Introduction

The diagnosis of tonsillar herniation has increased as the use of Magnetic Resonance Imaging (MRI) has become widespread. Hans Chiari first described the various alterations in the posterior fossa that became known as the Chiari Malformations. Type I was defined as elongation of the cerebellum into cone shape projections into the spinal canal. Chiari's original cases all had hydrocephalus. The first case of a Chiari Type I malformation in an adult without hydrocephalus was described by Aring in 1938. Larger series were published in the 1960s and 1970s and the term Adult Chiari Malformation (ACM) was coined for type I malformations in adults without hydrocephalus. ₁

The sagittal views provided by MRI demonstrate tonsillar ectopia of at least five millimeters (ACM) in 0.56-0.77 % of scans performed (Figure 1). The prevalence of symptomatic ACM is more difficult to characterize. The disorder occurs more commonly in women (3:1 gender ratio). The mean age of onset is approximately 25 years of age. The delay in diagnosis is at least five years in most studies. Symptoms include headache, imbalance, transient visual obscurations, intracranial noises and retro-orbital ache (Table 1) ₂,₃,₄,₅,₆,₇,₈,₉,₁₀,₁₁,₁₂ . Mueller and Oro, in a study of 265 patients, found that the five most frequently reported symptoms were headache (98%), dizziness (84%), difficulty sleeping (72%), weakness of an upper extremity (69%) and neck pain (67%). This study included both patients with and without syringomyelia. ₁₂ Reported clinical signs have varied widely (Table 2). ₁₃,₁₄,₁₅,₁₆,₁₇,₁₈,₁₉,₂₀,₂₁,₂₂

Figure 1

Table 1: Reported Symptoms of Adult Chiari Malformation (ACM)

Figure 2

Table 2: Reported Clinical Signs of Adult Chiari Malformation (ACM)

Ophthalmic findings in ACM have been poorly characterized but have included papilledema, decreased acuity, extraocular muscle palsy, convergence or divergence paresis, skew deviation, nystagmus and absent venous pulsations. ₂₁ Kumar and colleagues performed vestibular evaluation on 77 confirmed ACM patients symptomatic of hearing loss, dizziness, and tinnitus and found horizontal spontaneous nystagmus in over one third of patients. Other findings included vertical upbeat nystagmus, downbeat nystagmus, saccadic dysmetria, optokinetic nystagmus, and smooth-pursuit impairment. ₂₂

The cause of ACM is multifactorial. Craniocephalic disproportion is common. The skull dimensions can be congenitally small as seen with craniosynostosis. Increased intracranial pressure, as found in idiopathic intracranial hypertension, is also associated with ACM. Lumbar punctures or lumboperitoneal shunts can cause tonsillar descent. ₂₃,₂₄,₂₅,₂₆,₂₇,₂₈,₂₉

ACM becomes symptomatic due to two distinct effects: 1) direct compression of nervous tissue and 2) a valve effect by the tonsils at the foramen magnum. ₃₀ The direct compression leads to lower brainstem and cerebellar signs. The valve effect alters normal neural hydrodynamics. Normally, cerebral blood volume increases with systole and the relative brain engorgement is accommodated by the flow of cerebrospinal fluid into the spine. In ACM, intracranial pressure transiently increases and brain compliance is altered. Not surprisingly then, many of the symptoms are similar to IIH and even worsen with Valsalva maneuver. ₂₃,₂₄,₂₅,₂₆,₂₇,₂₈,₂₉,₃₀,₃₁,₃₂

Approximately 3500 surgical decompressions are performed per year in the US for symptomatic ACM. Arora et al. reported that the most dramatic symptomatic improvements in a group of Chiari I patients following surgical decompression were spasticity, neck pain, and cerebellar signs. ₃₃

Methods

This is a prospective observational case series of consecutive patients referred for standard neuro-ophthalmic evaluation of ACM. Inclusion criteria included tonsillar descent of at least five millimeters and characteristic symptoms.

A comprehensive neuro-ophthalmologic evaluation was performed that included best-corrected visual acuity, color vision (pseudoisochromatic color plates), pupillary size and function, anterior segment and posterior segment examination. The extraocular motility examination included assessment for cranial nerve dysfunction, nystagmus and vestibular ocular reflex impairment. In addition Humphrey visual fields (HVF 30-2) and Heidelberg Retinal Tomography (HRT) were performed to detect subclinical optic nerve dysfunction. (All examinations were performed by KPC).

Findings

56 patients with symptomatic Adult Chiari Malformation (ACM) were evaluated. The gender ratio was consistent with previous studies (40 women: 16 men). The mean age was 41.2 years old with a range of 8 to 73 years. This mean age is older than previous reports.

The most common visual symptom was transient visual obscurations (TVOs) lasting seconds (n= 35). The TVOs were unilateral (n = 3) or bilateral (n = 31), for a total of 65 affected eyes. Pain or fullness behind the eyes was present in 57 eyes of 32 patients, with 7 patients having unilateral discomfort. The discomfort increased with eye movement in some cases. 14 patients had floaters in the absence of any vitreous detachment. Neck pain (n = 40), dizziness (n = 35), numbness (n= 26) and facial pain (n = 12) were also common complaints. A wide variety of other symptoms were reported (Table 3). The severity of symptoms varied from moderate to extremely limiting.

Figure 3

Table 3: Neurologic Symptoms in Our ACM Series

Persistently decreased vision due to afferent dysfunction was not found. Fundus examination was remarkable for lack of venous pulsations in 51 eyes of 28 patients (50%), 1+ SVP in 24 eyes of 17 patients (7 unilateral) and 2 + SVP in 22 eyes of 14 patients (Table 4)(Figure 3) . One patient had 3+ SVP in one eye. Eight patients had dampened SVPs evoked only with digital pressure or Valsalva maneuver. Mild hyperemia with telangiectatic vessels was noted in 3 patients. No patients demonstrated disc edema, exudates, gliosis or reverse vessel taper. 3 patients demonstrated nystagmus, but no patients demonstrated cranial nerve deficits.

Figure 4

Table 4: Neuro-Ophthalmic Signs in Our ACM Series

Interpretation

Spontaneous venous pulsations are best seen with a direct ophthalmoscope or hand held lens (90 or 78 diopter) at the slit lamp. A vein overlying the optic disc, usually at the depth of the cup, is observed for brief collapse of the walls (pulsations). The physiology behind the pulsations is debated (Table 5). Pulsations are increased in patients with significant elevated intraocular pressure. Mydriatics have historically been noted to increase pulsations, but this is now controversial. In normals, digital pressure or Valsalva maneuver results in a transient increase in pulsations following release of the pressure or termination of the Valsalva. Intra-ocular hypotension and increased intracranial pressure have been associated with absent pulsations. Jugular pressure causes transient increase in intracranial pressure and has been noted to lead to suppression of venous pulsations.

Figure 5

Table 5: Theories on the Pathophysiology of the Retinal Venous Pulsation ,,

Hedges and colleagues further characterized the optic nerve head characteristics that limit the ability to detect venous pulsations in normals. If the veins were easily visible and a normal cup to disc ratio was present, spontaneous venous pulsations were present in 73%, if the arteries or gliosis partially obscured the veins or the cup to disc ratio was less than 0.1, spontaneous venous pulsations were present in 43 %. If the arteries or gliosis completely obscured the veins or the optic nerve is congenitally elevated and full, absent pulsations were very prevalent.

Adult Chiari Malformation (ACM) is diagnosed by performing sagittal MRI in patients with suspicious symptoms. Because the combination of vague symptoms can be difficult to objectively quantitate, many patients' symptoms are mistakenly labeled as psychogenic. This delay to diagnosis accompanies an increasing patient frustration. In this series of symptomatic patients with Adult Chiari Malformation, the most common visual symptoms were transient visual obscurations and retro-orbital ache. The most common objective finding was lack of venous pulsations even with Valsalva maneuvers. One possible explanation for this could be that the increased intracranial pressure in ACM leads to suppressed spontaneous venous pulsations.

A complete neuro-ophthalmic evaluation was helpful to exclude other diagnoses and confirm characteristic findings. A team of subspecialists, including a neuro-ophthalmologist, neurologist, neuro-otologist, and neurosurgeon best manages adult Chiari malformation.