Page 805 - Clinical Small Animal Internal Medicine

773

VetBooks.ir

71

Disorders of the Forebrain

Sam N. Long, BVSc, PhD, DipECVN

Centre for Animal Referral and Emergency, Melbourne, Australia

Anatomy/Function Sitting between and below the cerebral hemispheres,
and connecting them to the rest of the brainstem, is the
The forebrain, also known as the prosencephalon, diencephalon. The most important functional compo-
encompasses the two cerebral hemispheres, together nents of the diencephalon are the thalamus, a major
with the diencephalon (the most rostral part of the brain- gateway for all information entering and leaving the
stem, itself consisting of the thalamus, hypothalamus, cortex, and the hypothalamus, which houses higher
and epithalamus). It is regarded as a region of the brain control over the major endocrine systems such as the
separate from the rest of the brainstem because it is pituitary–adrenal axis and thyroid function, amongst
separated in part by the tentorium cerebelli from the others, as well as regulating temperature, appetite, and
other main compartment within the cranial vault, the sodium homeostasis.
caudal fossa. In addition, the forebrain is responsible for The clinical significance of these anatomic structures
those functions in both animals and humans which are is that disorders affecting the forebrain result in the
commonly thought of as the “higher” functions: mem- clinical signs detailed in Table 71.1.
ory, behavior, personality, learning, and integration. It is
the forebrain that receives information from the outside
world, integrates it with memory and other functions, Etiology/Pathophysiology
makes decisions based on this information and predicts
the outcomes of those decisions in order to select the Lesions within the forebrain can arise as one of a number
most appropriate one. Anatomically, this occurs within of pathologic processes, often categorized using the
what are known as the association areas. Rather than DAMNIT‐V system (Degenerative, Anomalous, Metabolic,
being a distinct area of the brain, the association areas Neoplastic/Nutritional, Inflammatory, Traumatic/Toxic
are a dense, interconnecting network of interneurons and Vascular). A complete list of differential diagnoses is
with short axons and widespread dendrites that essen- provided in Box 71.1.
tially have no beginning and no end. In dogs and cats, the It is important to note that clinical signs are dependent
association areas represent approximately 20% of the not on the nature of the lesion/disease, but on where it
cerebral cortex compared with approximately 85% in arises (and how rapidly). A patient presenting with signs
humans. of right‐sided forebrain disease (e.g., circling, unilateral
The other main areas within the cerebral cortex are the blindness in the left eye with normal pupillary light
projection areas. These regions are the primary sensory reflexes, left‐sided proprioceptive deficits) may be
and motor cortex (responsible for receiving sensation suffering from any one of the following diagnoses: an
and generating motor program output), the primary visual ischemic infarct, a primary brain tumor, granulomatous
and auditory centers (for receiving vision and hearing), meningoencephalitis, or a bacterial abscess. For this rea-
and the rhinencephalon (responsible for receiving son, details of history and signalment are extremely
olfactory information – evolutionarily this is the most important to collect since they provide the only way of
primitive part of the brain). narrowing this list of differentials.

Clinical Small Animal Internal Medicine Volume I, First Edition. Edited by David S. Bruyette.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.
Companion website: www.wiley.com/go/bruyette/clinical