Chapter 11: The Pupil

CHAPTER OUTLINE

• ANATOMY AND OVERVIEW OF PATHOPHYSIOLOGY

  • The iris

  • Sympathetic innervation of the pupil

  • Parasympathetic innervation of the Pupil

  • The afferent pupillary pathway and midbrain connections

• AFFERENT DISORDERS: THE RELATIVE AFFERENT PUPILLARY DEFECT

• MIDBRAIN DISORDERS AFFECTING THE PUPILS

  • Light-near dissociation

  • Midbrain relative afferent pupillary defects

• THE PUPIL AND EFFERENT DISORDERS OF THE VISUAL SYSTEM

  • Pupil states without anisocoria

  • Anisocoria

  • The iris and local factors

  • Oculosympathetic paresis (Horner syndrome)

  • Parasympathetic disorders of the pupil: oculomotor nerve

  • Parasympathetic disorders of the pupil: tonic pupil

  • The pharmacologically dilated pupil

  • Transient pupillary dilation

  • Clinical evaluation of anisocoria: overview

• KEY POINTS

INTRODUCTION

The pupils constrict relative to the amount of light perceived by the eye; therefore, the pupil can be used as a measure of afferent visual function. This principle is the basis of the relative afferent pupillary defect (RAPD) test. When the pupillary efferent system is normal, the pupils in the right and left eye are clinically the same size as each other, even though their size is constantly changing in response to ambient light and accommodative tone, and to other factors such as mood and state of alertness.

Pupil size is ultimately determined by the balance of efferent sympathetic and para-sympathetic flow to the pupil musculature. Disorders involving these autonomic systems can cause an asymmetry in pupil size (anisocoria). The pupil, therefore, is particularly important in neuroophthalmology because it is affected in both afferent and efferent disorders of the visual system.

ANATOMY AND OVERVIEW OF PATHOPHYSIOLOGY

Pupillary signs in neuro-ophthalmic disorders can be mastered by understanding the afferent and efferent systems that control the pupil. The neuroanatomic discussion begins with the iris and efferent pupillary systems followed by discussion of the pupillary light reflex and midbrain connections. Disease processes are briefly mentioned at pertinent points in the initial discussion of the neuroanatomy, but will be addressed in detail in the remainder of the chapter.

THE IRIS

The iris stroma readily contracts and expands in response to the forces generated by the intrinsic iris musculature (Figure 11–1). The pupillary constrictor muscle (iris sphincter) consists of a band of muscle fiber bundles encircling the pupillary aperture. Activation causes the diameter of the sphincter (and the pupil) to constrict (miosis). The muscle fiber bundles of the sphincter are segmentally innervated by parasympathetic autonomic nerves that travel from the midbrain to the orbit as part of the third cranial nerve (CN III).

The dilator muscle fiber bundles are radially oriented. The muscle fibers are anchored at ...