longer of truncal alopecia with sparing of the head and distal
extremities.
water deprivation may be necessary to attain concentrated
urine. Urine specific gravity may vary widely over time and
concentrated urine may be identified on random urine eval- such as scales, crusts, and papules, are absent. Seborrhea and
uation. Identification of concentrated urine implies hypotha- pyoderma may develop, depending on the underlying cause.
lamic AVP production, pituitary AVP secretion, and renal Causes of endocrine alopecia are listed in Table 46.4. In
tubular responsiveness to AVP. dogs the most common causes are hypothyroidism and glu-
Psychogenic PD is diagnosed by exclusion of other causes cocorticoid excess (iatrogenic or spontaneous). Hormone-
of PU and PD and by demonstrating that the dog or cat can induced causes of alopecia are not common in cats. The
concentrate urine to a specific gravity in excess of 1.030 diagnostic evaluation for endocrine alopecia begins with
during water deprivation. a complete history, physical examination, CBC, serum
Treatment is aimed at gradually limiting water intake to biochemistry panel, and urinalysis. Results of these tests
amounts in the upper end of the normal range. The client often provide evidence of hypothyroidism and hyperadre-
should determine the dog’s approximate water intake over a nocorticism; appropriate diagnostic tests can then be per-
24-hour period when free-choice water is allowed; this formed to confirm these diagnoses (see Chapters 48 and 50,
volume of water is then reduced by 10% per week until water respectively).
volumes of 60 to 80 mL/kg/24 h are reached. The total Once hypothyroidism and hyperadrenocorticism have
24-hour volume of water should be divided into several ali- been ruled out, the next diagnostic step is to rule out an excess
quots, with the last aliquot given at bedtime. Oral salt or less commonly a deficiency of one of the sex hormones,
(1 g/30 kg q12h) and/or oral sodium bicarbonate (0.6 g/30 kg most notably estrogen and progesterone. Dermatologic man-
q12h) may also be administered for 3 to 5 days to help rees- ifestations are similar for most sex hormone–induced der-
tablish the renal medullary concentration gradient. Changes matoses and include endocrine alopecia that initially begins
in the dog’s environment or daily routine should be consid- in the perineal, genital, and ventral abdominal regions and
ered, such as initiating a daily exercise routine; bringing a spreads cranially; dull, dry, easily epilated hair; failure of the
second pet into the home; providing some distraction, such haircoat to regrow after clipping; and variable presence of
as a radio playing when the clients are not at home; or seborrhea and hyperpigmentation. Additional clinical signs
moving the dog to an area that will ensure an increased of hyperestrogenism may include gynecomastia, a pendu-
amount of contact with humans. lous prepuce, the attraction of other male dogs, squatting to
urinate, and unilateral testicular atrophy (contralateral to the
testicular tumor) in the male dog, and vulvar enlargement
ENDOCRINE ALOPECIA and persistent proestrus or estrus in the female dog. Results
of a CBC may reveal aplastic anemia. Histologic assessment
Symmetric alopecia without historical or clinical evidence of of a skin biopsy specimen can be used to identify nonspecific
inflammation usually results from hair cycle arrest induced endocrine-related alterations and to support the diagnosis
by hormonal disease or imbalance—hence the term endo- of endocrine alopecia (Table 46.5). No pathognomonic his-
crine alopecia (Fig. 46.3). Hair follicles are atrophic, the hairs tologic changes are noted for sex hormone–induced derma-
are easily epilated, the skin is often thin and hypotonic, and toses. Identification of cornified epithelial cells on cytologic
hyperpigmentation is common. Other dermatologic lesions, examination of vaginal or preputial smears provides a clue