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Assessment of Maturity - Prematurity
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Editor's Comment - While there have been a greater number of successful outcomes from critical care of foals, the premature foal remains the most difficult condition to manage. Outcomes of very premature foals admitted to intensive care reveal a poor rate of survival (4060%). Of those surviving, limb angulation, lack of calcification of bones in carpus and tarsus, and poor growth continue to cause complications. When determining if a foal is premature, physical signs and hematology are more reliable than using gestational age.
I. Gestational Length
- Normal range = 320-360 days, no difference for sex, or time of year.
- Less than 320 days considered premature.
- Gestation length is poor indicator of individual readiness for birth.
Terms "Immature" and "dysmature" indicate gestational age is normal, but foal has signs, behavior and physiology of a premature foal. - Foals that are small for gestational age (term) due to in utero growth retardation.
- May or may not show signs of prematurity.
- Causes: in utero infections, twins or twin which was resorbed or mummified in early gestation, placentitis, malnutrition, other maternal uterine abnormalities.
II. Clinical Signs of Immaturity - Unreadiness for Birth [1]
- Weaker, take longer to rise and depressed sucking ability.
- Smaller size and reduced birth weight.
- Silky hair over back and rear quarters.
- Hooves do not dry out and separate from the "golden hoof".
- Domed forehead and soft lips.
- Floppy ears, red tongue.
- Hyperextension (hypoflexion) of fetlock joints.
- Reduced tolerance to oral feedings.
- Reduced body temperature and susceptibility to hypothermia.
- Incomplete ossification of carpal and tarsal bones. May predispose to angular limb deformity.
- Respiratory rate may increase following birth with some evidence of respiratory distress.
III. Laboratory Findings in Unreadiness for Birth
- Adrenocortical insufficiency.
- Seen as narrow neutrophil/lymphocyte ratio of 0.5-1:1 due to a neutropenia. Normal foals have a 2:1 ratio 3 hours following birth.
- Lymphocyte counts are higher than term foals (3500 5000/ul).
- Low plasma cortisol and no rise following administration of ACTH.
- Normal foals have a wider neutrophil/lymphocyte change in response to IM ACTH within 3 hours of birth.
- 0.01 mg cosyntropin (low dose) or 0.1 mg (high dose) (short-acting) IM total [2,3].
- Foals without HPA (hypothalamic-pituitary-adrenal axis) dysfunction should have a significant rise in plasma cortisol levels 30 minutes after administration [3].
- Increased plasma Vasopressin and ACTH concentrations in septic foals were associated with higher mortality in one study. Several septic foals had increased vasopressin/ACTH and ACTH/cortisol ratios, which indicates relative adenohypophyseal and adrenal insufficiency [4].
- Respiratory system compromise and blood gas abnormalities.
- Signs of hypoventilation - Decreased PaO2 increased PaCO2.
- Venous pH <7.25 (normal foal >7.3) and tendency for pH to decrease.
- May have decreased functional surfactant and respiratory distress.
- Depressed blood glucose 2 hours post foaling (<60 mg/dl; 3.3 mmol/l).
- Decreased absorption of colostral immunoglobulin. Overall immune function is decreased.
- Increased susceptibility to infection.
IV. Outcome of Premature Foals - Based on Induced Parturition Model [5]
- Survival rates of foals from induced parturition before 320 days gestation is poor.
- Spontaneous births in Thoroughbreds between 280-322 days gestation have 73% survival. Fetus is prepared for delivery and final maturation processes develop
- Correlation of post foaling behavior and outcome w/o treatment [2].
- If the foal does not establish righting reflexes and has weak sucking - Will die.
- If foal becomes sternal and has good head and neck tone, strong sucking reflex, but took 2 hours to stand, will do well for 24 hours and then fade and die unless intervention provided.
- If foal appears viable at birth and has normal righting reflexes at delivery, can stand and suck - Prognosis is better but a degree of fading still occurs in this group, 1-7 days following birth.
V. Diagnostic Evaluation
- Conduct physical exam.
- Complete blood count.
- Measure venous acid base.
- To assess respiratory maturity and function
- Administer face mask oxygen (10 L/min) and measure arterial oxygen after 5 minutes. PaO2 should be >200 mmHg. If less, have shunt or atelectasis.
- Monitor serum glucose levels.
- Assess serum IgG. Recommend giving supplemental colostrum and 1 liter plasma IV before assessment of IgG at 18 hrs because of the common finding of failure of passive transfer.
VI. Treatment [1]
- Supporting the foal by environmental temperature control, nutrition, circulatory and respiratory support with oxygen, or ventilation may be required.
- Adrenal insufficiency support.
- Depot long acting ACTH (Synacthen Depot®, Novartis) may provide stimulation after several days.
- 0.4 mg IM total dose. Repeat 6 and 12 hours with 0.2 mg IM total dose.
Editor's Comment - This is from older literature.
- Hydrocortisone -Sodium succinate (Cortelan® Glaxo Laboratories) or 50 mg BID Solu-Delta-Cortef® (Upjohn) has been used.
- Avoid pharmacological doses to prevent disruption of immune system.
- A short tapering course of hydrocortisone sodium succinate (Pfizer, New York) (1.3 mg/kg/day for 48h, 0.65mg/kg/d for 24h then 0.33mg/kg/d for 12h; total doses given in 6 doses as an IV bolus q4h) has potentially beneficial anti-inflammatory effects without significantly impairing innate immune function [7].
- Surfactant replacement and chemical closure of ductus arteriosus (flunixin-meglumine daily for 48 hrs) are now being attempted to increase survivability.
- Daily nursing care and maintenance as per the critical foal.
- Watch for entropion.
- These foals tolerate oral feeding very poorly initially.
- Start total parental nutrition early.
Editor's Comment - This can greatly increase survival.
- Prevention and control of sepsis.
- Therapy for limbs - MUST assess by radiography the degree of ossification of carpus and tarsus. Foals body weight must be supported during early ambulation.
- Heel extensions taped or glued on foot have provided increased support (See Cardiac Catheterization Pressure Measurements)
- Support wraps should not normally be used because they cause tendon relaxation.
- Radiograph carpus and tarsus and limit activity in foals with minimum ossification.
- Provide optimal nutrition up to 30% of BW per day in milk while confining foal and limiting movement.
- Many foals survive and then have damaged joints.
- Complete non weight bearing may be needed initially.
VII. Nutrition Support (See Parenteral Nutrition and Enteral Nutrition)
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1. Vaala WE. Diagnosis and treatment of prematurity and neonatal maladjustment syndrome in newborn foals. Compend Cont Educ Pract Vet 8:S211-S222, 1986.
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School of Veterinary Medicine, University of California-Davis, CA, USA.
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