23 Blindness
INITIAL PRESENTATION
Bumps into things, reluctant to move, hesitant to use stairs, stumbles on uneven ground, sleeps all day, unable to locate moving or stationary objects, cannot catch objects, always sniffing the ground, low head carriage, develops aggressive behaviour.
INTRODUCTION
Blindness
the lack of vision, is a result of pathology in the afferent pathway of the visual system (Fig. 23.1).
It may appear worse at night (nyctalopia) or during the day (hemeralopia: blindness with good night vision). Animals adapt remarkably well to a loss of vision as long as the environment remains constant. A lack of adaption in which the animal still behaves in a quiet tentative manner, bumping into objects, may indicate a cerebral lesion. Long-term blindness may appear to be sudden in onset following a change in living arrangements, e.g. staying in kennels or visiting new areas. Physical obstruction of the eyes by blepharospasm or bilaterally protruded third eyelids secondary to enophthalmos is capable of mechanically rendering the animal sightless. Localizing the anatomic site for blindness requires examination of the fundus, PLR and vision.
PLR can persist in an eye blinded by retinal detachment, retinal degeneration or optic neuritis. The pupil in such cases is more dilated than normal.
Enlargement of the optic disc occurs in papilloedema and, more commonly, optic neuritis. Papilloedema describes an enlarged pale pink oedematous optic nerve head (disc) with indistinct margins over which retinal blood vessels ‘kink’. It has been reported as a sign of increased intracranial pressure. Papilloedema does not cause visual deficits.
Optic neuritis results in acute bilateral blindness and may be retrobulbar and hence invisible on fundic exam, or may involve the optic disc.
Atrophy of the optic nerve is reflected in a shrunken pale grey optic disc.
CASE HISTORY
The dog had been inappetant for 2 weeks when it started to have periods of standing under the dining room table and staring into space. Ptosis of the right eye occurred 5 days later. No oral lesions were found. Haematology, biochemistry, urine analysis and abdominal radiographs had failed to find the cause of the reduced appetite.
CLINICAL EXAMINATION
The dog was mentally depressed with a menace response that was absent in the right eye and decreased in the left. PLRs were absent bilaterally with a mydriatic left pupil. There was decreased movement of the right eye in all directions, right enophthalmos and right ptosis. Sensation to the face was normal. Retractor bulbi reflex was present bilaterally.
Gait, posture, hopping, proprioception and spinal reflexes were normal. No spinal pain was detected.
NEUROANATOMICAL DIAGNOSIS
The lesion was localized to the right CNN II, III, IV and VI, and the sympathetic nerve supply to the right eye (Horner’s syndrome) and to the left CNN II and III. The lesion was thought to lie in or near the cavernous sinus, or optic and orbital canals (see page 42).
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